[PATCH v12 00/24] Enable CET Virtualization

[PATCH v12 00/24] Enable CET Virtualization

[Chao Gao]

The FPU support for CET virtualization has already been merged into 6.17-rc1.
Building on that, this series introduces Intel CET virtualization support for
KVM.

Changes in v12:
1. collect Tested-by tags from John and Mathias.
2. use less verbose names for KVM rdmsr/wrmsr emulation APIs in patch 1/2
   (Sean/Xin)
3. refer to s_cet, ssp, and ssp_table in a consistent order in patch 22
   (Xin)

Please note that I didn't include Mathias' patch, which makes CR4.CET
guest-owned. I expect that patch to be posted separately.

---
Control-flow Enforcement Technology (CET) is a kind of CPU feature used
to prevent Return/CALL/Jump-Oriented Programming (ROP/COP/JOP) attacks.
It provides two sub-features(SHSTK,IBT) to defend against ROP/COP/JOP
style control-flow subversion attacks.

Shadow Stack (SHSTK):
  A shadow stack is a second stack used exclusively for control transfer
  operations. The shadow stack is separate from the data/normal stack and
  can be enabled individually in user and kernel mode. When shadow stack
  is enabled, CALL pushes the return address on both the data and shadow
  stack. RET pops the return address from both stacks and compares them.
  If the return addresses from the two stacks do not match, the processor
  generates a #CP.

Indirect Branch Tracking (IBT):
  IBT introduces new instruction(ENDBRANCH)to mark valid target addresses
  of indirect branches (CALL, JMP etc...). If an indirect branch is
  executed and the next instruction is _not_ an ENDBRANCH, the processor
  generates a #CP. These instruction behaves as a NOP on platforms that
  doesn't support CET.

CET states management
=====================
KVM cooperates with host kernel FPU framework to manage guest CET registers.
With CET supervisor mode state support in this series, KVM can save/restore
full guest CET xsave-managed states.

CET user mode and supervisor mode xstates, i.e., MSR_IA32_{U_CET,PL3_SSP}
and MSR_IA32_PL{0,1,2}, depend on host FPU framework to swap guest and host
xstates. On VM-Exit, guest CET xstates are saved to guest fpu area and host
CET xstates are loaded from task/thread context before vCPU returns to
userspace, vice-versa on VM-Entry. See details in kvm_{load,put}_guest_fpu().

CET supervisor mode states are grouped into two categories : XSAVE-managed
and non-XSAVE-managed, the former includes MSR_IA32_PL{0,1,2}_SSP and are
controlled by CET supervisor mode bit(S_CET bit) in XSS, the later consists
of MSR_IA32_S_CET and MSR_IA32_INTR_SSP_TBL.

VMX introduces new VMCS fields, {GUEST|HOST}_{S_CET,SSP,INTR_SSP_TABL}, to
facilitate guest/host non-XSAVES-managed states. When VMX CET entry/exit load
bits are set, guest/host MSR_IA32_{S_CET,INTR_SSP_TBL,SSP} are loaded from
equivalent fields at VM-Exit/Entry. With these new fields, such supervisor
states require no addtional KVM save/reload actions.

Tests
======
This series has successfully passed the basic CET user shadow stack test
and kernel IBT test in both L1 and L2 guests. The newly added
KVM-unit-tests [2] also passed, and its v11 has been tested with the AMD
CET series by John [3].

For your convenience, you can use my WIP QEMU [1] for testing.

[1]: https://github.com/gaochaointel/qemu-dev qemu-cet
[2]: https://lore.kernel.org/kvm/20250626073459.12990-1-minipli@grsecurity.net/
[3]: https://lore.kernel.org/kvm/aH6CH+x5mCDrvtoz@AUSJOHALLEN.amd.com/

Chao Gao (3):
  KVM: x86: Zero XSTATE components on INIT by iterating over supported
    features
  KVM: nVMX: Add consistency checks for CR0.WP and CR4.CET
  KVM: nVMX: Add consistency checks for CET states

Sean Christopherson (4):
  KVM: x86: Use double-underscore read/write MSR helpers as appropriate
  KVM: x86: Manually clear MPX state only on INIT
  KVM: x86: Report XSS as to-be-saved if there are supported features
  KVM: x86: Load guest FPU state when access XSAVE-managed MSRs

Yang Weijiang (17):
  KVM: x86: Rename kvm_{g,s}et_msr()* to show that they emulate guest
    accesses
  KVM: x86: Add kvm_msr_{read,write}() helpers
  KVM: x86: Introduce KVM_{G,S}ET_ONE_REG uAPIs support
  KVM: x86: Refresh CPUID on write to guest MSR_IA32_XSS
  KVM: x86: Initialize kvm_caps.supported_xss
  KVM: x86: Add fault checks for guest CR4.CET setting
  KVM: x86: Report KVM supported CET MSRs as to-be-saved
  KVM: VMX: Introduce CET VMCS fields and control bits
  KVM: x86: Enable guest SSP read/write interface with new uAPIs
  KVM: VMX: Emulate read and write to CET MSRs
  KVM: x86: Save and reload SSP to/from SMRAM
  KVM: VMX: Set up interception for CET MSRs
  KVM: VMX: Set host constant supervisor states to VMCS fields
  KVM: x86: Don't emulate instructions guarded by CET
  KVM: x86: Enable CET virtualization for VMX and advertise to userspace
  KVM: nVMX: Virtualize NO_HW_ERROR_CODE_CC for L1 event injection to L2
  KVM: nVMX: Enable CET support for nested guest

 arch/x86/include/asm/kvm_host.h |  16 +-
 arch/x86/include/asm/vmx.h      |   9 +
 arch/x86/include/uapi/asm/kvm.h |  13 ++
 arch/x86/kvm/cpuid.c            |  19 +-
 arch/x86/kvm/emulate.c          |  46 +++--
 arch/x86/kvm/smm.c              |  12 +-
 arch/x86/kvm/smm.h              |   2 +-
 arch/x86/kvm/svm/svm.c          |   4 +
 arch/x86/kvm/vmx/capabilities.h |   9 +
 arch/x86/kvm/vmx/nested.c       | 175 +++++++++++++++--
 arch/x86/kvm/vmx/nested.h       |   5 +
 arch/x86/kvm/vmx/vmcs12.c       |   6 +
 arch/x86/kvm/vmx/vmcs12.h       |  14 +-
 arch/x86/kvm/vmx/vmx.c          |  85 +++++++-
 arch/x86/kvm/vmx/vmx.h          |   9 +-
 arch/x86/kvm/x86.c              | 339 +++++++++++++++++++++++++++-----
 arch/x86/kvm/x86.h              |  61 ++++++
 17 files changed, 732 insertions(+), 92 deletions(-)


base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
-- 
2.47.1

[PATCH v12 01/24] KVM: x86: Rename kvm_{g,s}et_msr()* to show that they emulate guest accesses

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

Rename
	kvm_{g,s}et_msr_with_filter()
	kvm_{g,s}et_msr()
to
	kvm_emulate_msr_{read,write}
	__kvm_emulate_msr_{read,write}

to make it more obvious that KVM uses these helpers to emulate guest
behaviors, i.e., host_initiated == false in these helpers.

Suggested-by: Sean Christopherson <seanjc@google.com>
Suggested-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
v12: use less verbose function names -- Sean/Xin
---
 arch/x86/include/asm/kvm_host.h |  8 ++++----
 arch/x86/kvm/smm.c              |  4 ++--
 arch/x86/kvm/vmx/nested.c       | 14 +++++++-------
 arch/x86/kvm/x86.c              | 26 +++++++++++++-------------
 4 files changed, 26 insertions(+), 26 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index f19a76d3ca0e..86e4d0b8469b 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2149,11 +2149,11 @@ void kvm_prepare_event_vectoring_exit(struct kvm_vcpu *vcpu, gpa_t gpa);
 
 void kvm_enable_efer_bits(u64);
 bool kvm_valid_efer(struct kvm_vcpu *vcpu, u64 efer);
-int kvm_get_msr_with_filter(struct kvm_vcpu *vcpu, u32 index, u64 *data);
-int kvm_set_msr_with_filter(struct kvm_vcpu *vcpu, u32 index, u64 data);
+int kvm_emulate_msr_read(struct kvm_vcpu *vcpu, u32 index, u64 *data);
+int kvm_emulate_msr_write(struct kvm_vcpu *vcpu, u32 index, u64 data);
 int __kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data, bool host_initiated);
-int kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data);
-int kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data);
+int __kvm_emulate_msr_read(struct kvm_vcpu *vcpu, u32 index, u64 *data);
+int __kvm_emulate_msr_write(struct kvm_vcpu *vcpu, u32 index, u64 data);
 int kvm_emulate_rdmsr(struct kvm_vcpu *vcpu);
 int kvm_emulate_wrmsr(struct kvm_vcpu *vcpu);
 int kvm_emulate_as_nop(struct kvm_vcpu *vcpu);
diff --git a/arch/x86/kvm/smm.c b/arch/x86/kvm/smm.c
index 9864c057187d..5dd8a1646800 100644
--- a/arch/x86/kvm/smm.c
+++ b/arch/x86/kvm/smm.c
@@ -529,7 +529,7 @@ static int rsm_load_state_64(struct x86_emulate_ctxt *ctxt,
 
 	vcpu->arch.smbase =         smstate->smbase;
 
-	if (kvm_set_msr(vcpu, MSR_EFER, smstate->efer & ~EFER_LMA))
+	if (__kvm_emulate_msr_write(vcpu, MSR_EFER, smstate->efer & ~EFER_LMA))
 		return X86EMUL_UNHANDLEABLE;
 
 	rsm_load_seg_64(vcpu, &smstate->tr, VCPU_SREG_TR);
@@ -620,7 +620,7 @@ int emulator_leave_smm(struct x86_emulate_ctxt *ctxt)
 
 		/* And finally go back to 32-bit mode.  */
 		efer = 0;
-		kvm_set_msr(vcpu, MSR_EFER, efer);
+		__kvm_emulate_msr_write(vcpu, MSR_EFER, efer);
 	}
 #endif
 
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index b8ea1969113d..7dc2e1c09ea6 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -997,7 +997,7 @@ static u32 nested_vmx_load_msr(struct kvm_vcpu *vcpu, u64 gpa, u32 count)
 				__func__, i, e.index, e.reserved);
 			goto fail;
 		}
-		if (kvm_set_msr_with_filter(vcpu, e.index, e.value)) {
+		if (kvm_emulate_msr_write(vcpu, e.index, e.value)) {
 			pr_debug_ratelimited(
 				"%s cannot write MSR (%u, 0x%x, 0x%llx)\n",
 				__func__, i, e.index, e.value);
@@ -1033,7 +1033,7 @@ static bool nested_vmx_get_vmexit_msr_value(struct kvm_vcpu *vcpu,
 		}
 	}
 
-	if (kvm_get_msr_with_filter(vcpu, msr_index, data)) {
+	if (kvm_emulate_msr_read(vcpu, msr_index, data)) {
 		pr_debug_ratelimited("%s cannot read MSR (0x%x)\n", __func__,
 			msr_index);
 		return false;
@@ -2770,8 +2770,8 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
 
 	if ((vmcs12->vm_entry_controls & VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL) &&
 	    kvm_pmu_has_perf_global_ctrl(vcpu_to_pmu(vcpu)) &&
-	    WARN_ON_ONCE(kvm_set_msr(vcpu, MSR_CORE_PERF_GLOBAL_CTRL,
-				     vmcs12->guest_ia32_perf_global_ctrl))) {
+	    WARN_ON_ONCE(__kvm_emulate_msr_write(vcpu, MSR_CORE_PERF_GLOBAL_CTRL,
+						 vmcs12->guest_ia32_perf_global_ctrl))) {
 		*entry_failure_code = ENTRY_FAIL_DEFAULT;
 		return -EINVAL;
 	}
@@ -4758,8 +4758,8 @@ static void load_vmcs12_host_state(struct kvm_vcpu *vcpu,
 	}
 	if ((vmcs12->vm_exit_controls & VM_EXIT_LOAD_IA32_PERF_GLOBAL_CTRL) &&
 	    kvm_pmu_has_perf_global_ctrl(vcpu_to_pmu(vcpu)))
-		WARN_ON_ONCE(kvm_set_msr(vcpu, MSR_CORE_PERF_GLOBAL_CTRL,
-					 vmcs12->host_ia32_perf_global_ctrl));
+		WARN_ON_ONCE(__kvm_emulate_msr_write(vcpu, MSR_CORE_PERF_GLOBAL_CTRL,
+						     vmcs12->host_ia32_perf_global_ctrl));
 
 	/* Set L1 segment info according to Intel SDM
 	    27.5.2 Loading Host Segment and Descriptor-Table Registers */
@@ -4937,7 +4937,7 @@ static void nested_vmx_restore_host_state(struct kvm_vcpu *vcpu)
 				goto vmabort;
 			}
 
-			if (kvm_set_msr_with_filter(vcpu, h.index, h.value)) {
+			if (kvm_emulate_msr_write(vcpu, h.index, h.value)) {
 				pr_debug_ratelimited(
 					"%s WRMSR failed (%u, 0x%x, 0x%llx)\n",
 					__func__, j, h.index, h.value);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a1c49bc681c4..09b106a5afdf 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1932,33 +1932,33 @@ static int kvm_get_msr_ignored_check(struct kvm_vcpu *vcpu,
 				 __kvm_get_msr);
 }
 
-int kvm_get_msr_with_filter(struct kvm_vcpu *vcpu, u32 index, u64 *data)
+int kvm_emulate_msr_read(struct kvm_vcpu *vcpu, u32 index, u64 *data)
 {
 	if (!kvm_msr_allowed(vcpu, index, KVM_MSR_FILTER_READ))
 		return KVM_MSR_RET_FILTERED;
 	return kvm_get_msr_ignored_check(vcpu, index, data, false);
 }
-EXPORT_SYMBOL_GPL(kvm_get_msr_with_filter);
+EXPORT_SYMBOL_GPL(kvm_emulate_msr_read);
 
-int kvm_set_msr_with_filter(struct kvm_vcpu *vcpu, u32 index, u64 data)
+int kvm_emulate_msr_write(struct kvm_vcpu *vcpu, u32 index, u64 data)
 {
 	if (!kvm_msr_allowed(vcpu, index, KVM_MSR_FILTER_WRITE))
 		return KVM_MSR_RET_FILTERED;
 	return kvm_set_msr_ignored_check(vcpu, index, data, false);
 }
-EXPORT_SYMBOL_GPL(kvm_set_msr_with_filter);
+EXPORT_SYMBOL_GPL(kvm_emulate_msr_write);
 
-int kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data)
+int __kvm_emulate_msr_read(struct kvm_vcpu *vcpu, u32 index, u64 *data)
 {
 	return kvm_get_msr_ignored_check(vcpu, index, data, false);
 }
-EXPORT_SYMBOL_GPL(kvm_get_msr);
+EXPORT_SYMBOL_GPL(__kvm_emulate_msr_read);
 
-int kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data)
+int __kvm_emulate_msr_write(struct kvm_vcpu *vcpu, u32 index, u64 data)
 {
 	return kvm_set_msr_ignored_check(vcpu, index, data, false);
 }
-EXPORT_SYMBOL_GPL(kvm_set_msr);
+EXPORT_SYMBOL_GPL(__kvm_emulate_msr_write);
 
 static void complete_userspace_rdmsr(struct kvm_vcpu *vcpu)
 {
@@ -2030,7 +2030,7 @@ int kvm_emulate_rdmsr(struct kvm_vcpu *vcpu)
 	u64 data;
 	int r;
 
-	r = kvm_get_msr_with_filter(vcpu, ecx, &data);
+	r = kvm_emulate_msr_read(vcpu, ecx, &data);
 
 	if (!r) {
 		trace_kvm_msr_read(ecx, data);
@@ -2055,7 +2055,7 @@ int kvm_emulate_wrmsr(struct kvm_vcpu *vcpu)
 	u64 data = kvm_read_edx_eax(vcpu);
 	int r;
 
-	r = kvm_set_msr_with_filter(vcpu, ecx, data);
+	r = kvm_emulate_msr_write(vcpu, ecx, data);
 
 	if (!r) {
 		trace_kvm_msr_write(ecx, data);
@@ -8353,7 +8353,7 @@ static int emulator_get_msr_with_filter(struct x86_emulate_ctxt *ctxt,
 	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
 	int r;
 
-	r = kvm_get_msr_with_filter(vcpu, msr_index, pdata);
+	r = kvm_emulate_msr_read(vcpu, msr_index, pdata);
 	if (r < 0)
 		return X86EMUL_UNHANDLEABLE;
 
@@ -8376,7 +8376,7 @@ static int emulator_set_msr_with_filter(struct x86_emulate_ctxt *ctxt,
 	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
 	int r;
 
-	r = kvm_set_msr_with_filter(vcpu, msr_index, data);
+	r = kvm_emulate_msr_write(vcpu, msr_index, data);
 	if (r < 0)
 		return X86EMUL_UNHANDLEABLE;
 
@@ -8396,7 +8396,7 @@ static int emulator_set_msr_with_filter(struct x86_emulate_ctxt *ctxt,
 static int emulator_get_msr(struct x86_emulate_ctxt *ctxt,
 			    u32 msr_index, u64 *pdata)
 {
-	return kvm_get_msr(emul_to_vcpu(ctxt), msr_index, pdata);
+	return __kvm_emulate_msr_read(emul_to_vcpu(ctxt), msr_index, pdata);
 }
 
 static int emulator_check_rdpmc_early(struct x86_emulate_ctxt *ctxt, u32 pmc)
-- 
2.47.1

Re: [PATCH v12 01/24] KVM: x86: Rename kvm_{g,s}et_msr()* to show that they emulate guest accesses

[Xiaoyao Li]

On 8/12/2025 10:55 AM, Chao Gao wrote:

...

> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index f19a76d3ca0e..86e4d0b8469b 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -2149,11 +2149,11 @@ void kvm_prepare_event_vectoring_exit(struct kvm_vcpu *vcpu, gpa_t gpa);
>   
>   void kvm_enable_efer_bits(u64);
>   bool kvm_valid_efer(struct kvm_vcpu *vcpu, u64 efer);
> -int kvm_get_msr_with_filter(struct kvm_vcpu *vcpu, u32 index, u64 *data);
> -int kvm_set_msr_with_filter(struct kvm_vcpu *vcpu, u32 index, u64 data);
> +int kvm_emulate_msr_read(struct kvm_vcpu *vcpu, u32 index, u64 *data);
> +int kvm_emulate_msr_write(struct kvm_vcpu *vcpu, u32 index, u64 data);
...
> -int kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data);
> -int kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data);
> +int __kvm_emulate_msr_read(struct kvm_vcpu *vcpu, u32 index, u64 *data);
> +int __kvm_emulate_msr_write(struct kvm_vcpu *vcpu, u32 index, u64 data);

I dislike the rename, it loses the information of filter.

can we keep the filer information and make them as below?

   kvm_get_msr_with_filter() -> kvm_emulate_msr_read_with_filter()
   kvm_get_msr() -> kvm_emulate_msr_read()
   kvm_set_msr_with_filter() -> kvm_emulate_msr_write_with_filter()
   kvm_set_msr() -> kvm_emulate_msr_write()

Re: [PATCH v12 01/24] KVM: x86: Rename kvm_{g,s}et_msr()* to show that they emulate guest accesses

[Sean Christopherson]

On Mon, Sep 01, 2025, Xiaoyao Li wrote:
> On 8/12/2025 10:55 AM, Chao Gao wrote:
> 
> ...
> 
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index f19a76d3ca0e..86e4d0b8469b 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -2149,11 +2149,11 @@ void kvm_prepare_event_vectoring_exit(struct kvm_vcpu *vcpu, gpa_t gpa);
> >   void kvm_enable_efer_bits(u64);
> >   bool kvm_valid_efer(struct kvm_vcpu *vcpu, u64 efer);
> > -int kvm_get_msr_with_filter(struct kvm_vcpu *vcpu, u32 index, u64 *data);
> > -int kvm_set_msr_with_filter(struct kvm_vcpu *vcpu, u32 index, u64 data);
> > +int kvm_emulate_msr_read(struct kvm_vcpu *vcpu, u32 index, u64 *data);
> > +int kvm_emulate_msr_write(struct kvm_vcpu *vcpu, u32 index, u64 data);
> ...
> > -int kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data);
> > -int kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data);
> > +int __kvm_emulate_msr_read(struct kvm_vcpu *vcpu, u32 index, u64 *data);
> > +int __kvm_emulate_msr_write(struct kvm_vcpu *vcpu, u32 index, u64 data);
> 
> I dislike the rename, it loses the information of filter.
> 
> can we keep the filer information and make them as below?
> 
>   kvm_get_msr_with_filter() -> kvm_emulate_msr_read_with_filter()
>   kvm_get_msr() -> kvm_emulate_msr_read()
>   kvm_set_msr_with_filter() -> kvm_emulate_msr_write_with_filter()
>   kvm_set_msr() -> kvm_emulate_msr_write()

Sorry, no.  I detest the "with_filter".  Requiring developers to add "with_filter"
suggests that emulating an access _with_ the filter is somehow exceptional.  That's
the exact opposite of the truth: emulating MSR accesses _without_ the filter should
only be done in very rare situations, e.g. when loading guest state as part of a
complex transition (RSM, SMI, VM-Enter, VM-Exit).

And if we keep "with_filter", the relationship between kvm_emulate_msr_{read,write}()
and __kvm_emulate_msr_{read,write}() will be non-obvious.

[PATCH v12 02/24] KVM: x86: Use double-underscore read/write MSR helpers as appropriate

[Chao Gao]

From: Sean Christopherson <seanjc@google.com>

Use the double-underscore helpers for emulating MSR reads and writes in
he no-underscore versions to better capture the relationship between the
two sets of APIs (the double-underscore versions don't honor userspace MSR
filters).

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/kvm/x86.c | 29 ++++++++++++++++-------------
 1 file changed, 16 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 09b106a5afdf..65c787bcfe8b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1932,11 +1932,24 @@ static int kvm_get_msr_ignored_check(struct kvm_vcpu *vcpu,
 				 __kvm_get_msr);
 }
 
+int __kvm_emulate_msr_read(struct kvm_vcpu *vcpu, u32 index, u64 *data)
+{
+	return kvm_get_msr_ignored_check(vcpu, index, data, false);
+}
+EXPORT_SYMBOL_GPL(__kvm_emulate_msr_read);
+
+int __kvm_emulate_msr_write(struct kvm_vcpu *vcpu, u32 index, u64 data)
+{
+	return kvm_set_msr_ignored_check(vcpu, index, data, false);
+}
+EXPORT_SYMBOL_GPL(__kvm_emulate_msr_write);
+
 int kvm_emulate_msr_read(struct kvm_vcpu *vcpu, u32 index, u64 *data)
 {
 	if (!kvm_msr_allowed(vcpu, index, KVM_MSR_FILTER_READ))
 		return KVM_MSR_RET_FILTERED;
-	return kvm_get_msr_ignored_check(vcpu, index, data, false);
+
+	return __kvm_emulate_msr_read(vcpu, index, data);
 }
 EXPORT_SYMBOL_GPL(kvm_emulate_msr_read);
 
@@ -1944,21 +1957,11 @@ int kvm_emulate_msr_write(struct kvm_vcpu *vcpu, u32 index, u64 data)
 {
 	if (!kvm_msr_allowed(vcpu, index, KVM_MSR_FILTER_WRITE))
 		return KVM_MSR_RET_FILTERED;
-	return kvm_set_msr_ignored_check(vcpu, index, data, false);
-}
-EXPORT_SYMBOL_GPL(kvm_emulate_msr_write);
 
-int __kvm_emulate_msr_read(struct kvm_vcpu *vcpu, u32 index, u64 *data)
-{
-	return kvm_get_msr_ignored_check(vcpu, index, data, false);
+	return __kvm_emulate_msr_write(vcpu, index, data);
 }
-EXPORT_SYMBOL_GPL(__kvm_emulate_msr_read);
+EXPORT_SYMBOL_GPL(kvm_emulate_msr_write);
 
-int __kvm_emulate_msr_write(struct kvm_vcpu *vcpu, u32 index, u64 data)
-{
-	return kvm_set_msr_ignored_check(vcpu, index, data, false);
-}
-EXPORT_SYMBOL_GPL(__kvm_emulate_msr_write);
 
 static void complete_userspace_rdmsr(struct kvm_vcpu *vcpu)
 {
-- 
2.47.1

[PATCH v12 03/24] KVM: x86: Add kvm_msr_{read,write}() helpers

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

Wrap __kvm_{get,set}_msr() into two new helpers for KVM usage and use the
helpers to replace existing usage of the raw functions.
kvm_msr_{read,write}() are KVM-internal helpers, i.e. used when KVM needs
to get/set a MSR value for emulating CPU behavior, i.e., host_initiated ==
%true in the helpers.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/include/asm/kvm_host.h |  3 ++-
 arch/x86/kvm/cpuid.c            |  2 +-
 arch/x86/kvm/x86.c              | 16 +++++++++++++---
 3 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 86e4d0b8469b..39b93642e7d2 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2151,9 +2151,10 @@ void kvm_enable_efer_bits(u64);
 bool kvm_valid_efer(struct kvm_vcpu *vcpu, u64 efer);
 int kvm_emulate_msr_read(struct kvm_vcpu *vcpu, u32 index, u64 *data);
 int kvm_emulate_msr_write(struct kvm_vcpu *vcpu, u32 index, u64 data);
-int __kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data, bool host_initiated);
 int __kvm_emulate_msr_read(struct kvm_vcpu *vcpu, u32 index, u64 *data);
 int __kvm_emulate_msr_write(struct kvm_vcpu *vcpu, u32 index, u64 data);
+int kvm_msr_read(struct kvm_vcpu *vcpu, u32 index, u64 *data);
+int kvm_msr_write(struct kvm_vcpu *vcpu, u32 index, u64 data);
 int kvm_emulate_rdmsr(struct kvm_vcpu *vcpu);
 int kvm_emulate_wrmsr(struct kvm_vcpu *vcpu);
 int kvm_emulate_as_nop(struct kvm_vcpu *vcpu);
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index e2836a255b16..30fd18700972 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -2001,7 +2001,7 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
 		if (function == 7 && index == 0) {
 			u64 data;
 			if ((*ebx & (feature_bit(RTM) | feature_bit(HLE))) &&
-			    !__kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data, true) &&
+			    !kvm_msr_read(vcpu, MSR_IA32_TSX_CTRL, &data) &&
 			    (data & TSX_CTRL_CPUID_CLEAR))
 				*ebx &= ~(feature_bit(RTM) | feature_bit(HLE));
 		} else if (function == 0x80000007) {
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 65c787bcfe8b..726028eb647b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1898,8 +1898,8 @@ static int kvm_set_msr_ignored_check(struct kvm_vcpu *vcpu,
  * Returns 0 on success, non-0 otherwise.
  * Assumes vcpu_load() was already called.
  */
-int __kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data,
-		  bool host_initiated)
+static int __kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data,
+			 bool host_initiated)
 {
 	struct msr_data msr;
 	int ret;
@@ -1925,6 +1925,16 @@ int __kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data,
 	return ret;
 }
 
+int kvm_msr_write(struct kvm_vcpu *vcpu, u32 index, u64 data)
+{
+	return __kvm_set_msr(vcpu, index, data, true);
+}
+
+int kvm_msr_read(struct kvm_vcpu *vcpu, u32 index, u64 *data)
+{
+	return __kvm_get_msr(vcpu, index, data, true);
+}
+
 static int kvm_get_msr_ignored_check(struct kvm_vcpu *vcpu,
 				     u32 index, u64 *data, bool host_initiated)
 {
@@ -12463,7 +12473,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
 						  MSR_IA32_MISC_ENABLE_BTS_UNAVAIL;
 
 		__kvm_set_xcr(vcpu, 0, XFEATURE_MASK_FP);
-		__kvm_set_msr(vcpu, MSR_IA32_XSS, 0, true);
+		kvm_msr_write(vcpu, MSR_IA32_XSS, 0);
 	}
 
 	/* All GPRs except RDX (handled below) are zeroed on RESET/INIT. */
-- 
2.47.1

[PATCH v12 04/24] KVM: x86: Manually clear MPX state only on INIT

[Chao Gao]

From: Sean Christopherson <seanjc@google.com>

Don't manually clear/zero MPX state on RESET, as the guest FPU state is
zero allocated and KVM only does RESET during vCPU creation, i.e. the
relevant state is guaranteed to be all zeroes.

Opportunistically move the relevant code into a helper in anticipation of
adding support for CET shadow stacks, which also has state that is zeroed
on INIT.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/kvm/x86.c | 46 ++++++++++++++++++++++++++++++----------------
 1 file changed, 30 insertions(+), 16 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 726028eb647b..6cf0d15a7a64 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -12389,6 +12389,35 @@ void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)
 	kvfree(vcpu->arch.cpuid_entries);
 }
 
+static void kvm_xstate_reset(struct kvm_vcpu *vcpu, bool init_event)
+{
+	struct fpstate *fpstate = vcpu->arch.guest_fpu.fpstate;
+
+	/*
+	 * Guest FPU state is zero allocated and so doesn't need to be manually
+	 * cleared on RESET, i.e. during vCPU creation.
+	 */
+	if (!init_event || !fpstate)
+		return;
+
+	/*
+	 * On INIT, only select XSTATE components are zeroed, most components
+	 * are unchanged.  Currently, the only components that are zeroed and
+	 * supported by KVM are MPX related.
+	 */
+	if (!kvm_mpx_supported())
+		return;
+
+	/*
+	 * All paths that lead to INIT are required to load the guest's FPU
+	 * state (because most paths are buried in KVM_RUN).
+	 */
+	kvm_put_guest_fpu(vcpu);
+	fpstate_clear_xstate_component(fpstate, XFEATURE_BNDREGS);
+	fpstate_clear_xstate_component(fpstate, XFEATURE_BNDCSR);
+	kvm_load_guest_fpu(vcpu);
+}
+
 void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
 {
 	struct kvm_cpuid_entry2 *cpuid_0x1;
@@ -12446,22 +12475,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
 	kvm_async_pf_hash_reset(vcpu);
 	vcpu->arch.apf.halted = false;
 
-	if (vcpu->arch.guest_fpu.fpstate && kvm_mpx_supported()) {
-		struct fpstate *fpstate = vcpu->arch.guest_fpu.fpstate;
-
-		/*
-		 * All paths that lead to INIT are required to load the guest's
-		 * FPU state (because most paths are buried in KVM_RUN).
-		 */
-		if (init_event)
-			kvm_put_guest_fpu(vcpu);
-
-		fpstate_clear_xstate_component(fpstate, XFEATURE_BNDREGS);
-		fpstate_clear_xstate_component(fpstate, XFEATURE_BNDCSR);
-
-		if (init_event)
-			kvm_load_guest_fpu(vcpu);
-	}
+	kvm_xstate_reset(vcpu, init_event);
 
 	if (!init_event) {
 		vcpu->arch.smbase = 0x30000;
-- 
2.47.1

[PATCH v12 05/24] KVM: x86: Zero XSTATE components on INIT by iterating over supported features

[Chao Gao]

Tweak the code a bit to facilitate resetting more xstate components in
the future, e.g., CET's xstate-managed MSRs.

No functional change intended.

Suggested-by: Sean Christopherson <seanjc@google.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/kvm/x86.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6cf0d15a7a64..0010aa45bf9f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -12392,6 +12392,8 @@ void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)
 static void kvm_xstate_reset(struct kvm_vcpu *vcpu, bool init_event)
 {
 	struct fpstate *fpstate = vcpu->arch.guest_fpu.fpstate;
+	u64 xfeatures_mask;
+	int i;
 
 	/*
 	 * Guest FPU state is zero allocated and so doesn't need to be manually
@@ -12405,16 +12407,20 @@ static void kvm_xstate_reset(struct kvm_vcpu *vcpu, bool init_event)
 	 * are unchanged.  Currently, the only components that are zeroed and
 	 * supported by KVM are MPX related.
 	 */
-	if (!kvm_mpx_supported())
+	xfeatures_mask = (kvm_caps.supported_xcr0 | kvm_caps.supported_xss) &
+			 (XFEATURE_MASK_BNDREGS | XFEATURE_MASK_BNDCSR);
+	if (!xfeatures_mask)
 		return;
 
+	BUILD_BUG_ON(sizeof(xfeatures_mask) * BITS_PER_BYTE <= XFEATURE_MAX);
+
 	/*
 	 * All paths that lead to INIT are required to load the guest's FPU
 	 * state (because most paths are buried in KVM_RUN).
 	 */
 	kvm_put_guest_fpu(vcpu);
-	fpstate_clear_xstate_component(fpstate, XFEATURE_BNDREGS);
-	fpstate_clear_xstate_component(fpstate, XFEATURE_BNDCSR);
+	for_each_set_bit(i, (unsigned long *)&xfeatures_mask, XFEATURE_MAX)
+		fpstate_clear_xstate_component(fpstate, i);
 	kvm_load_guest_fpu(vcpu);
 }
 
-- 
2.47.1

[PATCH v12 06/24] KVM: x86: Introduce KVM_{G,S}ET_ONE_REG uAPIs support

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

Enable KVM_{G,S}ET_ONE_REG uAPIs so that userspace can access HW MSR or
KVM synthetic MSR through it.

In CET KVM series [1], KVM "steals" an MSR from PV MSR space and access
it via KVM_{G,S}ET_MSRs uAPIs, but the approach pollutes PV MSR space
and hides the difference of synthetic MSRs and normal HW defined MSRs.

Now carve out a separate room in KVM-customized MSR address space for
synthetic MSRs. The synthetic MSRs are not exposed to userspace via
KVM_GET_MSR_INDEX_LIST, instead userspace complies with KVM's setup and
composes the uAPI params. KVM synthetic MSR indices start from 0 and
increase linearly. Userspace caller should tag MSR type correctly in
order to access intended HW or synthetic MSR.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Link: https://lore.kernel.org/all/20240219074733.122080-18-weijiang.yang@intel.com/ [1]
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/include/uapi/asm/kvm.h | 10 +++++
 arch/x86/kvm/x86.c              | 66 +++++++++++++++++++++++++++++++++
 2 files changed, 76 insertions(+)

diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index 0f15d683817d..e72d9e6c1739 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -411,6 +411,16 @@ struct kvm_xcrs {
 	__u64 padding[16];
 };
 
+#define KVM_X86_REG_MSR			(1 << 2)
+#define KVM_X86_REG_SYNTHETIC		(1 << 3)
+
+struct kvm_x86_reg_id {
+	__u32 index;
+	__u8 type;
+	__u8 rsvd;
+	__u16 rsvd16;
+};
+
 #define KVM_SYNC_X86_REGS      (1UL << 0)
 #define KVM_SYNC_X86_SREGS     (1UL << 1)
 #define KVM_SYNC_X86_EVENTS    (1UL << 2)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0010aa45bf9f..f77949c3e9dd 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2241,6 +2241,31 @@ static int do_set_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data)
 	return kvm_set_msr_ignored_check(vcpu, index, *data, true);
 }
 
+static int kvm_get_one_msr(struct kvm_vcpu *vcpu, u32 msr, u64 __user *value)
+{
+	u64 val;
+	int r;
+
+	r = do_get_msr(vcpu, msr, &val);
+	if (r)
+		return r;
+
+	if (put_user(val, value))
+		return -EFAULT;
+
+	return 0;
+}
+
+static int kvm_set_one_msr(struct kvm_vcpu *vcpu, u32 msr, u64 __user *value)
+{
+	u64 val;
+
+	if (get_user(val, value))
+		return -EFAULT;
+
+	return do_set_msr(vcpu, msr, &val);
+}
+
 #ifdef CONFIG_X86_64
 struct pvclock_clock {
 	int vclock_mode;
@@ -5902,6 +5927,11 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
 	}
 }
 
+static int kvm_translate_synthetic_msr(struct kvm_x86_reg_id *reg)
+{
+	return -EINVAL;
+}
+
 long kvm_arch_vcpu_ioctl(struct file *filp,
 			 unsigned int ioctl, unsigned long arg)
 {
@@ -6018,6 +6048,42 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 		srcu_read_unlock(&vcpu->kvm->srcu, idx);
 		break;
 	}
+	case KVM_GET_ONE_REG:
+	case KVM_SET_ONE_REG: {
+		struct kvm_x86_reg_id *id;
+		struct kvm_one_reg reg;
+		u64 __user *value;
+
+		r = -EFAULT;
+		if (copy_from_user(&reg, argp, sizeof(reg)))
+			break;
+
+		r = -EINVAL;
+		id = (struct kvm_x86_reg_id *)&reg.id;
+		if (id->rsvd || id->rsvd16)
+			break;
+
+		if (id->type != KVM_X86_REG_MSR &&
+		    id->type != KVM_X86_REG_SYNTHETIC)
+			break;
+
+		if (id->type == KVM_X86_REG_SYNTHETIC) {
+			r = kvm_translate_synthetic_msr(id);
+			if (r)
+				break;
+		}
+
+		r = -EINVAL;
+		if (id->type != KVM_X86_REG_MSR)
+			break;
+
+		value = u64_to_user_ptr(reg.addr);
+		if (ioctl == KVM_GET_ONE_REG)
+			r = kvm_get_one_msr(vcpu, id->index, value);
+		else
+			r = kvm_set_one_msr(vcpu, id->index, value);
+		break;
+	}
 	case KVM_TPR_ACCESS_REPORTING: {
 		struct kvm_tpr_access_ctl tac;
 
-- 
2.47.1

Re: [PATCH v12 06/24] KVM: x86: Introduce KVM_{G,S}ET_ONE_REG uAPIs support

[Sean Christopherson]

On Mon, Aug 11, 2025, Chao Gao wrote:
> From: Yang Weijiang <weijiang.yang@intel.com>
> 
> Enable KVM_{G,S}ET_ONE_REG uAPIs so that userspace can access HW MSR or
> KVM synthetic MSR through it.
> 
> In CET KVM series [1], KVM "steals" an MSR from PV MSR space and access
> it via KVM_{G,S}ET_MSRs uAPIs, but the approach pollutes PV MSR space
> and hides the difference of synthetic MSRs and normal HW defined MSRs.
> 
> Now carve out a separate room in KVM-customized MSR address space for
> synthetic MSRs. The synthetic MSRs are not exposed to userspace via
> KVM_GET_MSR_INDEX_LIST, instead userspace complies with KVM's setup and
> composes the uAPI params. KVM synthetic MSR indices start from 0 and
> increase linearly. Userspace caller should tag MSR type correctly in
> order to access intended HW or synthetic MSR.
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> Link: https://lore.kernel.org/all/20240219074733.122080-18-weijiang.yang@intel.com/ [1]
> Tested-by: Mathias Krause <minipli@grsecurity.net>
> Tested-by: John Allen <john.allen@amd.com>
> Signed-off-by: Chao Gao <chao.gao@intel.com>
> ---
>  arch/x86/include/uapi/asm/kvm.h | 10 +++++
>  arch/x86/kvm/x86.c              | 66 +++++++++++++++++++++++++++++++++
>  2 files changed, 76 insertions(+)
> 
> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
> index 0f15d683817d..e72d9e6c1739 100644
> --- a/arch/x86/include/uapi/asm/kvm.h
> +++ b/arch/x86/include/uapi/asm/kvm.h
> @@ -411,6 +411,16 @@ struct kvm_xcrs {
>  	__u64 padding[16];
>  };
>  
> +#define KVM_X86_REG_MSR			(1 << 2)
> +#define KVM_X86_REG_SYNTHETIC		(1 << 3)
> +
> +struct kvm_x86_reg_id {
> +	__u32 index;
> +	__u8 type;
> +	__u8 rsvd;
> +	__u16 rsvd16;
> +};

Some feedback from a while back never got addressed[*].  That feedback still
looks sane/good, so this for the uAPI:

--
#define KVM_X86_REG_TYPE_MSR	2ull

#define KVM_x86_REG_TYPE_SIZE(type) 						\
{(										\
	__u64 type_size = type;							\
										\
	type_size |= type == KVM_X86_REG_TYPE_MSR ? KVM_REG_SIZE_U64 :		\
		     type == KVM_X86_REG_TYPE_SYNTHETIC_MSR ? KVM_REG_SIZE_U64 :\
		     0;								\
	type_size;								\
})

#define KVM_X86_REG_ENCODE(type, index)				\
	(KVM_REG_X86 | KVM_X86_REG_TYPE_SIZE(type) | index)

#define KVM_X86_REG_MSR(index) KVM_X86_REG_ENCODE(KVM_X86_REG_TYPE_MSR, index)
--

And then the kernel-only struct overlay becomes:

--
struct kvm_x86_reg_id {
	__u32 index;
	__u8  type;
	__u8  rsvd;
	__u8  rsvd4:4;
	__u8  size:4;
	__u8  x86;
}
--

[*] https://lore.kernel.org/all/ZuGpJtEPv1NtdYwM@google.com

Re: [PATCH v12 06/24] KVM: x86: Introduce KVM_{G,S}ET_ONE_REG uAPIs support

[Chao Gao]

>> +#define KVM_X86_REG_MSR			(1 << 2)
>> +#define KVM_X86_REG_SYNTHETIC		(1 << 3)
>> +
>> +struct kvm_x86_reg_id {
>> +	__u32 index;
>> +	__u8 type;
>> +	__u8 rsvd;
>> +	__u16 rsvd16;
>> +};
>
>Some feedback from a while back never got addressed[*].  That feedback still
>looks sane/good, so this for the uAPI:

I missed that comment. Below is the diff I end up with. I moved struct
kvm_x86_reg_id to x86.c and added checks for ARCH (i.e., x86) and size.

diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index e72d9e6c1739..bb17b7a85159 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -411,15 +411,23 @@ struct kvm_xcrs {
	__u64 padding[16];
 };
 
-#define KVM_X86_REG_MSR			(1 << 2)
-#define KVM_X86_REG_SYNTHETIC		(1 << 3)
-
-struct kvm_x86_reg_id {
-	__u32 index;
-	__u8 type;
-	__u8 rsvd;
-	__u16 rsvd16;
-};
+#define KVM_X86_REG_TYPE_MSR		2
+#define KVM_X86_REG_TYPE_SYNTHETIC_MSR	3
+
+#define KVM_x86_REG_TYPE_SIZE(type)						\
+{(										\
+	__u64 type_size = type;							\
+										\
+	type_size |= type == KVM_X86_REG_TYPE_MSR ? KVM_REG_SIZE_U64 :		\
+		     type == KVM_X86_REG_TYPE_SYNTHETIC_MSR ? KVM_REG_SIZE_U64 :\
+		     0;								\
+	type_size;								\
+})
+
+#define KVM_X86_REG_ENCODE(type, index)				\
+	(KVM_REG_X86 | KVM_X86_REG_TYPE_SIZE(type) | index)
+
+#define KVM_X86_REG_MSR(index) KVM_X86_REG_ENCODE(KVM_X86_REG_TYPE_MSR, index)
 
 #define KVM_SYNC_X86_REGS      (1UL << 0)
 #define KVM_SYNC_X86_SREGS     (1UL << 1)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 3bf098a1183a..28e33269c1e9 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5940,6 +5940,15 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
	}
 }
 
+struct kvm_x86_reg_id {
+	__u32 index;
+	__u8  type;
+	__u8  rsvd;
+	__u8  rsvd4:4;
+	__u8  size:4;
+	__u8  x86;
+};
+
 static int kvm_translate_synthetic_msr(struct kvm_x86_reg_id *reg)
 {
	return -EINVAL;
@@ -6072,22 +6081,28 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
			break;
 
		r = -EINVAL;
+		if ((reg.id & KVM_REG_ARCH_MASK) != KVM_REG_X86)
+			break;
+
		id = (struct kvm_x86_reg_id *)&reg.id;
-		if (id->rsvd || id->rsvd16)
+		if (id->rsvd || id->rsvd4)
+			break;
+
+		if (id->type != KVM_X86_REG_TYPE_MSR &&
+		    id->type != KVM_X86_REG_TYPE_SYNTHETIC_MSR)
			break;
 
-		if (id->type != KVM_X86_REG_MSR &&
-		    id->type != KVM_X86_REG_SYNTHETIC)
+		if ((reg.id & KVM_REG_SIZE_MASK) != KVM_REG_SIZE_U64)
			break;
 
-		if (id->type == KVM_X86_REG_SYNTHETIC) {
+		if (id->type == KVM_X86_REG_TYPE_SYNTHETIC_MSR) {
			r = kvm_translate_synthetic_msr(id);
			if (r)
				break;
		}
 
		r = -EINVAL;
-		if (id->type != KVM_X86_REG_MSR)
+		if (id->type != KVM_X86_REG_TYPE_MSR)
			break;
 
		value = u64_to_user_ptr(reg.addr);


>
>--
>#define KVM_X86_REG_TYPE_MSR	2ull
>
>#define KVM_x86_REG_TYPE_SIZE(type) 						\
>{(										\
>	__u64 type_size = type;							\
>										\
>	type_size |= type == KVM_X86_REG_TYPE_MSR ? KVM_REG_SIZE_U64 :		\
>		     type == KVM_X86_REG_TYPE_SYNTHETIC_MSR ? KVM_REG_SIZE_U64 :\
>		     0;								\
>	type_size;								\
>})
>
>#define KVM_X86_REG_ENCODE(type, index)				\
>	(KVM_REG_X86 | KVM_X86_REG_TYPE_SIZE(type) | index)
>
>#define KVM_X86_REG_MSR(index) KVM_X86_REG_ENCODE(KVM_X86_REG_TYPE_MSR, index)
>--
>
>And then the kernel-only struct overlay becomes:
>
>--
>struct kvm_x86_reg_id {
>	__u32 index;
>	__u8  type;
>	__u8  rsvd;
>	__u8  rsvd4:4;
>	__u8  size:4;
>	__u8  x86;
>}
>--
>
>[*] https://lore.kernel.org/all/ZuGpJtEPv1NtdYwM@google.com

Re: [PATCH v12 06/24] KVM: x86: Introduce KVM_{G,S}ET_ONE_REG uAPIs support

[Chao Gao]

On Wed, Aug 20, 2025 at 04:29:04PM +0800, Chao Gao wrote:
>>> +#define KVM_X86_REG_MSR			(1 << 2)
>>> +#define KVM_X86_REG_SYNTHETIC		(1 << 3)
>>> +
>>> +struct kvm_x86_reg_id {
>>> +	__u32 index;
>>> +	__u8 type;
>>> +	__u8 rsvd;
>>> +	__u16 rsvd16;
>>> +};
>>
>>Some feedback from a while back never got addressed[*].  That feedback still
>>looks sane/good, so this for the uAPI:
>
>I missed that comment. Below is the diff I end up with. I moved struct
>kvm_x86_reg_id to x86.c and added checks for ARCH (i.e., x86) and size.
>
>diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
>index e72d9e6c1739..bb17b7a85159 100644
>--- a/arch/x86/include/uapi/asm/kvm.h
>+++ b/arch/x86/include/uapi/asm/kvm.h
>@@ -411,15 +411,23 @@ struct kvm_xcrs {
>	__u64 padding[16];
> };
> 
>-#define KVM_X86_REG_MSR			(1 << 2)
>-#define KVM_X86_REG_SYNTHETIC		(1 << 3)
>-
>-struct kvm_x86_reg_id {
>-	__u32 index;
>-	__u8 type;
>-	__u8 rsvd;
>-	__u16 rsvd16;
>-};
>+#define KVM_X86_REG_TYPE_MSR		2
>+#define KVM_X86_REG_TYPE_SYNTHETIC_MSR	3
>+
>+#define KVM_x86_REG_TYPE_SIZE(type)						\
>+{(										\

There are two typos here. s/x86/X86 and s/{(/({

>+	__u64 type_size = type;							\

and this should be __u64 type_size = (__u64)type << 32; 

When I tried to add a kselftest for the new ioctls, I found this patch didn't
advertise KVM_CAP_ONE_REG support for x86.

[PATCH v12 07/24] KVM: x86: Report XSS as to-be-saved if there are supported features

[Chao Gao]

From: Sean Christopherson <seanjc@google.com>

Add MSR_IA32_XSS to list of MSRs reported to userspace if supported_xss
is non-zero, i.e. KVM supports at least one XSS based feature.

Before enabling CET virtualization series, guest IA32_MSR_XSS is
guaranteed to be 0, i.e., XSAVES/XRSTORS is executed in non-root mode
with XSS == 0, which equals to the effect of XSAVE/XRSTOR.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/kvm/x86.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f77949c3e9dd..e0e440607e02 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -335,7 +335,7 @@ static const u32 msrs_to_save_base[] = {
 	MSR_IA32_RTIT_ADDR3_A, MSR_IA32_RTIT_ADDR3_B,
 	MSR_IA32_UMWAIT_CONTROL,
 
-	MSR_IA32_XFD, MSR_IA32_XFD_ERR,
+	MSR_IA32_XFD, MSR_IA32_XFD_ERR, MSR_IA32_XSS,
 };
 
 static const u32 msrs_to_save_pmu[] = {
@@ -7444,6 +7444,10 @@ static void kvm_probe_msr_to_save(u32 msr_index)
 		if (!(kvm_get_arch_capabilities() & ARCH_CAP_TSX_CTRL_MSR))
 			return;
 		break;
+	case MSR_IA32_XSS:
+		if (!kvm_caps.supported_xss)
+			return;
+		break;
 	default:
 		break;
 	}
-- 
2.47.1

[PATCH v12 08/24] KVM: x86: Refresh CPUID on write to guest MSR_IA32_XSS

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

Update CPUID.(EAX=0DH,ECX=1).EBX to reflect current required xstate size
due to XSS MSR modification.
CPUID(EAX=0DH,ECX=1).EBX reports the required storage size of all enabled
xstate features in (XCR0 | IA32_XSS). The CPUID value can be used by guest
before allocate sufficient xsave buffer.

Note, KVM does not yet support any XSS based features, i.e. supported_xss
is guaranteed to be zero at this time.

Opportunistically return KVM_MSR_RET_UNSUPPORTED if guest CPUID doesn't
enumerate it. Since KVM_MSR_RET_UNSUPPORTED takes care of host_initiated
cases, drop the host_initiated check.

Suggested-by: Sean Christopherson <seanjc@google.com>
Co-developed-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
Signed-off-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/include/asm/kvm_host.h |  3 ++-
 arch/x86/kvm/cpuid.c            | 15 ++++++++++++++-
 arch/x86/kvm/x86.c              |  9 +++++----
 3 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 39b93642e7d2..1faf53df6259 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -811,7 +811,6 @@ struct kvm_vcpu_arch {
 	bool at_instruction_boundary;
 	bool tpr_access_reporting;
 	bool xfd_no_write_intercept;
-	u64 ia32_xss;
 	u64 microcode_version;
 	u64 arch_capabilities;
 	u64 perf_capabilities;
@@ -872,6 +871,8 @@ struct kvm_vcpu_arch {
 
 	u64 xcr0;
 	u64 guest_supported_xcr0;
+	u64 guest_supported_xss;
+	u64 ia32_xss;
 
 	struct kvm_pio_request pio;
 	void *pio_data;
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 30fd18700972..85079caaf507 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -263,6 +263,17 @@ static u64 cpuid_get_supported_xcr0(struct kvm_vcpu *vcpu)
 	return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
 }
 
+static u64 cpuid_get_supported_xss(struct kvm_vcpu *vcpu)
+{
+	struct kvm_cpuid_entry2 *best;
+
+	best = kvm_find_cpuid_entry_index(vcpu, 0xd, 1);
+	if (!best)
+		return 0;
+
+	return (best->ecx | ((u64)best->edx << 32)) & kvm_caps.supported_xss;
+}
+
 static __always_inline void kvm_update_feature_runtime(struct kvm_vcpu *vcpu,
 						       struct kvm_cpuid_entry2 *entry,
 						       unsigned int x86_feature,
@@ -305,7 +316,8 @@ static void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
 	best = kvm_find_cpuid_entry_index(vcpu, 0xD, 1);
 	if (best && (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||
 		     cpuid_entry_has(best, X86_FEATURE_XSAVEC)))
-		best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
+		best->ebx = xstate_required_size(vcpu->arch.xcr0 |
+						 vcpu->arch.ia32_xss, true);
 }
 
 static bool kvm_cpuid_has_hyperv(struct kvm_vcpu *vcpu)
@@ -424,6 +436,7 @@ void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 	}
 
 	vcpu->arch.guest_supported_xcr0 = cpuid_get_supported_xcr0(vcpu);
+	vcpu->arch.guest_supported_xss = cpuid_get_supported_xss(vcpu);
 
 	vcpu->arch.pv_cpuid.features = kvm_apply_cpuid_pv_features_quirk(vcpu);
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e0e440607e02..c91472d36717 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3998,16 +3998,17 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		}
 		break;
 	case MSR_IA32_XSS:
-		if (!msr_info->host_initiated &&
-		    !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
-			return 1;
+		if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
+			return KVM_MSR_RET_UNSUPPORTED;
 		/*
 		 * KVM supports exposing PT to the guest, but does not support
 		 * IA32_XSS[bit 8]. Guests have to use RDMSR/WRMSR rather than
 		 * XSAVES/XRSTORS to save/restore PT MSRs.
 		 */
-		if (data & ~kvm_caps.supported_xss)
+		if (data & ~vcpu->arch.guest_supported_xss)
 			return 1;
+		if (vcpu->arch.ia32_xss == data)
+			break;
 		vcpu->arch.ia32_xss = data;
 		vcpu->arch.cpuid_dynamic_bits_dirty = true;
 		break;
-- 
2.47.1

[PATCH v12 09/24] KVM: x86: Initialize kvm_caps.supported_xss

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

Set original kvm_caps.supported_xss to (host_xss & KVM_SUPPORTED_XSS) if
XSAVES is supported. host_xss contains the host supported xstate feature
bits for thread FPU context switch, KVM_SUPPORTED_XSS includes all KVM
enabled XSS feature bits, the resulting value represents the supervisor
xstates that are available to guest and are backed by host FPU framework
for swapping {guest,host} XSAVE-managed registers/MSRs.

Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/kvm/x86.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c91472d36717..235a5de66e68 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -220,6 +220,8 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs;
 				| XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
 				| XFEATURE_MASK_PKRU | XFEATURE_MASK_XTILE)
 
+#define KVM_SUPPORTED_XSS     0
+
 bool __read_mostly allow_smaller_maxphyaddr = 0;
 EXPORT_SYMBOL_GPL(allow_smaller_maxphyaddr);
 
@@ -9762,14 +9764,17 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
 		kvm_host.xcr0 = xgetbv(XCR_XFEATURE_ENABLED_MASK);
 		kvm_caps.supported_xcr0 = kvm_host.xcr0 & KVM_SUPPORTED_XCR0;
 	}
+
+	if (boot_cpu_has(X86_FEATURE_XSAVES)) {
+		rdmsrq(MSR_IA32_XSS, kvm_host.xss);
+		kvm_caps.supported_xss = kvm_host.xss & KVM_SUPPORTED_XSS;
+	}
+
 	kvm_caps.supported_quirks = KVM_X86_VALID_QUIRKS;
 	kvm_caps.inapplicable_quirks = KVM_X86_CONDITIONAL_QUIRKS;
 
 	rdmsrq_safe(MSR_EFER, &kvm_host.efer);
 
-	if (boot_cpu_has(X86_FEATURE_XSAVES))
-		rdmsrq(MSR_IA32_XSS, kvm_host.xss);
-
 	kvm_init_pmu_capability(ops->pmu_ops);
 
 	if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES))
-- 
2.47.1

[PATCH v12 10/24] KVM: x86: Load guest FPU state when access XSAVE-managed MSRs

[Chao Gao]

From: Sean Christopherson <seanjc@google.com>

Load the guest's FPU state if userspace is accessing MSRs whose values
are managed by XSAVES. Introduce two helpers, kvm_{get,set}_xstate_msr(),
to facilitate access to such kind of MSRs.

If MSRs supported in kvm_caps.supported_xss are passed through to guest,
the guest MSRs are swapped with host's before vCPU exits to userspace and
after it reenters kernel before next VM-entry.

Because the modified code is also used for the KVM_GET_MSRS device ioctl(),
explicitly check @vcpu is non-null before attempting to load guest state.
The XSAVE-managed MSRs cannot be retrieved via the device ioctl() without
loading guest FPU state (which doesn't exist).

Note that guest_cpuid_has() is not queried as host userspace is allowed to
access MSRs that have not been exposed to the guest, e.g. it might do
KVM_SET_MSRS prior to KVM_SET_CPUID2.

The two helpers are put here in order to manifest accessing xsave-managed
MSRs requires special check and handling to guarantee the correctness of
read/write to the MSRs.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Co-developed-by: Yang Weijiang <weijiang.yang@intel.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/kvm/x86.c | 35 ++++++++++++++++++++++++++++++++++-
 arch/x86/kvm/x86.h | 24 ++++++++++++++++++++++++
 2 files changed, 58 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 235a5de66e68..f87974e0682b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -136,6 +136,9 @@ static int __set_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
 static void __get_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
 
 static DEFINE_MUTEX(vendor_module_lock);
+static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu);
+static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu);
+
 struct kvm_x86_ops kvm_x86_ops __read_mostly;
 
 #define KVM_X86_OP(func)					     \
@@ -4553,6 +4556,21 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 }
 EXPORT_SYMBOL_GPL(kvm_get_msr_common);
 
+/*
+ *  Returns true if the MSR in question is managed via XSTATE, i.e. is context
+ *  switched with the rest of guest FPU state.
+ */
+static bool is_xstate_managed_msr(u32 index)
+{
+	switch (index) {
+	case MSR_IA32_U_CET:
+	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
+		return true;
+	default:
+		return false;
+	}
+}
+
 /*
  * Read or write a bunch of msrs. All parameters are kernel addresses.
  *
@@ -4563,11 +4581,26 @@ static int __msr_io(struct kvm_vcpu *vcpu, struct kvm_msrs *msrs,
 		    int (*do_msr)(struct kvm_vcpu *vcpu,
 				  unsigned index, u64 *data))
 {
+	bool fpu_loaded = false;
 	int i;
 
-	for (i = 0; i < msrs->nmsrs; ++i)
+	for (i = 0; i < msrs->nmsrs; ++i) {
+		/*
+		 * If userspace is accessing one or more XSTATE-managed MSRs,
+		 * temporarily load the guest's FPU state so that the guest's
+		 * MSR value(s) is resident in hardware, i.e. so that KVM can
+		 * get/set the MSR via RDMSR/WRMSR.
+		 */
+		if (vcpu && !fpu_loaded && kvm_caps.supported_xss &&
+		    is_xstate_managed_msr(entries[i].index)) {
+			kvm_load_guest_fpu(vcpu);
+			fpu_loaded = true;
+		}
 		if (do_msr(vcpu, entries[i].index, &entries[i].data))
 			break;
+	}
+	if (fpu_loaded)
+		kvm_put_guest_fpu(vcpu);
 
 	return i;
 }
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index bcfd9b719ada..31ce76369cc7 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -699,4 +699,28 @@ int ____kvm_emulate_hypercall(struct kvm_vcpu *vcpu, int cpl,
 
 int kvm_emulate_hypercall(struct kvm_vcpu *vcpu);
 
+/*
+ * Lock and/or reload guest FPU and access xstate MSRs. For accesses initiated
+ * by host, guest FPU is loaded in __msr_io(). For accesses initiated by guest,
+ * guest FPU should have been loaded already.
+ */
+
+static inline void kvm_get_xstate_msr(struct kvm_vcpu *vcpu,
+				      struct msr_data *msr_info)
+{
+	KVM_BUG_ON(!vcpu->arch.guest_fpu.fpstate->in_use, vcpu->kvm);
+	kvm_fpu_get();
+	rdmsrl(msr_info->index, msr_info->data);
+	kvm_fpu_put();
+}
+
+static inline void kvm_set_xstate_msr(struct kvm_vcpu *vcpu,
+				      struct msr_data *msr_info)
+{
+	KVM_BUG_ON(!vcpu->arch.guest_fpu.fpstate->in_use, vcpu->kvm);
+	kvm_fpu_get();
+	wrmsrl(msr_info->index, msr_info->data);
+	kvm_fpu_put();
+}
+
 #endif
-- 
2.47.1

[PATCH v12 11/24] KVM: x86: Add fault checks for guest CR4.CET setting

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

Check potential faults for CR4.CET setting per Intel SDM requirements.
CET can be enabled if and only if CR0.WP == 1, i.e. setting CR4.CET ==
1 faults if CR0.WP == 0 and setting CR0.WP == 0 fails if CR4.CET == 1.

Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/kvm/x86.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f87974e0682b..cca05908f989 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1172,6 +1172,9 @@ int kvm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
 	    (is_64_bit_mode(vcpu) || kvm_is_cr4_bit_set(vcpu, X86_CR4_PCIDE)))
 		return 1;
 
+	if (!(cr0 & X86_CR0_WP) && kvm_is_cr4_bit_set(vcpu, X86_CR4_CET))
+		return 1;
+
 	kvm_x86_call(set_cr0)(vcpu, cr0);
 
 	kvm_post_set_cr0(vcpu, old_cr0, cr0);
@@ -1371,6 +1374,9 @@ int kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 			return 1;
 	}
 
+	if ((cr4 & X86_CR4_CET) && !kvm_is_cr0_bit_set(vcpu, X86_CR0_WP))
+		return 1;
+
 	kvm_x86_call(set_cr4)(vcpu, cr4);
 
 	kvm_post_set_cr4(vcpu, old_cr4, cr4);
-- 
2.47.1

[PATCH v12 12/24] KVM: x86: Report KVM supported CET MSRs as to-be-saved

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

Add CET MSRs to the list of MSRs reported to userspace if the feature,
i.e. IBT or SHSTK, associated with the MSRs is supported by KVM.

Suggested-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/kvm/x86.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index cca05908f989..7134f428b9f7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -341,6 +341,10 @@ static const u32 msrs_to_save_base[] = {
 	MSR_IA32_UMWAIT_CONTROL,
 
 	MSR_IA32_XFD, MSR_IA32_XFD_ERR, MSR_IA32_XSS,
+
+	MSR_IA32_U_CET, MSR_IA32_S_CET,
+	MSR_IA32_PL0_SSP, MSR_IA32_PL1_SSP, MSR_IA32_PL2_SSP,
+	MSR_IA32_PL3_SSP, MSR_IA32_INT_SSP_TAB,
 };
 
 static const u32 msrs_to_save_pmu[] = {
@@ -7490,6 +7494,20 @@ static void kvm_probe_msr_to_save(u32 msr_index)
 		if (!kvm_caps.supported_xss)
 			return;
 		break;
+	case MSR_IA32_U_CET:
+	case MSR_IA32_S_CET:
+		if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK) &&
+		    !kvm_cpu_cap_has(X86_FEATURE_IBT))
+			return;
+		break;
+	case MSR_IA32_INT_SSP_TAB:
+		if (!kvm_cpu_cap_has(X86_FEATURE_LM))
+			return;
+		fallthrough;
+	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
+		if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK))
+			return;
+		break;
 	default:
 		break;
 	}
-- 
2.47.1

[PATCH v12 13/24] KVM: VMX: Introduce CET VMCS fields and control bits

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

Control-flow Enforcement Technology (CET) is a kind of CPU feature used
to prevent Return/CALL/Jump-Oriented Programming (ROP/COP/JOP) attacks.
It provides two sub-features(SHSTK,IBT) to defend against ROP/COP/JOP
style control-flow subversion attacks.

Shadow Stack (SHSTK):
  A shadow stack is a second stack used exclusively for control transfer
  operations. The shadow stack is separate from the data/normal stack and
  can be enabled individually in user and kernel mode. When shadow stack
  is enabled, CALL pushes the return address on both the data and shadow
  stack. RET pops the return address from both stacks and compares them.
  If the return addresses from the two stacks do not match, the processor
  generates a #CP.

Indirect Branch Tracking (IBT):
  IBT introduces instruction(ENDBRANCH)to mark valid target addresses of
  indirect branches (CALL, JMP etc...). If an indirect branch is executed
  and the next instruction is _not_ an ENDBRANCH, the processor generates
  a #CP. These instruction behaves as a NOP on platforms that have no CET.

Several new CET MSRs are defined to support CET:
  MSR_IA32_{U,S}_CET: CET settings for {user,supervisor} CET respectively.

  MSR_IA32_PL{0,1,2,3}_SSP: SHSTK pointer linear address for CPL{0,1,2,3}.

  MSR_IA32_INT_SSP_TAB: Linear address of SHSTK pointer table, whose entry
			is indexed by IST of interrupt gate desc.

Two XSAVES state bits are introduced for CET:
  IA32_XSS:[bit 11]: Control saving/restoring user mode CET states
  IA32_XSS:[bit 12]: Control saving/restoring supervisor mode CET states.

Six VMCS fields are introduced for CET:
  {HOST,GUEST}_S_CET: Stores CET settings for kernel mode.
  {HOST,GUEST}_SSP: Stores current active SSP.
  {HOST,GUEST}_INTR_SSP_TABLE: Stores current active MSR_IA32_INT_SSP_TAB.

On Intel platforms, two additional bits are defined in VM_EXIT and VM_ENTRY
control fields:
If VM_EXIT_LOAD_CET_STATE = 1, host CET states are loaded from following
VMCS fields at VM-Exit:
  HOST_S_CET
  HOST_SSP
  HOST_INTR_SSP_TABLE

If VM_ENTRY_LOAD_CET_STATE = 1, guest CET states are loaded from following
VMCS fields at VM-Entry:
  GUEST_S_CET
  GUEST_SSP
  GUEST_INTR_SSP_TABLE

Co-developed-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
Signed-off-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/include/asm/vmx.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
index cca7d6641287..ce10a7e2d3d9 100644
--- a/arch/x86/include/asm/vmx.h
+++ b/arch/x86/include/asm/vmx.h
@@ -106,6 +106,7 @@
 #define VM_EXIT_CLEAR_BNDCFGS                   0x00800000
 #define VM_EXIT_PT_CONCEAL_PIP			0x01000000
 #define VM_EXIT_CLEAR_IA32_RTIT_CTL		0x02000000
+#define VM_EXIT_LOAD_CET_STATE                  0x10000000
 
 #define VM_EXIT_ALWAYSON_WITHOUT_TRUE_MSR	0x00036dff
 
@@ -119,6 +120,7 @@
 #define VM_ENTRY_LOAD_BNDCFGS                   0x00010000
 #define VM_ENTRY_PT_CONCEAL_PIP			0x00020000
 #define VM_ENTRY_LOAD_IA32_RTIT_CTL		0x00040000
+#define VM_ENTRY_LOAD_CET_STATE                 0x00100000
 
 #define VM_ENTRY_ALWAYSON_WITHOUT_TRUE_MSR	0x000011ff
 
@@ -369,6 +371,9 @@ enum vmcs_field {
 	GUEST_PENDING_DBG_EXCEPTIONS    = 0x00006822,
 	GUEST_SYSENTER_ESP              = 0x00006824,
 	GUEST_SYSENTER_EIP              = 0x00006826,
+	GUEST_S_CET                     = 0x00006828,
+	GUEST_SSP                       = 0x0000682a,
+	GUEST_INTR_SSP_TABLE            = 0x0000682c,
 	HOST_CR0                        = 0x00006c00,
 	HOST_CR3                        = 0x00006c02,
 	HOST_CR4                        = 0x00006c04,
@@ -381,6 +386,9 @@ enum vmcs_field {
 	HOST_IA32_SYSENTER_EIP          = 0x00006c12,
 	HOST_RSP                        = 0x00006c14,
 	HOST_RIP                        = 0x00006c16,
+	HOST_S_CET                      = 0x00006c18,
+	HOST_SSP                        = 0x00006c1a,
+	HOST_INTR_SSP_TABLE             = 0x00006c1c
 };
 
 /*
-- 
2.47.1

[PATCH v12 14/24] KVM: x86: Enable guest SSP read/write interface with new uAPIs

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

Enable guest shadow stack pointer(SSP) access interface with new uAPIs.
CET guest SSP is HW register which has corresponding VMCS field to save
/restore guest values when VM-{Exit,Entry} happens. KVM handles SSP as
a synthetic MSR for userspace access.

Use a translation helper to set up mapping for SSP synthetic index and
KVM-internal MSR index so that userspace doesn't need to take care of
KVM's management for synthetic MSRs and avoid conflicts.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/include/uapi/asm/kvm.h |  3 +++
 arch/x86/kvm/x86.c              | 10 +++++++++-
 arch/x86/kvm/x86.h              | 10 ++++++++++
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index e72d9e6c1739..a4870d9c9279 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -421,6 +421,9 @@ struct kvm_x86_reg_id {
 	__u16 rsvd16;
 };
 
+/* KVM synthetic MSR index staring from 0 */
+#define KVM_SYNTHETIC_GUEST_SSP 0
+
 #define KVM_SYNC_X86_REGS      (1UL << 0)
 #define KVM_SYNC_X86_SREGS     (1UL << 1)
 #define KVM_SYNC_X86_EVENTS    (1UL << 2)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7134f428b9f7..b5c4db4b7e04 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5975,7 +5975,15 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
 
 static int kvm_translate_synthetic_msr(struct kvm_x86_reg_id *reg)
 {
-	return -EINVAL;
+	switch (reg->index) {
+	case KVM_SYNTHETIC_GUEST_SSP:
+		reg->type = KVM_X86_REG_MSR;
+		reg->index = MSR_KVM_INTERNAL_GUEST_SSP;
+		break;
+	default:
+		return -EINVAL;
+	}
+	return 0;
 }
 
 long kvm_arch_vcpu_ioctl(struct file *filp,
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 31ce76369cc7..f8fbd33db067 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -101,6 +101,16 @@ do {											\
 #define KVM_SVM_DEFAULT_PLE_WINDOW_MAX	USHRT_MAX
 #define KVM_SVM_DEFAULT_PLE_WINDOW	3000
 
+/*
+ * KVM's internal, non-ABI indices for synthetic MSRs. The values themselves
+ * are arbitrary and have no meaning, the only requirement is that they don't
+ * conflict with "real" MSRs that KVM supports. Use values at the upper end
+ * of KVM's reserved paravirtual MSR range to minimize churn, i.e. these values
+ * will be usable until KVM exhausts its supply of paravirtual MSR indices.
+ */
+
+#define MSR_KVM_INTERNAL_GUEST_SSP	0x4b564dff
+
 static inline unsigned int __grow_ple_window(unsigned int val,
 		unsigned int base, unsigned int modifier, unsigned int max)
 {
-- 
2.47.1

[PATCH v12 15/24] KVM: VMX: Emulate read and write to CET MSRs

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

Add emulation interface for CET MSR access. The emulation code is split
into common part and vendor specific part. The former does common checks
for MSRs, e.g., accessibility, data validity etc., then passes operation
to either XSAVE-managed MSRs via the helpers or CET VMCS fields.

SSP can only be read via RDSSP. Writing even requires destructive and
potentially faulting operations such as SAVEPREVSSP/RSTORSSP or
SETSSBSY/CLRSSBSY. Let the host use a pseudo-MSR that is just a wrapper
for the GUEST_SSP field of the VMCS.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/kvm/vmx/vmx.c | 18 ++++++++++++++++++
 arch/x86/kvm/x86.c     | 43 ++++++++++++++++++++++++++++++++++++++++++
 arch/x86/kvm/x86.h     | 23 ++++++++++++++++++++++
 3 files changed, 84 insertions(+)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index aa157fe5b7b3..bd572c8c7bc3 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2093,6 +2093,15 @@ int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		else
 			msr_info->data = vmx->pt_desc.guest.addr_a[index / 2];
 		break;
+	case MSR_IA32_S_CET:
+		msr_info->data = vmcs_readl(GUEST_S_CET);
+		break;
+	case MSR_KVM_INTERNAL_GUEST_SSP:
+		msr_info->data = vmcs_readl(GUEST_SSP);
+		break;
+	case MSR_IA32_INT_SSP_TAB:
+		msr_info->data = vmcs_readl(GUEST_INTR_SSP_TABLE);
+		break;
 	case MSR_IA32_DEBUGCTLMSR:
 		msr_info->data = vmx_guest_debugctl_read();
 		break;
@@ -2411,6 +2420,15 @@ int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		else
 			vmx->pt_desc.guest.addr_a[index / 2] = data;
 		break;
+	case MSR_IA32_S_CET:
+		vmcs_writel(GUEST_S_CET, data);
+		break;
+	case MSR_KVM_INTERNAL_GUEST_SSP:
+		vmcs_writel(GUEST_SSP, data);
+		break;
+	case MSR_IA32_INT_SSP_TAB:
+		vmcs_writel(GUEST_INTR_SSP_TABLE, data);
+		break;
 	case MSR_IA32_PERF_CAPABILITIES:
 		if (data & PMU_CAP_LBR_FMT) {
 			if ((data & PMU_CAP_LBR_FMT) !=
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b5c4db4b7e04..cc39ace47262 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1885,6 +1885,27 @@ static int __kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data,
 
 		data = (u32)data;
 		break;
+	case MSR_IA32_U_CET:
+	case MSR_IA32_S_CET:
+		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK) &&
+		    !guest_cpu_cap_has(vcpu, X86_FEATURE_IBT))
+			return KVM_MSR_RET_UNSUPPORTED;
+		if (!is_cet_msr_valid(vcpu, data))
+			return 1;
+		break;
+	case MSR_KVM_INTERNAL_GUEST_SSP:
+		if (!host_initiated)
+			return 1;
+		fallthrough;
+	case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
+		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK))
+			return KVM_MSR_RET_UNSUPPORTED;
+		if (is_noncanonical_msr_address(data, vcpu))
+			return 1;
+		/* All SSP MSRs except MSR_IA32_INT_SSP_TAB must be 4-byte aligned */
+		if (index != MSR_IA32_INT_SSP_TAB && !IS_ALIGNED(data, 4))
+			return 1;
+		break;
 	}
 
 	msr.data = data;
@@ -1929,6 +1950,20 @@ static int __kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data,
 		    !guest_cpu_cap_has(vcpu, X86_FEATURE_RDPID))
 			return 1;
 		break;
+	case MSR_IA32_U_CET:
+	case MSR_IA32_S_CET:
+		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK) &&
+		    !guest_cpu_cap_has(vcpu, X86_FEATURE_IBT))
+			return KVM_MSR_RET_UNSUPPORTED;
+		break;
+	case MSR_KVM_INTERNAL_GUEST_SSP:
+		if (!host_initiated)
+			return 1;
+		fallthrough;
+	case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
+		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK))
+			return KVM_MSR_RET_UNSUPPORTED;
+		break;
 	}
 
 	msr.index = index;
@@ -4207,6 +4242,10 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		vcpu->arch.guest_fpu.xfd_err = data;
 		break;
 #endif
+	case MSR_IA32_U_CET:
+	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
+		kvm_set_xstate_msr(vcpu, msr_info);
+		break;
 	default:
 		if (kvm_pmu_is_valid_msr(vcpu, msr))
 			return kvm_pmu_set_msr(vcpu, msr_info);
@@ -4556,6 +4595,10 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		msr_info->data = vcpu->arch.guest_fpu.xfd_err;
 		break;
 #endif
+	case MSR_IA32_U_CET:
+	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
+		kvm_get_xstate_msr(vcpu, msr_info);
+		break;
 	default:
 		if (kvm_pmu_is_valid_msr(vcpu, msr_info->index))
 			return kvm_pmu_get_msr(vcpu, msr_info);
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index f8fbd33db067..d5b039addd11 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -733,4 +733,27 @@ static inline void kvm_set_xstate_msr(struct kvm_vcpu *vcpu,
 	kvm_fpu_put();
 }
 
+#define CET_US_RESERVED_BITS		GENMASK(9, 6)
+#define CET_US_SHSTK_MASK_BITS		GENMASK(1, 0)
+#define CET_US_IBT_MASK_BITS		(GENMASK_ULL(5, 2) | GENMASK_ULL(63, 10))
+#define CET_US_LEGACY_BITMAP_BASE(data)	((data) >> 12)
+
+static inline bool is_cet_msr_valid(struct kvm_vcpu *vcpu, u64 data)
+{
+	if (data & CET_US_RESERVED_BITS)
+		return false;
+	if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK) &&
+	    (data & CET_US_SHSTK_MASK_BITS))
+		return false;
+	if (!guest_cpu_cap_has(vcpu, X86_FEATURE_IBT) &&
+	    (data & CET_US_IBT_MASK_BITS))
+		return false;
+	if (!IS_ALIGNED(CET_US_LEGACY_BITMAP_BASE(data), 4))
+		return false;
+	/* IBT can be suppressed iff the TRACKER isn't WAIT_ENDBR. */
+	if ((data & CET_SUPPRESS) && (data & CET_WAIT_ENDBR))
+		return false;
+
+	return true;
+}
 #endif
-- 
2.47.1

Re: [PATCH v12 15/24] KVM: VMX: Emulate read and write to CET MSRs

[Sean Christopherson]

On Mon, Aug 11, 2025, Chao Gao wrote:
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b5c4db4b7e04..cc39ace47262 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1885,6 +1885,27 @@ static int __kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data,
>  
>  		data = (u32)data;
>  		break;
> +	case MSR_IA32_U_CET:
> +	case MSR_IA32_S_CET:
> +		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK) &&
> +		    !guest_cpu_cap_has(vcpu, X86_FEATURE_IBT))
> +			return KVM_MSR_RET_UNSUPPORTED;
> +		if (!is_cet_msr_valid(vcpu, data))
> +			return 1;
> +		break;
> +	case MSR_KVM_INTERNAL_GUEST_SSP:
> +		if (!host_initiated)
> +			return 1;
> +		fallthrough;
> +	case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
> +		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK))
> +			return KVM_MSR_RET_UNSUPPORTED;
> +		if (is_noncanonical_msr_address(data, vcpu))

This emulation is wrong (in no small part because the architecture sucks).  From
the SDM:

  If the processor does not support Intel 64 architecture, these fields have only
  32 bits; bits 63:32 of the MSRs are reserved.

  On processors that support Intel 64 architecture this value cannot represent a
  non-canonical address.

  In protected mode, only 31:0 are loaded.

That means KVM needs to drop bits 63:32 if the vCPU doesn't have LM or if the vCPU
isn't in 64-bit mode.  The last one is especially frustrating, because software
can still get a 64-bit value into the MSRs while running in protected, e.g. by
switching to 64-bit mode, doing WRMSRs, then switching back to 32-bit mode.

But, there's probably no point in actually trying to correctly emulate/virtualize
the Protected Mode behavior, because the MSRs can be written via XRSTOR, and to
close that hole KVM would need to trap-and-emulate XRSTOR.  No thanks.

Unless someone has a better idea, I'm inclined to take an erratum for this, i.e.
just sweep it under the rug.

> +			return 1;
> +		/* All SSP MSRs except MSR_IA32_INT_SSP_TAB must be 4-byte aligned */
> +		if (index != MSR_IA32_INT_SSP_TAB && !IS_ALIGNED(data, 4))
> +			return 1;
> +		break;
>  	}
>  
>  	msr.data = data;

...

> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index f8fbd33db067..d5b039addd11 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -733,4 +733,27 @@ static inline void kvm_set_xstate_msr(struct kvm_vcpu *vcpu,
>  	kvm_fpu_put();
>  }
>  
> +#define CET_US_RESERVED_BITS		GENMASK(9, 6)
> +#define CET_US_SHSTK_MASK_BITS		GENMASK(1, 0)
> +#define CET_US_IBT_MASK_BITS		(GENMASK_ULL(5, 2) | GENMASK_ULL(63, 10))
> +#define CET_US_LEGACY_BITMAP_BASE(data)	((data) >> 12)
> +
> +static inline bool is_cet_msr_valid(struct kvm_vcpu *vcpu, u64 data)

This name is misleading, e.g. it reads "is this CET MSR valid", whereas the helper
is checking "is this value for U_CET or S_CET valid".  Maybe kvm_is_valid_u_s_cet()?

> +{
> +	if (data & CET_US_RESERVED_BITS)
> +		return false;
> +	if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK) &&
> +	    (data & CET_US_SHSTK_MASK_BITS))
> +		return false;
> +	if (!guest_cpu_cap_has(vcpu, X86_FEATURE_IBT) &&
> +	    (data & CET_US_IBT_MASK_BITS))
> +		return false;
> +	if (!IS_ALIGNED(CET_US_LEGACY_BITMAP_BASE(data), 4))
> +		return false;
> +	/* IBT can be suppressed iff the TRACKER isn't WAIT_ENDBR. */
> +	if ((data & CET_SUPPRESS) && (data & CET_WAIT_ENDBR))
> +		return false;
> +
> +	return true;
> +}
>  #endif
> -- 
> 2.47.1
> 

Re: [PATCH v12 15/24] KVM: VMX: Emulate read and write to CET MSRs

[Edgecombe, Rick P]

On Tue, 2025-08-19 at 09:09 -0700, Sean Christopherson wrote:
> This emulation is wrong (in no small part because the architecture sucks).  From
> the SDM:
> 
>   If the processor does not support Intel 64 architecture, these fields have only
>   32 bits; bits 63:32 of the MSRs are reserved.
> 
>   On processors that support Intel 64 architecture this value cannot represent a
>   non-canonical address.
> 
>   In protected mode, only 31:0 are loaded.
> 
> That means KVM needs to drop bits 63:32 if the vCPU doesn't have LM or if the vCPU
> isn't in 64-bit mode.  The last one is especially frustrating, because software
> can still get a 64-bit value into the MSRs while running in protected, e.g. by
> switching to 64-bit mode, doing WRMSRs, then switching back to 32-bit mode.
> 
> But, there's probably no point in actually trying to correctly emulate/virtualize
> the Protected Mode behavior, because the MSRs can be written via XRSTOR, and to
> close that hole KVM would need to trap-and-emulate XRSTOR.  No thanks.
> 
> Unless someone has a better idea, I'm inclined to take an erratum for this, i.e.
> just sweep it under the rug.

Sounds ok to me. All I could think would be something like use the CR/EFER
interceptions and just exit to userspace if (CR0.PE && !EFER.LM && CR4.CET). But
this would require some rototilling and then likely remain un-exercised. Not
sure it's worth it.

Re: [PATCH v12 15/24] KVM: VMX: Emulate read and write to CET MSRs

[Sean Christopherson]

On Tue, Aug 19, 2025, Rick P Edgecombe wrote:
> On Tue, 2025-08-19 at 09:09 -0700, Sean Christopherson wrote:
> > This emulation is wrong (in no small part because the architecture sucks).  From
> > the SDM:
> > 
> >   If the processor does not support Intel 64 architecture, these fields have only
> >   32 bits; bits 63:32 of the MSRs are reserved.
> > 
> >   On processors that support Intel 64 architecture this value cannot represent a
> >   non-canonical address.
> > 
> >   In protected mode, only 31:0 are loaded.
> > 
> > That means KVM needs to drop bits 63:32 if the vCPU doesn't have LM or if the vCPU
> > isn't in 64-bit mode.  The last one is especially frustrating, because software
> > can still get a 64-bit value into the MSRs while running in protected, e.g. by
> > switching to 64-bit mode, doing WRMSRs, then switching back to 32-bit mode.
> > 
> > But, there's probably no point in actually trying to correctly emulate/virtualize
> > the Protected Mode behavior, because the MSRs can be written via XRSTOR, and to
> > close that hole KVM would need to trap-and-emulate XRSTOR.  No thanks.
> > 
> > Unless someone has a better idea, I'm inclined to take an erratum for this, i.e.
> > just sweep it under the rug.
> 
> Sounds ok to me. All I could think would be something like use the CR/EFER
> interceptions and just exit to userspace if (CR0.PE && !EFER.LM && CR4.CET). But
> this would require some rototilling and then likely remain un-exercised.

And a far worse experience if a guest did ever trip that combo.  Letting the guest
set bits 63:32 would only cause problems if the guest is being deliberately weird,
whereas exiting to userspace would terminate even well-behaved guests (though as
you note, the odds of a 32-bit guest using CET is quite small...).

> Not sure it's worth it.

Re: [PATCH v12 15/24] KVM: VMX: Emulate read and write to CET MSRs

[Xin Li]

On 8/19/2025 9:09 AM, Sean Christopherson wrote:
>> +	case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
>> +		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK))
>> +			return KVM_MSR_RET_UNSUPPORTED;
>> +		if (is_noncanonical_msr_address(data, vcpu))
> This emulation is wrong (in no small part because the architecture sucks).  From
> the SDM:
> 
>    If the processor does not support Intel 64 architecture, these fields have only
>    32 bits; bits 63:32 of the MSRs are reserved.
> 
>    On processors that support Intel 64 architecture this value cannot represent a
>    non-canonical address.
> 
>    In protected mode, only 31:0 are loaded.
> 
> That means KVM needs to drop bits 63:32 if the vCPU doesn't have LM or if the vCPU
> isn't in 64-bit mode.  The last one is especially frustrating, because software
> can still get a 64-bit value into the MSRs while running in protected, e.g. by
> switching to 64-bit mode, doing WRMSRs, then switching back to 32-bit mode.
> 
> But, there's probably no point in actually trying to correctly emulate/virtualize
> the Protected Mode behavior, because the MSRs can be written via XRSTOR, and to
> close that hole KVM would need to trap-and-emulate XRSTOR.  No thanks.
> 
> Unless someone has a better idea, I'm inclined to take an erratum for this, i.e.
> just sweep it under the rug.

Since WRMSR (WRMSRNS) and XRSTORS are the two instructions that write to
MSRs in CPL0, Why KVM doesn't use the XSS-exiting bitmap?

Re: [PATCH v12 15/24] KVM: VMX: Emulate read and write to CET MSRs

[Sean Christopherson]

On Tue, Aug 19, 2025, Xin Li wrote:
> On 8/19/2025 9:09 AM, Sean Christopherson wrote:
> > > +	case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
> > > +		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK))
> > > +			return KVM_MSR_RET_UNSUPPORTED;
> > > +		if (is_noncanonical_msr_address(data, vcpu))
> > This emulation is wrong (in no small part because the architecture sucks).  From
> > the SDM:
> > 
> >    If the processor does not support Intel 64 architecture, these fields have only
> >    32 bits; bits 63:32 of the MSRs are reserved.
> > 
> >    On processors that support Intel 64 architecture this value cannot represent a
> >    non-canonical address.
> > 
> >    In protected mode, only 31:0 are loaded.
> > 
> > That means KVM needs to drop bits 63:32 if the vCPU doesn't have LM or if the vCPU
> > isn't in 64-bit mode.  The last one is especially frustrating, because software
> > can still get a 64-bit value into the MSRs while running in protected, e.g. by
> > switching to 64-bit mode, doing WRMSRs, then switching back to 32-bit mode.
> > 
> > But, there's probably no point in actually trying to correctly emulate/virtualize
> > the Protected Mode behavior, because the MSRs can be written via XRSTOR, and to
> > close that hole KVM would need to trap-and-emulate XRSTOR.  No thanks.
> > 
> > Unless someone has a better idea, I'm inclined to take an erratum for this, i.e.
> > just sweep it under the rug.
> 
> Since WRMSR (WRMSRNS) and XRSTORS are the two instructions that write to
> MSRs in CPL0, Why KVM doesn't use the XSS-exiting bitmap?

Intercepting XRSTORS would introduce (likely) unacceptable performance overhead
if a guest actually used CET in PM, I have zero desire to try and proxy/emulate
XRSTORS from guest memory in KVM, and CET already has big gaping virtualization
holes.

Re: [PATCH v12 15/24] KVM: VMX: Emulate read and write to CET MSRs

[Chao Gao]

>> +	case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
>> +		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK))
>> +			return KVM_MSR_RET_UNSUPPORTED;
>> +		if (is_noncanonical_msr_address(data, vcpu))
>
>This emulation is wrong (in no small part because the architecture sucks).  From
>the SDM:
>
>  If the processor does not support Intel 64 architecture, these fields have only
>  32 bits; bits 63:32 of the MSRs are reserved.
>
>  On processors that support Intel 64 architecture this value cannot represent a
>  non-canonical address.
>
>  In protected mode, only 31:0 are loaded.
>
>That means KVM needs to drop bits 63:32 if the vCPU doesn't have LM or if the vCPU
>isn't in 64-bit mode.  The last one is especially frustrating, because software
>can still get a 64-bit value into the MSRs while running in protected, e.g. by
>switching to 64-bit mode, doing WRMSRs, then switching back to 32-bit mode.
>
>But, there's probably no point in actually trying to correctly emulate/virtualize
>the Protected Mode behavior, because the MSRs can be written via XRSTOR, and to
>close that hole KVM would need to trap-and-emulate XRSTOR.  No thanks.

I don't get why we need to trap-and-emulate XRSTOR. if XRSTOR instruction in
protection mode can change higher 32 bits of CET MSRs, it is the hardware
behavior. why KVM needs to clear the higher 32 bits?

>
>Unless someone has a better idea, I'm inclined to take an erratum for this, i.e.
>just sweep it under the rug.
>
>> +			return 1;
>> +		/* All SSP MSRs except MSR_IA32_INT_SSP_TAB must be 4-byte aligned */
>> +		if (index != MSR_IA32_INT_SSP_TAB && !IS_ALIGNED(data, 4))
>> +			return 1;
>> +		break;
>>  	}
>>  
>>  	msr.data = data;
>
>...
>
>> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
>> index f8fbd33db067..d5b039addd11 100644
>> --- a/arch/x86/kvm/x86.h
>> +++ b/arch/x86/kvm/x86.h
>> @@ -733,4 +733,27 @@ static inline void kvm_set_xstate_msr(struct kvm_vcpu *vcpu,
>>  	kvm_fpu_put();
>>  }
>>  
>> +#define CET_US_RESERVED_BITS		GENMASK(9, 6)
>> +#define CET_US_SHSTK_MASK_BITS		GENMASK(1, 0)
>> +#define CET_US_IBT_MASK_BITS		(GENMASK_ULL(5, 2) | GENMASK_ULL(63, 10))
>> +#define CET_US_LEGACY_BITMAP_BASE(data)	((data) >> 12)
>> +
>> +static inline bool is_cet_msr_valid(struct kvm_vcpu *vcpu, u64 data)
>
>This name is misleading, e.g. it reads "is this CET MSR valid", whereas the helper
>is checking "is this value for U_CET or S_CET valid".  Maybe kvm_is_valid_u_s_cet()?

Will do.

Re: [PATCH v12 15/24] KVM: VMX: Emulate read and write to CET MSRs

[Sean Christopherson]

On Wed, Aug 20, 2025, Chao Gao wrote:
> >> +	case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
> >> +		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK))
> >> +			return KVM_MSR_RET_UNSUPPORTED;
> >> +		if (is_noncanonical_msr_address(data, vcpu))
> >
> >This emulation is wrong (in no small part because the architecture sucks).  From
> >the SDM:
> >
> >  If the processor does not support Intel 64 architecture, these fields have only
> >  32 bits; bits 63:32 of the MSRs are reserved.
> >
> >  On processors that support Intel 64 architecture this value cannot represent a
> >  non-canonical address.
> >
> >  In protected mode, only 31:0 are loaded.
> >
> >That means KVM needs to drop bits 63:32 if the vCPU doesn't have LM or if the vCPU
> >isn't in 64-bit mode.  The last one is especially frustrating, because software
> >can still get a 64-bit value into the MSRs while running in protected, e.g. by
> >switching to 64-bit mode, doing WRMSRs, then switching back to 32-bit mode.
> >
> >But, there's probably no point in actually trying to correctly emulate/virtualize
> >the Protected Mode behavior, because the MSRs can be written via XRSTOR, and to
> >close that hole KVM would need to trap-and-emulate XRSTOR.  No thanks.
> 
> I don't get why we need to trap-and-emulate XRSTOR. if XRSTOR instruction in
> protection mode can change higher 32 bits of CET MSRs, it is the hardware
> behavior. why KVM needs to clear the higher 32 bits?

If the VMM configures the virtual CPU model to be a 32-bit CPU, i.e. a vCPU that
"does not support Intel 64 architecture", i.e. CPUID.0x80000001.EDX.LM = 0, then
the behavior of the "hardware" that the guest sees should be that the upper 32
bits do not exist and should always read '0'.  But because the actual harware on
which the virtual CPU is running _does_ support Intel 64, XRSTORS/XSAVES in the
guest could observe behavior that violates the architecture.

[PATCH v12 16/24] KVM: x86: Save and reload SSP to/from SMRAM

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

Save CET SSP to SMRAM on SMI and reload it on RSM. KVM emulates HW arch
behavior when guest enters/leaves SMM mode,i.e., save registers to SMRAM
at the entry of SMM and reload them at the exit to SMM. Per SDM, SSP is
one of such registers on 64-bit Arch, and add the support for SSP.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/kvm/smm.c | 8 ++++++++
 arch/x86/kvm/smm.h | 2 +-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/smm.c b/arch/x86/kvm/smm.c
index 5dd8a1646800..b0b14ba37f9a 100644
--- a/arch/x86/kvm/smm.c
+++ b/arch/x86/kvm/smm.c
@@ -269,6 +269,10 @@ static void enter_smm_save_state_64(struct kvm_vcpu *vcpu,
 	enter_smm_save_seg_64(vcpu, &smram->gs, VCPU_SREG_GS);
 
 	smram->int_shadow = kvm_x86_call(get_interrupt_shadow)(vcpu);
+
+	if (guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK) &&
+	    kvm_msr_read(vcpu, MSR_KVM_INTERNAL_GUEST_SSP, &smram->ssp))
+		kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
 }
 #endif
 
@@ -558,6 +562,10 @@ static int rsm_load_state_64(struct x86_emulate_ctxt *ctxt,
 	kvm_x86_call(set_interrupt_shadow)(vcpu, 0);
 	ctxt->interruptibility = (u8)smstate->int_shadow;
 
+	if (guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK) &&
+	    kvm_msr_write(vcpu, MSR_KVM_INTERNAL_GUEST_SSP, smstate->ssp))
+		return X86EMUL_UNHANDLEABLE;
+
 	return X86EMUL_CONTINUE;
 }
 #endif
diff --git a/arch/x86/kvm/smm.h b/arch/x86/kvm/smm.h
index 551703fbe200..db3c88f16138 100644
--- a/arch/x86/kvm/smm.h
+++ b/arch/x86/kvm/smm.h
@@ -116,8 +116,8 @@ struct kvm_smram_state_64 {
 	u32 smbase;
 	u32 reserved4[5];
 
-	/* ssp and svm_* fields below are not implemented by KVM */
 	u64 ssp;
+	/* svm_* fields below are not implemented by KVM */
 	u64 svm_guest_pat;
 	u64 svm_host_efer;
 	u64 svm_host_cr4;
-- 
2.47.1

[PATCH v12 17/24] KVM: VMX: Set up interception for CET MSRs

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

Enable/disable CET MSRs interception per associated feature configuration.

Shadow Stack feature requires all CET MSRs passed through to guest to make
it supported in user and supervisor mode while IBT feature only depends on
MSR_IA32_{U,S}_CETS_CET to enable user and supervisor IBT.

Note, this MSR design introduced an architectural limitation of SHSTK and
IBT control for guest, i.e., when SHSTK is exposed, IBT is also available
to guest from architectural perspective since IBT relies on subset of SHSTK
relevant MSRs.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/kvm/vmx/vmx.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index bd572c8c7bc3..130ffbe7dc1a 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4088,6 +4088,8 @@ void pt_update_intercept_for_msr(struct kvm_vcpu *vcpu)
 
 void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
 {
+	bool set;
+
 	if (!cpu_has_vmx_msr_bitmap())
 		return;
 
@@ -4133,6 +4135,24 @@ void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
 		vmx_set_intercept_for_msr(vcpu, MSR_IA32_FLUSH_CMD, MSR_TYPE_W,
 					  !guest_cpu_cap_has(vcpu, X86_FEATURE_FLUSH_L1D));
 
+	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
+		set = !guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK);
+
+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP, MSR_TYPE_RW, set);
+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP, MSR_TYPE_RW, set);
+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP, MSR_TYPE_RW, set);
+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP, MSR_TYPE_RW, set);
+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB, MSR_TYPE_RW, set);
+	}
+
+	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK) || kvm_cpu_cap_has(X86_FEATURE_IBT)) {
+		set = !guest_cpu_cap_has(vcpu, X86_FEATURE_IBT) &&
+		      !guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK);
+
+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET, MSR_TYPE_RW, set);
+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET, MSR_TYPE_RW, set);
+	}
+
 	/*
 	 * x2APIC and LBR MSR intercepts are modified on-demand and cannot be
 	 * filtered by userspace.
-- 
2.47.1

Re: [PATCH v12 17/24] KVM: VMX: Set up interception for CET MSRs

[Sean Christopherson]

On Mon, Aug 11, 2025, Chao Gao wrote:
> From: Yang Weijiang <weijiang.yang@intel.com>
> 
> Enable/disable CET MSRs interception per associated feature configuration.
> 
> Shadow Stack feature requires all CET MSRs passed through to guest to make
> it supported in user and supervisor mode 

I doubt that SS _requires_ CET MSRs to be passed through.  IIRC, the actual
reason for passing through most of the MSRs is that they are managed via XSAVE,
i.e. _can't_ be intercepted without also intercepting XRSTOR.

> while IBT feature only depends on
> MSR_IA32_{U,S}_CETS_CET to enable user and supervisor IBT.
> 
> Note, this MSR design introduced an architectural limitation of SHSTK and
> IBT control for guest, i.e., when SHSTK is exposed, IBT is also available
> to guest from architectural perspective since IBT relies on subset of SHSTK
> relevant MSRs.
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> Tested-by: Mathias Krause <minipli@grsecurity.net>
> Tested-by: John Allen <john.allen@amd.com>
> Signed-off-by: Chao Gao <chao.gao@intel.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index bd572c8c7bc3..130ffbe7dc1a 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4088,6 +4088,8 @@ void pt_update_intercept_for_msr(struct kvm_vcpu *vcpu)
>  
>  void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
>  {
> +	bool set;

s/set/intercept

> +
>  	if (!cpu_has_vmx_msr_bitmap())
>  		return;
>  
> @@ -4133,6 +4135,24 @@ void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
>  		vmx_set_intercept_for_msr(vcpu, MSR_IA32_FLUSH_CMD, MSR_TYPE_W,
>  					  !guest_cpu_cap_has(vcpu, X86_FEATURE_FLUSH_L1D));
>  
> +	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
> +		set = !guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK);
> +
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP, MSR_TYPE_RW, set);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP, MSR_TYPE_RW, set);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP, MSR_TYPE_RW, set);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP, MSR_TYPE_RW, set);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB, MSR_TYPE_RW, set);

MSR_IA32_INT_SSP_TAB isn't managed via XSAVE, so why is it being passed through?

> +	}
> +
> +	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK) || kvm_cpu_cap_has(X86_FEATURE_IBT)) {
> +		set = !guest_cpu_cap_has(vcpu, X86_FEATURE_IBT) &&
> +		      !guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK);
> +
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET, MSR_TYPE_RW, set);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET, MSR_TYPE_RW, set);
> +	}
> +
>  	/*
>  	 * x2APIC and LBR MSR intercepts are modified on-demand and cannot be
>  	 * filtered by userspace.
> -- 
> 2.47.1
> 

Re: [PATCH v12 17/24] KVM: VMX: Set up interception for CET MSRs

[Xin Li]

On 8/19/2025 9:11 AM, Sean Christopherson wrote:
> On Mon, Aug 11, 2025, Chao Gao wrote:
>> From: Yang Weijiang <weijiang.yang@intel.com>
>>
>> Enable/disable CET MSRs interception per associated feature configuration.
>>
>> Shadow Stack feature requires all CET MSRs passed through to guest to make
>> it supported in user and supervisor mode
> 
> I doubt that SS _requires_ CET MSRs to be passed through.  IIRC, the actual
> reason for passing through most of the MSRs is that they are managed via XSAVE,
> i.e. _can't_ be intercepted without also intercepting XRSTOR.
> 
>> while IBT feature only depends on
>> MSR_IA32_{U,S}_CETS_CET to enable user and supervisor IBT.
>>
>> Note, this MSR design introduced an architectural limitation of SHSTK and
>> IBT control for guest, i.e., when SHSTK is exposed, IBT is also available
>> to guest from architectural perspective since IBT relies on subset of SHSTK
>> relevant MSRs.
>>
>> Suggested-by: Sean Christopherson <seanjc@google.com>
>> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
>> Tested-by: Mathias Krause <minipli@grsecurity.net>
>> Tested-by: John Allen <john.allen@amd.com>
>> Signed-off-by: Chao Gao <chao.gao@intel.com>
>> ---
>>   arch/x86/kvm/vmx/vmx.c | 20 ++++++++++++++++++++
>>   1 file changed, 20 insertions(+)
>>
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index bd572c8c7bc3..130ffbe7dc1a 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -4088,6 +4088,8 @@ void pt_update_intercept_for_msr(struct kvm_vcpu *vcpu)
>>   
>>   void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
>>   {
>> +	bool set;
> 
> s/set/intercept
> 

Maybe because you asked me to change "flag" to "set" when reviewing FRED
patches, however "intercept" does sound better, and I just changed it :)

>> +
>>   	if (!cpu_has_vmx_msr_bitmap())
>>   		return;
>>   
>> @@ -4133,6 +4135,24 @@ void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
>>   		vmx_set_intercept_for_msr(vcpu, MSR_IA32_FLUSH_CMD, MSR_TYPE_W,
>>   					  !guest_cpu_cap_has(vcpu, X86_FEATURE_FLUSH_L1D));
>>   
>> +	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
>> +		set = !guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK);
>> +
>> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP, MSR_TYPE_RW, set);
>> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP, MSR_TYPE_RW, set);
>> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP, MSR_TYPE_RW, set);
>> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP, MSR_TYPE_RW, set);
>> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB, MSR_TYPE_RW, set);
> 
> MSR_IA32_INT_SSP_TAB isn't managed via XSAVE, so why is it being passed through?

It's managed in VMCS host and guest areas, i.e. HOST_INTR_SSP_TABLE and
GUEST_INTR_SSP_TABLE, if the "load CET" bits are set in both VM entry
and exit controls.

FRED MSRs are also passed through to guest in such cases.

no?

Re: [PATCH v12 17/24] KVM: VMX: Set up interception for CET MSRs

[Sean Christopherson]

On Tue, Aug 19, 2025, Xin Li wrote:
> On 8/19/2025 9:11 AM, Sean Christopherson wrote:
> > On Mon, Aug 11, 2025, Chao Gao wrote:
> > > From: Yang Weijiang <weijiang.yang@intel.com>
> > > 
> > > Enable/disable CET MSRs interception per associated feature configuration.
> > > 
> > > Shadow Stack feature requires all CET MSRs passed through to guest to make
> > > it supported in user and supervisor mode
> > 
> > I doubt that SS _requires_ CET MSRs to be passed through.  IIRC, the actual
> > reason for passing through most of the MSRs is that they are managed via XSAVE,
> > i.e. _can't_ be intercepted without also intercepting XRSTOR.
> > 
> > > while IBT feature only depends on
> > > MSR_IA32_{U,S}_CETS_CET to enable user and supervisor IBT.
> > > 
> > > Note, this MSR design introduced an architectural limitation of SHSTK and
> > > IBT control for guest, i.e., when SHSTK is exposed, IBT is also available
> > > to guest from architectural perspective since IBT relies on subset of SHSTK
> > > relevant MSRs.
> > > 
> > > Suggested-by: Sean Christopherson <seanjc@google.com>
> > > Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> > > Tested-by: Mathias Krause <minipli@grsecurity.net>
> > > Tested-by: John Allen <john.allen@amd.com>
> > > Signed-off-by: Chao Gao <chao.gao@intel.com>
> > > ---
> > >   arch/x86/kvm/vmx/vmx.c | 20 ++++++++++++++++++++
> > >   1 file changed, 20 insertions(+)
> > > 
> > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > > index bd572c8c7bc3..130ffbe7dc1a 100644
> > > --- a/arch/x86/kvm/vmx/vmx.c
> > > +++ b/arch/x86/kvm/vmx/vmx.c
> > > @@ -4088,6 +4088,8 @@ void pt_update_intercept_for_msr(struct kvm_vcpu *vcpu)
> > >   void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
> > >   {
> > > +	bool set;
> > 
> > s/set/intercept
> > 
> 
> Maybe because you asked me to change "flag" to "set" when reviewing FRED
> patches, however "intercept" does sound better, and I just changed it :)

Ah crud.  I had a feeling I was flip-flopping.  I obviously don't have a strong
preference.

> > > +
> > >   	if (!cpu_has_vmx_msr_bitmap())
> > >   		return;
> > > @@ -4133,6 +4135,24 @@ void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
> > >   		vmx_set_intercept_for_msr(vcpu, MSR_IA32_FLUSH_CMD, MSR_TYPE_W,
> > >   					  !guest_cpu_cap_has(vcpu, X86_FEATURE_FLUSH_L1D));
> > > +	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
> > > +		set = !guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK);
> > > +
> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP, MSR_TYPE_RW, set);
> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP, MSR_TYPE_RW, set);
> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP, MSR_TYPE_RW, set);
> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP, MSR_TYPE_RW, set);
> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB, MSR_TYPE_RW, set);
> > 
> > MSR_IA32_INT_SSP_TAB isn't managed via XSAVE, so why is it being passed through?
> 
> It's managed in VMCS host and guest areas, i.e. HOST_INTR_SSP_TABLE and
> GUEST_INTR_SSP_TABLE, if the "load CET" bits are set in both VM entry
> and exit controls.

Ah, "because it's essentially free".  Unless there's a true need to pass it through,
I think it makes sense to intercept.  Merging KVM's bitmap with vmcs12's bitmap
isn't completely free (though it's quite cheap).  More importantly, this is technically
wrong due to MSR_IA32_INT_SSP_TAB not existing if the vCPU doesn't have LM.  That's
obviously easy to solve, I just don't see the point.

Re: [PATCH v12 17/24] KVM: VMX: Set up interception for CET MSRs

[Chao Gao]

On Tue, Aug 19, 2025 at 11:45:42AM -0700, Sean Christopherson wrote:
>On Tue, Aug 19, 2025, Xin Li wrote:
>> On 8/19/2025 9:11 AM, Sean Christopherson wrote:
>> > On Mon, Aug 11, 2025, Chao Gao wrote:
>> > > From: Yang Weijiang <weijiang.yang@intel.com>
>> > > 
>> > > Enable/disable CET MSRs interception per associated feature configuration.
>> > > 
>> > > Shadow Stack feature requires all CET MSRs passed through to guest to make
>> > > it supported in user and supervisor mode
>> > 
>> > I doubt that SS _requires_ CET MSRs to be passed through.  IIRC, the actual
>> > reason for passing through most of the MSRs is that they are managed via XSAVE,
>> > i.e. _can't_ be intercepted without also intercepting XRSTOR.

Agreed. Will update the changelog.

>> > 
>> > > while IBT feature only depends on
>> > > MSR_IA32_{U,S}_CETS_CET to enable user and supervisor IBT.
>> > > 
>> > > Note, this MSR design introduced an architectural limitation of SHSTK and
>> > > IBT control for guest, i.e., when SHSTK is exposed, IBT is also available
>> > > to guest from architectural perspective since IBT relies on subset of SHSTK
>> > > relevant MSRs.
>> > > 
>> > > Suggested-by: Sean Christopherson <seanjc@google.com>
>> > > Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
>> > > Tested-by: Mathias Krause <minipli@grsecurity.net>
>> > > Tested-by: John Allen <john.allen@amd.com>
>> > > Signed-off-by: Chao Gao <chao.gao@intel.com>
>> > > ---
>> > >   arch/x86/kvm/vmx/vmx.c | 20 ++++++++++++++++++++
>> > >   1 file changed, 20 insertions(+)
>> > > 
>> > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> > > index bd572c8c7bc3..130ffbe7dc1a 100644
>> > > --- a/arch/x86/kvm/vmx/vmx.c
>> > > +++ b/arch/x86/kvm/vmx/vmx.c
>> > > @@ -4088,6 +4088,8 @@ void pt_update_intercept_for_msr(struct kvm_vcpu *vcpu)
>> > >   void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
>> > >   {
>> > > +	bool set;
>> > 
>> > s/set/intercept
>> > 
>> 
>> Maybe because you asked me to change "flag" to "set" when reviewing FRED
>> patches, however "intercept" does sound better, and I just changed it :)
>
>Ah crud.  I had a feeling I was flip-flopping.  I obviously don't have a strong
>preference.

Anyway, I will use "intercept".

>
>> > > +
>> > >   	if (!cpu_has_vmx_msr_bitmap())
>> > >   		return;
>> > > @@ -4133,6 +4135,24 @@ void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
>> > >   		vmx_set_intercept_for_msr(vcpu, MSR_IA32_FLUSH_CMD, MSR_TYPE_W,
>> > >   					  !guest_cpu_cap_has(vcpu, X86_FEATURE_FLUSH_L1D));
>> > > +	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
>> > > +		set = !guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK);
>> > > +
>> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP, MSR_TYPE_RW, set);
>> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP, MSR_TYPE_RW, set);
>> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP, MSR_TYPE_RW, set);
>> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP, MSR_TYPE_RW, set);
>> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB, MSR_TYPE_RW, set);
>> > 
>> > MSR_IA32_INT_SSP_TAB isn't managed via XSAVE, so why is it being passed through?
>> 
>> It's managed in VMCS host and guest areas, i.e. HOST_INTR_SSP_TABLE and
>> GUEST_INTR_SSP_TABLE, if the "load CET" bits are set in both VM entry
>> and exit controls.
>
>Ah, "because it's essentially free".  Unless there's a true need to pass it through,
>I think it makes sense to intercept.  Merging KVM's bitmap with vmcs12's bitmap
>isn't completely free (though it's quite cheap).  More importantly, this is technically
>wrong due to MSR_IA32_INT_SSP_TAB not existing if the vCPU doesn't have LM.  That's
>obviously easy to solve, I just don't see the point.

Sure. I will leave MSR_IA32_INT_SSP_TAB intercept and add a LM check on its emulation.

[PATCH v12 18/24] KVM: VMX: Set host constant supervisor states to VMCS fields

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

Save constant values to HOST_{S_CET,SSP,INTR_SSP_TABLE} field explicitly.
Kernel IBT is supported and the setting in MSR_IA32_S_CET is static after
post-boot(The exception is BIOS call case but vCPU thread never across it)
and KVM doesn't need to refresh HOST_S_CET field before every VM-Enter/
VM-Exit sequence.

Host supervisor shadow stack is not enabled now and SSP is not accessible
to kernel mode, thus it's safe to set host IA32_INT_SSP_TAB/SSP VMCS field
to 0s. When shadow stack is enabled for CPL3, SSP is reloaded from PL3_SSP
before it exits to userspace. Check SDM Vol 2A/B Chapter 3/4 for SYSCALL/
SYSRET/SYSENTER SYSEXIT/RDSSP/CALL etc.

Prevent KVM module loading if host supervisor shadow stack SHSTK_EN is set
in MSR_IA32_S_CET as KVM cannot co-exit with it correctly.

Suggested-by: Sean Christopherson <seanjc@google.com>
Suggested-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/kvm/vmx/capabilities.h |  4 ++++
 arch/x86/kvm/vmx/vmx.c          | 15 +++++++++++++++
 arch/x86/kvm/x86.c              | 12 ++++++++++++
 arch/x86/kvm/x86.h              |  1 +
 4 files changed, 32 insertions(+)

diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
index 5316c27f6099..7d290b2cb0f4 100644
--- a/arch/x86/kvm/vmx/capabilities.h
+++ b/arch/x86/kvm/vmx/capabilities.h
@@ -103,6 +103,10 @@ static inline bool cpu_has_load_perf_global_ctrl(void)
 	return vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL;
 }
 
+static inline bool cpu_has_load_cet_ctrl(void)
+{
+	return (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_CET_STATE);
+}
 static inline bool cpu_has_vmx_mpx(void)
 {
 	return vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_BNDCFGS;
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 130ffbe7dc1a..ba44223405cd 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4308,6 +4308,21 @@ void vmx_set_constant_host_state(struct vcpu_vmx *vmx)
 
 	if (cpu_has_load_ia32_efer())
 		vmcs_write64(HOST_IA32_EFER, kvm_host.efer);
+
+	/*
+	 * Supervisor shadow stack is not enabled on host side, i.e.,
+	 * host IA32_S_CET.SHSTK_EN bit is guaranteed to 0 now, per SDM
+	 * description(RDSSP instruction), SSP is not readable in CPL0,
+	 * so resetting the two registers to 0s at VM-Exit does no harm
+	 * to kernel execution. When execution flow exits to userspace,
+	 * SSP is reloaded from IA32_PL3_SSP. Check SDM Vol.2A/B Chapter
+	 * 3 and 4 for details.
+	 */
+	if (cpu_has_load_cet_ctrl()) {
+		vmcs_writel(HOST_S_CET, kvm_host.s_cet);
+		vmcs_writel(HOST_SSP, 0);
+		vmcs_writel(HOST_INTR_SSP_TABLE, 0);
+	}
 }
 
 void set_cr4_guest_host_mask(struct vcpu_vmx *vmx)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index cc39ace47262..91e78c506105 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9845,6 +9845,18 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
 		return -EIO;
 	}
 
+	if (boot_cpu_has(X86_FEATURE_SHSTK)) {
+		rdmsrl(MSR_IA32_S_CET, kvm_host.s_cet);
+		/*
+		 * Linux doesn't yet support supervisor shadow stacks (SSS), so
+		 * KVM doesn't save/restore the associated MSRs, i.e. KVM may
+		 * clobber the host values.  Yell and refuse to load if SSS is
+		 * unexpectedly enabled, e.g. to avoid crashing the host.
+		 */
+		if (WARN_ON_ONCE(kvm_host.s_cet & CET_SHSTK_EN))
+			return -EIO;
+	}
+
 	memset(&kvm_caps, 0, sizeof(kvm_caps));
 
 	x86_emulator_cache = kvm_alloc_emulator_cache();
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index d5b039addd11..d612ddcae247 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -50,6 +50,7 @@ struct kvm_host_values {
 	u64 efer;
 	u64 xcr0;
 	u64 xss;
+	u64 s_cet;
 	u64 arch_capabilities;
 };
 
-- 
2.47.1

Re: [PATCH v12 18/24] KVM: VMX: Set host constant supervisor states to VMCS fields

[Chao Gao]

> void set_cr4_guest_host_mask(struct vcpu_vmx *vmx)
>diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>index cc39ace47262..91e78c506105 100644
>--- a/arch/x86/kvm/x86.c
>+++ b/arch/x86/kvm/x86.c
>@@ -9845,6 +9845,18 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
> 		return -EIO;
> 	}
> 
>+	if (boot_cpu_has(X86_FEATURE_SHSTK)) {
>+		rdmsrl(MSR_IA32_S_CET, kvm_host.s_cet);

This should be rdmsrq for consistency with other call sites in this file.

>+		/*
>+		 * Linux doesn't yet support supervisor shadow stacks (SSS), so
>+		 * KVM doesn't save/restore the associated MSRs, i.e. KVM may
>+		 * clobber the host values.  Yell and refuse to load if SSS is
>+		 * unexpectedly enabled, e.g. to avoid crashing the host.
>+		 */
>+		if (WARN_ON_ONCE(kvm_host.s_cet & CET_SHSTK_EN))
>+			return -EIO;
>+	}
>+
> 	memset(&kvm_caps, 0, sizeof(kvm_caps));
> 
> 	x86_emulator_cache = kvm_alloc_emulator_cache();
>diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
>index d5b039addd11..d612ddcae247 100644
>--- a/arch/x86/kvm/x86.h
>+++ b/arch/x86/kvm/x86.h
>@@ -50,6 +50,7 @@ struct kvm_host_values {
> 	u64 efer;
> 	u64 xcr0;
> 	u64 xss;
>+	u64 s_cet;
> 	u64 arch_capabilities;
> };
> 
>-- 
>2.47.1
>

[PATCH v12 19/24] KVM: x86: Don't emulate instructions guarded by CET

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

Don't emulate the branch instructions, e.g., CALL/RET/JMP etc., when CET
is active in guest, return KVM_INTERNAL_ERROR_EMULATION to userspace to
handle it.

KVM doesn't emulate CPU behaviors to check CET protected stuffs while
emulating guest instructions, instead it stops emulation on detecting
the instructions in process are CET protected. By doing so, it can avoid
generating bogus #CP in guest and preventing CET protected execution flow
subversion from guest side.

Suggested-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/kvm/emulate.c | 46 ++++++++++++++++++++++++++++++++----------
 1 file changed, 35 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 1349e278cd2a..80b9d1e4a50a 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -178,6 +178,8 @@
 #define IncSP       ((u64)1 << 54)  /* SP is incremented before ModRM calc */
 #define TwoMemOp    ((u64)1 << 55)  /* Instruction has two memory operand */
 #define IsBranch    ((u64)1 << 56)  /* Instruction is considered a branch. */
+#define ShadowStack ((u64)1 << 57)  /* Instruction protected by Shadow Stack. */
+#define IndirBrnTrk ((u64)1 << 58)  /* Instruction protected by IBT. */
 
 #define DstXacc     (DstAccLo | SrcAccHi | SrcWrite)
 
@@ -4068,9 +4070,11 @@ static const struct opcode group4[] = {
 static const struct opcode group5[] = {
 	F(DstMem | SrcNone | Lock,		em_inc),
 	F(DstMem | SrcNone | Lock,		em_dec),
-	I(SrcMem | NearBranch | IsBranch,       em_call_near_abs),
-	I(SrcMemFAddr | ImplicitOps | IsBranch, em_call_far),
-	I(SrcMem | NearBranch | IsBranch,       em_jmp_abs),
+	I(SrcMem | NearBranch | IsBranch | ShadowStack | IndirBrnTrk,
+	em_call_near_abs),
+	I(SrcMemFAddr | ImplicitOps | IsBranch | ShadowStack | IndirBrnTrk,
+	em_call_far),
+	I(SrcMem | NearBranch | IsBranch | IndirBrnTrk, em_jmp_abs),
 	I(SrcMemFAddr | ImplicitOps | IsBranch, em_jmp_far),
 	I(SrcMem | Stack | TwoMemOp,		em_push), D(Undefined),
 };
@@ -4332,11 +4336,11 @@ static const struct opcode opcode_table[256] = {
 	/* 0xC8 - 0xCF */
 	I(Stack | SrcImmU16 | Src2ImmByte | IsBranch, em_enter),
 	I(Stack | IsBranch, em_leave),
-	I(ImplicitOps | SrcImmU16 | IsBranch, em_ret_far_imm),
-	I(ImplicitOps | IsBranch, em_ret_far),
-	D(ImplicitOps | IsBranch), DI(SrcImmByte | IsBranch, intn),
+	I(ImplicitOps | SrcImmU16 | IsBranch | ShadowStack, em_ret_far_imm),
+	I(ImplicitOps | IsBranch | ShadowStack, em_ret_far),
+	D(ImplicitOps | IsBranch), DI(SrcImmByte | IsBranch | ShadowStack, intn),
 	D(ImplicitOps | No64 | IsBranch),
-	II(ImplicitOps | IsBranch, em_iret, iret),
+	II(ImplicitOps | IsBranch | ShadowStack, em_iret, iret),
 	/* 0xD0 - 0xD7 */
 	G(Src2One | ByteOp, group2), G(Src2One, group2),
 	G(Src2CL | ByteOp, group2), G(Src2CL, group2),
@@ -4352,7 +4356,7 @@ static const struct opcode opcode_table[256] = {
 	I2bvIP(SrcImmUByte | DstAcc, em_in,  in,  check_perm_in),
 	I2bvIP(SrcAcc | DstImmUByte, em_out, out, check_perm_out),
 	/* 0xE8 - 0xEF */
-	I(SrcImm | NearBranch | IsBranch, em_call),
+	I(SrcImm | NearBranch | IsBranch | ShadowStack, em_call),
 	D(SrcImm | ImplicitOps | NearBranch | IsBranch),
 	I(SrcImmFAddr | No64 | IsBranch, em_jmp_far),
 	D(SrcImmByte | ImplicitOps | NearBranch | IsBranch),
@@ -4371,7 +4375,8 @@ static const struct opcode opcode_table[256] = {
 static const struct opcode twobyte_table[256] = {
 	/* 0x00 - 0x0F */
 	G(0, group6), GD(0, &group7), N, N,
-	N, I(ImplicitOps | EmulateOnUD | IsBranch, em_syscall),
+	N, I(ImplicitOps | EmulateOnUD | IsBranch | ShadowStack | IndirBrnTrk,
+	em_syscall),
 	II(ImplicitOps | Priv, em_clts, clts), N,
 	DI(ImplicitOps | Priv, invd), DI(ImplicitOps | Priv, wbinvd), N, N,
 	N, D(ImplicitOps | ModRM | SrcMem | NoAccess), N, N,
@@ -4402,8 +4407,9 @@ static const struct opcode twobyte_table[256] = {
 	IIP(ImplicitOps, em_rdtsc, rdtsc, check_rdtsc),
 	II(ImplicitOps | Priv, em_rdmsr, rdmsr),
 	IIP(ImplicitOps, em_rdpmc, rdpmc, check_rdpmc),
-	I(ImplicitOps | EmulateOnUD | IsBranch, em_sysenter),
-	I(ImplicitOps | Priv | EmulateOnUD | IsBranch, em_sysexit),
+	I(ImplicitOps | EmulateOnUD | IsBranch | ShadowStack | IndirBrnTrk,
+	em_sysenter),
+	I(ImplicitOps | Priv | EmulateOnUD | IsBranch | ShadowStack, em_sysexit),
 	N, N,
 	N, N, N, N, N, N, N, N,
 	/* 0x40 - 0x4F */
@@ -4941,6 +4947,24 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len, int
 	if (ctxt->d == 0)
 		return EMULATION_FAILED;
 
+	if (ctxt->ops->get_cr(ctxt, 4) & X86_CR4_CET) {
+		u64 u_cet, s_cet;
+		bool stop_em;
+
+		if (ctxt->ops->get_msr(ctxt, MSR_IA32_U_CET, &u_cet) ||
+		    ctxt->ops->get_msr(ctxt, MSR_IA32_S_CET, &s_cet))
+			return EMULATION_FAILED;
+
+		stop_em = ((u_cet & CET_SHSTK_EN) || (s_cet & CET_SHSTK_EN)) &&
+			  (opcode.flags & ShadowStack);
+
+		stop_em |= ((u_cet & CET_ENDBR_EN) || (s_cet & CET_ENDBR_EN)) &&
+			   (opcode.flags & IndirBrnTrk);
+
+		if (stop_em)
+			return EMULATION_FAILED;
+	}
+
 	ctxt->execute = opcode.u.execute;
 
 	if (unlikely(emulation_type & EMULTYPE_TRAP_UD) &&
-- 
2.47.1

[PATCH v12 20/24] KVM: x86: Enable CET virtualization for VMX and advertise to userspace

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

Expose CET features to guest if KVM/host can support them, clear CPUID
feature bits if KVM/host cannot support.

Set CPUID feature bits so that CET features are available in guest CPUID.
Add CR4.CET bit support in order to allow guest set CET master control
bit.

Disable KVM CET feature if unrestricted_guest is unsupported/disabled as
KVM does not support emulating CET.

The CET load-bits in VM_ENTRY/VM_EXIT control fields should be set to make
guest CET xstates isolated from host's.

On platforms with VMX_BASIC[bit56] == 0, inject #CP at VMX entry with error
code will fail, and if VMX_BASIC[bit56] == 1, #CP injection with or without
error code is allowed. Disable CET feature bits if the MSR bit is cleared
so that nested VMM can inject #CP if and only if VMX_BASIC[bit56] == 1.

Don't expose CET feature if either of {U,S}_CET xstate bits is cleared
in host XSS or if XSAVES isn't supported.

CET MSRs are reset to 0s after RESET, power-up and INIT, clear guest CET
xsave-area fields so that guest CET MSRs are reset to 0s after the events.

Meanwhile explicitly disable SHSTK and IBT for SVM because CET KVM enabling
for SVM is not ready.

Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/include/asm/kvm_host.h |  2 +-
 arch/x86/include/asm/vmx.h      |  1 +
 arch/x86/kvm/cpuid.c            |  2 ++
 arch/x86/kvm/svm/svm.c          |  4 ++++
 arch/x86/kvm/vmx/capabilities.h |  5 +++++
 arch/x86/kvm/vmx/vmx.c          | 30 +++++++++++++++++++++++++++++-
 arch/x86/kvm/vmx/vmx.h          |  6 ++++--
 arch/x86/kvm/x86.c              | 22 +++++++++++++++++++---
 arch/x86/kvm/x86.h              |  3 +++
 9 files changed, 68 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 1faf53df6259..9a667d7efe83 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -142,7 +142,7 @@
 			  | X86_CR4_OSXSAVE | X86_CR4_SMEP | X86_CR4_FSGSBASE \
 			  | X86_CR4_OSXMMEXCPT | X86_CR4_LA57 | X86_CR4_VMXE \
 			  | X86_CR4_SMAP | X86_CR4_PKE | X86_CR4_UMIP \
-			  | X86_CR4_LAM_SUP))
+			  | X86_CR4_LAM_SUP | X86_CR4_CET))
 
 #define CR8_RESERVED_BITS (~(unsigned long)X86_CR8_TPR)
 
diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
index ce10a7e2d3d9..c85c50019523 100644
--- a/arch/x86/include/asm/vmx.h
+++ b/arch/x86/include/asm/vmx.h
@@ -134,6 +134,7 @@
 #define VMX_BASIC_DUAL_MONITOR_TREATMENT	BIT_ULL(49)
 #define VMX_BASIC_INOUT				BIT_ULL(54)
 #define VMX_BASIC_TRUE_CTLS			BIT_ULL(55)
+#define VMX_BASIC_NO_HW_ERROR_CODE_CC		BIT_ULL(56)
 
 static inline u32 vmx_basic_vmcs_revision_id(u64 vmx_basic)
 {
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 85079caaf507..2515b2623fb1 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -944,6 +944,7 @@ void kvm_set_cpu_caps(void)
 		VENDOR_F(WAITPKG),
 		F(SGX_LC),
 		F(BUS_LOCK_DETECT),
+		F(SHSTK),
 	);
 
 	/*
@@ -970,6 +971,7 @@ void kvm_set_cpu_caps(void)
 		F(AMX_INT8),
 		F(AMX_BF16),
 		F(FLUSH_L1D),
+		F(IBT),
 	);
 
 	if (boot_cpu_has(X86_FEATURE_AMD_IBPB_RET) &&
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index d9931c6c4bc6..fb36774b8046 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5231,6 +5231,10 @@ static __init void svm_set_cpu_caps(void)
 	kvm_caps.supported_perf_cap = 0;
 	kvm_caps.supported_xss = 0;
 
+	/* KVM doesn't yet support CET virtualization for SVM. */
+	kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
+	kvm_cpu_cap_clear(X86_FEATURE_IBT);
+
 	/* CPUID 0x80000001 and 0x8000000A (SVM features) */
 	if (nested) {
 		kvm_cpu_cap_set(X86_FEATURE_SVM);
diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
index 7d290b2cb0f4..47b0dec8665a 100644
--- a/arch/x86/kvm/vmx/capabilities.h
+++ b/arch/x86/kvm/vmx/capabilities.h
@@ -76,6 +76,11 @@ static inline bool cpu_has_vmx_basic_inout(void)
 	return	vmcs_config.basic & VMX_BASIC_INOUT;
 }
 
+static inline bool cpu_has_vmx_basic_no_hw_errcode(void)
+{
+	return	vmcs_config.basic & VMX_BASIC_NO_HW_ERROR_CODE_CC;
+}
+
 static inline bool cpu_has_virtual_nmis(void)
 {
 	return vmcs_config.pin_based_exec_ctrl & PIN_BASED_VIRTUAL_NMIS &&
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index ba44223405cd..ae8be898e1df 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2602,6 +2602,7 @@ static int setup_vmcs_config(struct vmcs_config *vmcs_conf,
 		{ VM_ENTRY_LOAD_IA32_EFER,		VM_EXIT_LOAD_IA32_EFER },
 		{ VM_ENTRY_LOAD_BNDCFGS,		VM_EXIT_CLEAR_BNDCFGS },
 		{ VM_ENTRY_LOAD_IA32_RTIT_CTL,		VM_EXIT_CLEAR_IA32_RTIT_CTL },
+		{ VM_ENTRY_LOAD_CET_STATE,		VM_EXIT_LOAD_CET_STATE },
 	};
 
 	memset(vmcs_conf, 0, sizeof(*vmcs_conf));
@@ -4870,6 +4871,14 @@ void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
 
 	vmcs_write32(VM_ENTRY_INTR_INFO_FIELD, 0);  /* 22.2.1 */
 
+	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
+		vmcs_writel(GUEST_SSP, 0);
+		vmcs_writel(GUEST_INTR_SSP_TABLE, 0);
+	}
+	if (kvm_cpu_cap_has(X86_FEATURE_IBT) ||
+	    kvm_cpu_cap_has(X86_FEATURE_SHSTK))
+		vmcs_writel(GUEST_S_CET, 0);
+
 	kvm_make_request(KVM_REQ_APIC_PAGE_RELOAD, vcpu);
 
 	vpid_sync_context(vmx->vpid);
@@ -6318,6 +6327,10 @@ void dump_vmcs(struct kvm_vcpu *vcpu)
 	if (vmcs_read32(VM_EXIT_MSR_STORE_COUNT) > 0)
 		vmx_dump_msrs("guest autostore", &vmx->msr_autostore.guest);
 
+	if (vmentry_ctl & VM_ENTRY_LOAD_CET_STATE)
+		pr_err("S_CET = 0x%016lx, SSP = 0x%016lx, SSP TABLE = 0x%016lx\n",
+		       vmcs_readl(GUEST_S_CET), vmcs_readl(GUEST_SSP),
+		       vmcs_readl(GUEST_INTR_SSP_TABLE));
 	pr_err("*** Host State ***\n");
 	pr_err("RIP = 0x%016lx  RSP = 0x%016lx\n",
 	       vmcs_readl(HOST_RIP), vmcs_readl(HOST_RSP));
@@ -6348,6 +6361,10 @@ void dump_vmcs(struct kvm_vcpu *vcpu)
 		       vmcs_read64(HOST_IA32_PERF_GLOBAL_CTRL));
 	if (vmcs_read32(VM_EXIT_MSR_LOAD_COUNT) > 0)
 		vmx_dump_msrs("host autoload", &vmx->msr_autoload.host);
+	if (vmexit_ctl & VM_EXIT_LOAD_CET_STATE)
+		pr_err("S_CET = 0x%016lx, SSP = 0x%016lx, SSP TABLE = 0x%016lx\n",
+		       vmcs_readl(HOST_S_CET), vmcs_readl(HOST_SSP),
+		       vmcs_readl(HOST_INTR_SSP_TABLE));
 
 	pr_err("*** Control State ***\n");
 	pr_err("CPUBased=0x%08x SecondaryExec=0x%08x TertiaryExec=0x%016llx\n",
@@ -7925,7 +7942,6 @@ static __init void vmx_set_cpu_caps(void)
 		kvm_cpu_cap_set(X86_FEATURE_UMIP);
 
 	/* CPUID 0xD.1 */
-	kvm_caps.supported_xss = 0;
 	if (!cpu_has_vmx_xsaves())
 		kvm_cpu_cap_clear(X86_FEATURE_XSAVES);
 
@@ -7937,6 +7953,18 @@ static __init void vmx_set_cpu_caps(void)
 
 	if (cpu_has_vmx_waitpkg())
 		kvm_cpu_cap_check_and_set(X86_FEATURE_WAITPKG);
+
+	/*
+	 * Disable CET if unrestricted_guest is unsupported as KVM doesn't
+	 * enforce CET HW behaviors in emulator. On platforms with
+	 * VMX_BASIC[bit56] == 0, inject #CP at VMX entry with error code
+	 * fails, so disable CET in this case too.
+	 */
+	if (!cpu_has_load_cet_ctrl() || !enable_unrestricted_guest ||
+	    !cpu_has_vmx_basic_no_hw_errcode()) {
+		kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
+		kvm_cpu_cap_clear(X86_FEATURE_IBT);
+	}
 }
 
 static bool vmx_is_io_intercepted(struct kvm_vcpu *vcpu,
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index d3389baf3ab3..89200586b35a 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -484,7 +484,8 @@ static inline u8 vmx_get_rvi(void)
 	 VM_ENTRY_LOAD_IA32_EFER |					\
 	 VM_ENTRY_LOAD_BNDCFGS |					\
 	 VM_ENTRY_PT_CONCEAL_PIP |					\
-	 VM_ENTRY_LOAD_IA32_RTIT_CTL)
+	 VM_ENTRY_LOAD_IA32_RTIT_CTL |					\
+	 VM_ENTRY_LOAD_CET_STATE)
 
 #define __KVM_REQUIRED_VMX_VM_EXIT_CONTROLS				\
 	(VM_EXIT_SAVE_DEBUG_CONTROLS |					\
@@ -506,7 +507,8 @@ static inline u8 vmx_get_rvi(void)
 	       VM_EXIT_LOAD_IA32_EFER |					\
 	       VM_EXIT_CLEAR_BNDCFGS |					\
 	       VM_EXIT_PT_CONCEAL_PIP |					\
-	       VM_EXIT_CLEAR_IA32_RTIT_CTL)
+	       VM_EXIT_CLEAR_IA32_RTIT_CTL |				\
+	       VM_EXIT_LOAD_CET_STATE)
 
 #define KVM_REQUIRED_VMX_PIN_BASED_VM_EXEC_CONTROL			\
 	(PIN_BASED_EXT_INTR_MASK |					\
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 91e78c506105..081d676fd259 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -223,7 +223,8 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs;
 				| XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
 				| XFEATURE_MASK_PKRU | XFEATURE_MASK_XTILE)
 
-#define KVM_SUPPORTED_XSS     0
+#define KVM_SUPPORTED_XSS	(XFEATURE_MASK_CET_USER | \
+				 XFEATURE_MASK_CET_KERNEL)
 
 bool __read_mostly allow_smaller_maxphyaddr = 0;
 EXPORT_SYMBOL_GPL(allow_smaller_maxphyaddr);
@@ -9943,6 +9944,20 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
 	if (!kvm_cpu_cap_has(X86_FEATURE_XSAVES))
 		kvm_caps.supported_xss = 0;
 
+	if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK) &&
+	    !kvm_cpu_cap_has(X86_FEATURE_IBT))
+		kvm_caps.supported_xss &= ~(XFEATURE_MASK_CET_USER |
+					    XFEATURE_MASK_CET_KERNEL);
+
+	if ((kvm_caps.supported_xss & (XFEATURE_MASK_CET_USER |
+	     XFEATURE_MASK_CET_KERNEL)) !=
+	    (XFEATURE_MASK_CET_USER | XFEATURE_MASK_CET_KERNEL)) {
+		kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
+		kvm_cpu_cap_clear(X86_FEATURE_IBT);
+		kvm_caps.supported_xss &= ~(XFEATURE_MASK_CET_USER |
+					    XFEATURE_MASK_CET_KERNEL);
+	}
+
 	if (kvm_caps.has_tsc_control) {
 		/*
 		 * Make sure the user can only configure tsc_khz values that
@@ -12601,10 +12616,11 @@ static void kvm_xstate_reset(struct kvm_vcpu *vcpu, bool init_event)
 	/*
 	 * On INIT, only select XSTATE components are zeroed, most components
 	 * are unchanged.  Currently, the only components that are zeroed and
-	 * supported by KVM are MPX related.
+	 * supported by KVM are MPX and CET related.
 	 */
 	xfeatures_mask = (kvm_caps.supported_xcr0 | kvm_caps.supported_xss) &
-			 (XFEATURE_MASK_BNDREGS | XFEATURE_MASK_BNDCSR);
+			 (XFEATURE_MASK_BNDREGS | XFEATURE_MASK_BNDCSR |
+			  XFEATURE_MASK_CET_USER | XFEATURE_MASK_CET_KERNEL);
 	if (!xfeatures_mask)
 		return;
 
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index d612ddcae247..c2053b753980 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -679,6 +679,9 @@ static inline bool __kvm_is_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 		__reserved_bits |= X86_CR4_PCIDE;       \
 	if (!__cpu_has(__c, X86_FEATURE_LAM))           \
 		__reserved_bits |= X86_CR4_LAM_SUP;     \
+	if (!__cpu_has(__c, X86_FEATURE_SHSTK) &&       \
+	    !__cpu_has(__c, X86_FEATURE_IBT))           \
+		__reserved_bits |= X86_CR4_CET;         \
 	__reserved_bits;                                \
 })
 
-- 
2.47.1

Re: [PATCH v12 20/24] KVM: x86: Enable CET virtualization for VMX and advertise to userspace

[Sean Christopherson]

On Mon, Aug 11, 2025, Chao Gao wrote:
> diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
> index ce10a7e2d3d9..c85c50019523 100644
> --- a/arch/x86/include/asm/vmx.h
> +++ b/arch/x86/include/asm/vmx.h
> @@ -134,6 +134,7 @@
>  #define VMX_BASIC_DUAL_MONITOR_TREATMENT	BIT_ULL(49)
>  #define VMX_BASIC_INOUT				BIT_ULL(54)
>  #define VMX_BASIC_TRUE_CTLS			BIT_ULL(55)
> +#define VMX_BASIC_NO_HW_ERROR_CODE_CC		BIT_ULL(56)
>  
>  static inline u32 vmx_basic_vmcs_revision_id(u64 vmx_basic)
>  {
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 85079caaf507..2515b2623fb1 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -944,6 +944,7 @@ void kvm_set_cpu_caps(void)
>  		VENDOR_F(WAITPKG),
>  		F(SGX_LC),
>  		F(BUS_LOCK_DETECT),
> +		F(SHSTK),

I think we should limit this to 64-bit kernels with X86_64_F().  It probably
won't change anything in practice, but we mitigate the virtualization holes related
to 32-bit vCPUs at least a little bit.

>  	);
>  
>  	/*
> @@ -970,6 +971,7 @@ void kvm_set_cpu_caps(void)
>  		F(AMX_INT8),
>  		F(AMX_BF16),
>  		F(FLUSH_L1D),
> +		F(IBT),
>  	);
>  

[PATCH v12 21/24] KVM: nVMX: Virtualize NO_HW_ERROR_CODE_CC for L1 event injection to L2

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

Per SDM description(Vol.3D, Appendix A.1):
"If bit 56 is read as 1, software can use VM entry to deliver a hardware
exception with or without an error code, regardless of vector"

Modify has_error_code check before inject events to nested guest. Only
enforce the check when guest is in real mode, the exception is not hard
exception and the platform doesn't enumerate bit56 in VMX_BASIC, in all
other case ignore the check to make the logic consistent with SDM.

Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/kvm/vmx/nested.c | 28 +++++++++++++++++++---------
 arch/x86/kvm/vmx/nested.h |  5 +++++
 2 files changed, 24 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 7dc2e1c09ea6..618cc6c6425c 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -1272,9 +1272,10 @@ static int vmx_restore_vmx_basic(struct vcpu_vmx *vmx, u64 data)
 {
 	const u64 feature_bits = VMX_BASIC_DUAL_MONITOR_TREATMENT |
 				 VMX_BASIC_INOUT |
-				 VMX_BASIC_TRUE_CTLS;
+				 VMX_BASIC_TRUE_CTLS |
+				 VMX_BASIC_NO_HW_ERROR_CODE_CC;
 
-	const u64 reserved_bits = GENMASK_ULL(63, 56) |
+	const u64 reserved_bits = GENMASK_ULL(63, 57) |
 				  GENMASK_ULL(47, 45) |
 				  BIT_ULL(31);
 
@@ -2949,7 +2950,6 @@ static int nested_check_vm_entry_controls(struct kvm_vcpu *vcpu,
 		u8 vector = intr_info & INTR_INFO_VECTOR_MASK;
 		u32 intr_type = intr_info & INTR_INFO_INTR_TYPE_MASK;
 		bool has_error_code = intr_info & INTR_INFO_DELIVER_CODE_MASK;
-		bool should_have_error_code;
 		bool urg = nested_cpu_has2(vmcs12,
 					   SECONDARY_EXEC_UNRESTRICTED_GUEST);
 		bool prot_mode = !urg || vmcs12->guest_cr0 & X86_CR0_PE;
@@ -2966,12 +2966,20 @@ static int nested_check_vm_entry_controls(struct kvm_vcpu *vcpu,
 		    CC(intr_type == INTR_TYPE_OTHER_EVENT && vector != 0))
 			return -EINVAL;
 
-		/* VM-entry interruption-info field: deliver error code */
-		should_have_error_code =
-			intr_type == INTR_TYPE_HARD_EXCEPTION && prot_mode &&
-			x86_exception_has_error_code(vector);
-		if (CC(has_error_code != should_have_error_code))
-			return -EINVAL;
+		/*
+		 * Cannot deliver error code in real mode or if the interrupt
+		 * type is not hardware exception. For other cases, do the
+		 * consistency check only if the vCPU doesn't enumerate
+		 * VMX_BASIC_NO_HW_ERROR_CODE_CC.
+		 */
+		if (!prot_mode || intr_type != INTR_TYPE_HARD_EXCEPTION) {
+			if (CC(has_error_code))
+				return -EINVAL;
+		} else if (!nested_cpu_has_no_hw_errcode_cc(vcpu)) {
+			if (CC(has_error_code !=
+			       x86_exception_has_error_code(vector)))
+				return -EINVAL;
+		}
 
 		/* VM-entry exception error code */
 		if (CC(has_error_code &&
@@ -7205,6 +7213,8 @@ static void nested_vmx_setup_basic(struct nested_vmx_msrs *msrs)
 	msrs->basic |= VMX_BASIC_TRUE_CTLS;
 	if (cpu_has_vmx_basic_inout())
 		msrs->basic |= VMX_BASIC_INOUT;
+	if (cpu_has_vmx_basic_no_hw_errcode())
+		msrs->basic |= VMX_BASIC_NO_HW_ERROR_CODE_CC;
 }
 
 static void nested_vmx_setup_cr_fixed(struct nested_vmx_msrs *msrs)
diff --git a/arch/x86/kvm/vmx/nested.h b/arch/x86/kvm/vmx/nested.h
index 6eedcfc91070..983484d42ebf 100644
--- a/arch/x86/kvm/vmx/nested.h
+++ b/arch/x86/kvm/vmx/nested.h
@@ -309,6 +309,11 @@ static inline bool nested_cr4_valid(struct kvm_vcpu *vcpu, unsigned long val)
 	       __kvm_is_valid_cr4(vcpu, val);
 }
 
+static inline bool nested_cpu_has_no_hw_errcode_cc(struct kvm_vcpu *vcpu)
+{
+	return to_vmx(vcpu)->nested.msrs.basic & VMX_BASIC_NO_HW_ERROR_CODE_CC;
+}
+
 /* No difference in the restrictions on guest and host CR4 in VMX operation. */
 #define nested_guest_cr4_valid	nested_cr4_valid
 #define nested_host_cr4_valid	nested_cr4_valid
-- 
2.47.1

[PATCH v12 22/24] KVM: nVMX: Enable CET support for nested guest

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

Set up CET MSRs, related VM_ENTRY/EXIT control bits and fixed CR4 setting
to enable CET for nested VM.

vmcs12 and vmcs02 needs to be synced when L2 exits to L1 or when L1 wants
to resume L2, that way correct CET states can be observed by one another.

Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
v12: use a consistent order when referring to s_cet, ssp, and ssp_table
     throughout the patch this patch. -- Xin
---
 arch/x86/kvm/vmx/nested.c | 80 ++++++++++++++++++++++++++++++++++++++-
 arch/x86/kvm/vmx/vmcs12.c |  6 +++
 arch/x86/kvm/vmx/vmcs12.h | 14 ++++++-
 arch/x86/kvm/vmx/vmx.c    |  2 +
 arch/x86/kvm/vmx/vmx.h    |  3 ++
 5 files changed, 102 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 618cc6c6425c..f20f205c6560 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -721,6 +721,27 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
 	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
 					 MSR_IA32_MPERF, MSR_TYPE_R);
 
+	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
+					 MSR_IA32_U_CET, MSR_TYPE_RW);
+
+	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
+					 MSR_IA32_S_CET, MSR_TYPE_RW);
+
+	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
+					 MSR_IA32_PL0_SSP, MSR_TYPE_RW);
+
+	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
+					 MSR_IA32_PL1_SSP, MSR_TYPE_RW);
+
+	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
+					 MSR_IA32_PL2_SSP, MSR_TYPE_RW);
+
+	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
+					 MSR_IA32_PL3_SSP, MSR_TYPE_RW);
+
+	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
+					 MSR_IA32_INT_SSP_TAB, MSR_TYPE_RW);
+
 	kvm_vcpu_unmap(vcpu, &map);
 
 	vmx->nested.force_msr_bitmap_recalc = false;
@@ -2521,6 +2542,32 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct loaded_vmcs *vmcs0
 	}
 }
 
+static inline void cet_vmcs_fields_get(struct kvm_vcpu *vcpu, u64 *s_cet,
+				       u64 *ssp, u64 *ssp_tbl)
+{
+	if (guest_cpu_cap_has(vcpu, X86_FEATURE_IBT) ||
+	    guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK))
+		*s_cet = vmcs_readl(GUEST_S_CET);
+
+	if (guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK)) {
+		*ssp = vmcs_readl(GUEST_SSP);
+		*ssp_tbl = vmcs_readl(GUEST_INTR_SSP_TABLE);
+	}
+}
+
+static inline void cet_vmcs_fields_set(struct kvm_vcpu *vcpu, u64 s_cet,
+				       u64 ssp, u64 ssp_tbl)
+{
+	if (guest_cpu_cap_has(vcpu, X86_FEATURE_IBT) ||
+	    guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK))
+		vmcs_writel(GUEST_S_CET, s_cet);
+
+	if (guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK)) {
+		vmcs_writel(GUEST_SSP, ssp);
+		vmcs_writel(GUEST_INTR_SSP_TABLE, ssp_tbl);
+	}
+}
+
 static void prepare_vmcs02_rare(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
 {
 	struct hv_enlightened_vmcs *hv_evmcs = nested_vmx_evmcs(vmx);
@@ -2637,6 +2684,10 @@ static void prepare_vmcs02_rare(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
 	vmcs_write32(VM_EXIT_MSR_LOAD_COUNT, vmx->msr_autoload.host.nr);
 	vmcs_write32(VM_ENTRY_MSR_LOAD_COUNT, vmx->msr_autoload.guest.nr);
 
+	if (vmcs12->vm_entry_controls & VM_ENTRY_LOAD_CET_STATE)
+		cet_vmcs_fields_set(&vmx->vcpu, vmcs12->guest_s_cet,
+				    vmcs12->guest_ssp, vmcs12->guest_ssp_tbl);
+
 	set_cr4_guest_host_mask(vmx);
 }
 
@@ -2676,6 +2727,13 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
 		kvm_set_dr(vcpu, 7, vcpu->arch.dr7);
 		vmx_guest_debugctl_write(vcpu, vmx->nested.pre_vmenter_debugctl);
 	}
+
+	if (!vmx->nested.nested_run_pending ||
+	    !(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_CET_STATE))
+		cet_vmcs_fields_set(vcpu, vmx->nested.pre_vmenter_s_cet,
+				    vmx->nested.pre_vmenter_ssp,
+				    vmx->nested.pre_vmenter_ssp_tbl);
+
 	if (kvm_mpx_supported() && (!vmx->nested.nested_run_pending ||
 	    !(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_BNDCFGS)))
 		vmcs_write64(GUEST_BNDCFGS, vmx->nested.pre_vmenter_bndcfgs);
@@ -3552,6 +3610,12 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
 	     !(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_BNDCFGS)))
 		vmx->nested.pre_vmenter_bndcfgs = vmcs_read64(GUEST_BNDCFGS);
 
+	if (!vmx->nested.nested_run_pending ||
+	    !(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_CET_STATE))
+		cet_vmcs_fields_get(vcpu, &vmx->nested.pre_vmenter_s_cet,
+				    &vmx->nested.pre_vmenter_ssp,
+				    &vmx->nested.pre_vmenter_ssp_tbl);
+
 	/*
 	 * Overwrite vmcs01.GUEST_CR3 with L1's CR3 if EPT is disabled *and*
 	 * nested early checks are disabled.  In the event of a "late" VM-Fail,
@@ -4479,6 +4543,9 @@ static bool is_vmcs12_ext_field(unsigned long field)
 	case GUEST_IDTR_BASE:
 	case GUEST_PENDING_DBG_EXCEPTIONS:
 	case GUEST_BNDCFGS:
+	case GUEST_S_CET:
+	case GUEST_SSP:
+	case GUEST_INTR_SSP_TABLE:
 		return true;
 	default:
 		break;
@@ -4529,6 +4596,10 @@ static void sync_vmcs02_to_vmcs12_rare(struct kvm_vcpu *vcpu,
 	vmcs12->guest_pending_dbg_exceptions =
 		vmcs_readl(GUEST_PENDING_DBG_EXCEPTIONS);
 
+	cet_vmcs_fields_get(&vmx->vcpu, &vmcs12->guest_s_cet,
+			    &vmcs12->guest_ssp,
+			    &vmcs12->guest_ssp_tbl);
+
 	vmx->nested.need_sync_vmcs02_to_vmcs12_rare = false;
 }
 
@@ -4760,6 +4831,10 @@ static void load_vmcs12_host_state(struct kvm_vcpu *vcpu,
 	if (vmcs12->vm_exit_controls & VM_EXIT_CLEAR_BNDCFGS)
 		vmcs_write64(GUEST_BNDCFGS, 0);
 
+	if (vmcs12->vm_exit_controls & VM_EXIT_LOAD_CET_STATE)
+		cet_vmcs_fields_set(vcpu, vmcs12->host_s_cet, vmcs12->host_ssp,
+				    vmcs12->host_ssp_tbl);
+
 	if (vmcs12->vm_exit_controls & VM_EXIT_LOAD_IA32_PAT) {
 		vmcs_write64(GUEST_IA32_PAT, vmcs12->host_ia32_pat);
 		vcpu->arch.pat = vmcs12->host_ia32_pat;
@@ -7037,7 +7112,7 @@ static void nested_vmx_setup_exit_ctls(struct vmcs_config *vmcs_conf,
 		VM_EXIT_HOST_ADDR_SPACE_SIZE |
 #endif
 		VM_EXIT_LOAD_IA32_PAT | VM_EXIT_SAVE_IA32_PAT |
-		VM_EXIT_CLEAR_BNDCFGS;
+		VM_EXIT_CLEAR_BNDCFGS | VM_EXIT_LOAD_CET_STATE;
 	msrs->exit_ctls_high |=
 		VM_EXIT_ALWAYSON_WITHOUT_TRUE_MSR |
 		VM_EXIT_LOAD_IA32_EFER | VM_EXIT_SAVE_IA32_EFER |
@@ -7059,7 +7134,8 @@ static void nested_vmx_setup_entry_ctls(struct vmcs_config *vmcs_conf,
 #ifdef CONFIG_X86_64
 		VM_ENTRY_IA32E_MODE |
 #endif
-		VM_ENTRY_LOAD_IA32_PAT | VM_ENTRY_LOAD_BNDCFGS;
+		VM_ENTRY_LOAD_IA32_PAT | VM_ENTRY_LOAD_BNDCFGS |
+		VM_ENTRY_LOAD_CET_STATE;
 	msrs->entry_ctls_high |=
 		(VM_ENTRY_ALWAYSON_WITHOUT_TRUE_MSR | VM_ENTRY_LOAD_IA32_EFER |
 		 VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL);
diff --git a/arch/x86/kvm/vmx/vmcs12.c b/arch/x86/kvm/vmx/vmcs12.c
index 106a72c923ca..4233b5ca9461 100644
--- a/arch/x86/kvm/vmx/vmcs12.c
+++ b/arch/x86/kvm/vmx/vmcs12.c
@@ -139,6 +139,9 @@ const unsigned short vmcs12_field_offsets[] = {
 	FIELD(GUEST_PENDING_DBG_EXCEPTIONS, guest_pending_dbg_exceptions),
 	FIELD(GUEST_SYSENTER_ESP, guest_sysenter_esp),
 	FIELD(GUEST_SYSENTER_EIP, guest_sysenter_eip),
+	FIELD(GUEST_S_CET, guest_s_cet),
+	FIELD(GUEST_SSP, guest_ssp),
+	FIELD(GUEST_INTR_SSP_TABLE, guest_ssp_tbl),
 	FIELD(HOST_CR0, host_cr0),
 	FIELD(HOST_CR3, host_cr3),
 	FIELD(HOST_CR4, host_cr4),
@@ -151,5 +154,8 @@ const unsigned short vmcs12_field_offsets[] = {
 	FIELD(HOST_IA32_SYSENTER_EIP, host_ia32_sysenter_eip),
 	FIELD(HOST_RSP, host_rsp),
 	FIELD(HOST_RIP, host_rip),
+	FIELD(HOST_S_CET, host_s_cet),
+	FIELD(HOST_SSP, host_ssp),
+	FIELD(HOST_INTR_SSP_TABLE, host_ssp_tbl),
 };
 const unsigned int nr_vmcs12_fields = ARRAY_SIZE(vmcs12_field_offsets);
diff --git a/arch/x86/kvm/vmx/vmcs12.h b/arch/x86/kvm/vmx/vmcs12.h
index 56fd150a6f24..4ad6b16525b9 100644
--- a/arch/x86/kvm/vmx/vmcs12.h
+++ b/arch/x86/kvm/vmx/vmcs12.h
@@ -117,7 +117,13 @@ struct __packed vmcs12 {
 	natural_width host_ia32_sysenter_eip;
 	natural_width host_rsp;
 	natural_width host_rip;
-	natural_width paddingl[8]; /* room for future expansion */
+	natural_width host_s_cet;
+	natural_width host_ssp;
+	natural_width host_ssp_tbl;
+	natural_width guest_s_cet;
+	natural_width guest_ssp;
+	natural_width guest_ssp_tbl;
+	natural_width paddingl[2]; /* room for future expansion */
 	u32 pin_based_vm_exec_control;
 	u32 cpu_based_vm_exec_control;
 	u32 exception_bitmap;
@@ -294,6 +300,12 @@ static inline void vmx_check_vmcs12_offsets(void)
 	CHECK_OFFSET(host_ia32_sysenter_eip, 656);
 	CHECK_OFFSET(host_rsp, 664);
 	CHECK_OFFSET(host_rip, 672);
+	CHECK_OFFSET(host_s_cet, 680);
+	CHECK_OFFSET(host_ssp, 688);
+	CHECK_OFFSET(host_ssp_tbl, 696);
+	CHECK_OFFSET(guest_s_cet, 704);
+	CHECK_OFFSET(guest_ssp, 712);
+	CHECK_OFFSET(guest_ssp_tbl, 720);
 	CHECK_OFFSET(pin_based_vm_exec_control, 744);
 	CHECK_OFFSET(cpu_based_vm_exec_control, 748);
 	CHECK_OFFSET(exception_bitmap, 752);
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index ae8be898e1df..3ab0fe6e47c9 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7711,6 +7711,8 @@ static void nested_vmx_cr_fixed1_bits_update(struct kvm_vcpu *vcpu)
 	cr4_fixed1_update(X86_CR4_PKE,        ecx, feature_bit(PKU));
 	cr4_fixed1_update(X86_CR4_UMIP,       ecx, feature_bit(UMIP));
 	cr4_fixed1_update(X86_CR4_LA57,       ecx, feature_bit(LA57));
+	cr4_fixed1_update(X86_CR4_CET,	      ecx, feature_bit(SHSTK));
+	cr4_fixed1_update(X86_CR4_CET,	      edx, feature_bit(IBT));
 
 	entry = kvm_find_cpuid_entry_index(vcpu, 0x7, 1);
 	cr4_fixed1_update(X86_CR4_LAM_SUP,    eax, feature_bit(LAM));
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index 89200586b35a..3bf7748ddfc2 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -181,6 +181,9 @@ struct nested_vmx {
 	 */
 	u64 pre_vmenter_debugctl;
 	u64 pre_vmenter_bndcfgs;
+	u64 pre_vmenter_s_cet;
+	u64 pre_vmenter_ssp;
+	u64 pre_vmenter_ssp_tbl;
 
 	/* to migrate it to L1 if L2 writes to L1's CR8 directly */
 	int l1_tpr_threshold;
-- 
2.47.1

Re: [PATCH v12 22/24] KVM: nVMX: Enable CET support for nested guest

[Chao Gao]

>@@ -4479,6 +4543,9 @@ static bool is_vmcs12_ext_field(unsigned long field)
> 	case GUEST_IDTR_BASE:
> 	case GUEST_PENDING_DBG_EXCEPTIONS:
> 	case GUEST_BNDCFGS:
>+	case GUEST_S_CET:
>+	case GUEST_SSP:
>+	case GUEST_INTR_SSP_TABLE:
> 		return true;
> 	default:
> 		break;
>@@ -4529,6 +4596,10 @@ static void sync_vmcs02_to_vmcs12_rare(struct kvm_vcpu *vcpu,
> 	vmcs12->guest_pending_dbg_exceptions =
> 		vmcs_readl(GUEST_PENDING_DBG_EXCEPTIONS);
> 
>+	cet_vmcs_fields_get(&vmx->vcpu, &vmcs12->guest_s_cet,
>+			    &vmcs12->guest_ssp,
>+			    &vmcs12->guest_ssp_tbl);
>+
> 	vmx->nested.need_sync_vmcs02_to_vmcs12_rare = false;
> }
> 
>@@ -4760,6 +4831,10 @@ static void load_vmcs12_host_state(struct kvm_vcpu *vcpu,
> 	if (vmcs12->vm_exit_controls & VM_EXIT_CLEAR_BNDCFGS)
> 		vmcs_write64(GUEST_BNDCFGS, 0);
> 
>+	if (vmcs12->vm_exit_controls & VM_EXIT_LOAD_CET_STATE)
>+		cet_vmcs_fields_set(vcpu, vmcs12->host_s_cet, vmcs12->host_ssp,
>+				    vmcs12->host_ssp_tbl);
>+

Xin wrote a new test [*] and found a bug here. If VM_EXIT_LOAD_CET_STATE is not
set, the guest values should be retained after the nested vm-exit.

so here should be

	if (vmcs12->vm_exit_controls & VM_EXIT_LOAD_CET_STATE)
		cet_vmcs_fields_set(vcpu, vmcs12->host_s_cet, vmcs12->host_ssp,
				    vmcs12->host_ssp_tbl);
	else
		cet_vmcs_fields_set(vcpu, vmcs12->guest_s_cet, vmcs12->guest_ssp,
				    vmcs12->guest_ssp_tbl);

This creates a dependency that vmcs12->guest_s_cet/ssp/ssp_tbl must be
up-to-date here. So, vmcs12->guest_s_cet/ssp/ssp_tbl should be synced from
vmcs02 on each nested VM-exit rather than lazily in sync_vmcs02_to_vmcs12_rare().
Specifically, the cet_vmcs_fields_get() in sync_vmcs02_to_vmcs12_rare() should
be moved to sync_vmcs02_to_vmcs12(), and is_vmcs12_ext_field() should return
false for CET VMCS fields.

Note that Xin's test differs from the test I wrote [**] in L2 guest behavior.
His test writes to the S_CET MSR, while my test reads the S_CET MSR, which
is why this bug escaped from my CET test.

[*]: https://github.com/xinli-intel/kvm-unit-tests/commit/f1df81c3189a3328adb47c7dd6cd985830fe738f
[**]: https://lore.kernel.org/kvm/20250626073459.12990-9-minipli@grsecurity.net/

below diff can fix this issue:

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 7c88fedc27c7..eadd659ae22f 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -4596,9 +4596,6 @@ static bool is_vmcs12_ext_field(unsigned long field)
	case GUEST_IDTR_BASE:
	case GUEST_PENDING_DBG_EXCEPTIONS:
	case GUEST_BNDCFGS:
-	case GUEST_S_CET:
-	case GUEST_SSP:
-	case GUEST_INTR_SSP_TABLE:
		return true;
	default:
		break;
@@ -4649,10 +4646,6 @@ static void sync_vmcs02_to_vmcs12_rare(struct kvm_vcpu *vcpu,
	vmcs12->guest_pending_dbg_exceptions =
		vmcs_readl(GUEST_PENDING_DBG_EXCEPTIONS);
 
-	cet_vmcs_fields_get(&vmx->vcpu, &vmcs12->guest_s_cet,
-			    &vmcs12->guest_ssp,
-			    &vmcs12->guest_ssp_tbl);
-
	vmx->nested.need_sync_vmcs02_to_vmcs12_rare = false;
 }
 
@@ -4759,6 +4752,10 @@ static void sync_vmcs02_to_vmcs12(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
 
	if (vmcs12->vm_exit_controls & VM_EXIT_SAVE_IA32_EFER)
		vmcs12->guest_ia32_efer = vcpu->arch.efer;
+
+	cet_vmcs_fields_get(&vmx->vcpu, &vmcs12->guest_s_cet,
+			    &vmcs12->guest_ssp,
+			    &vmcs12->guest_ssp_tbl);
 }
 
 /*
@@ -4884,9 +4881,17 @@ static void load_vmcs12_host_state(struct kvm_vcpu *vcpu,
	if (vmcs12->vm_exit_controls & VM_EXIT_CLEAR_BNDCFGS)
		vmcs_write64(GUEST_BNDCFGS, 0);
 
+	/*
+	 * Load CET state from host state if VM_EXIT_LOAD_CET_STATE is set.
+	 * otherwise CET state should be retained across VM-exit, i.e.,
+	 * guest values should be propagated from vmcs12 to vmcs01.
+	 */
	if (vmcs12->vm_exit_controls & VM_EXIT_LOAD_CET_STATE)
		cet_vmcs_fields_set(vcpu, vmcs12->host_s_cet, vmcs12->host_ssp,
				    vmcs12->host_ssp_tbl);
+	else
+		cet_vmcs_fields_set(vcpu, vmcs12->guest_s_cet, vmcs12->guest_ssp,
+				    vmcs12->guest_ssp_tbl);
 
	if (vmcs12->vm_exit_controls & VM_EXIT_LOAD_IA32_PAT) {
		vmcs_write64(GUEST_IA32_PAT, vmcs12->host_ia32_pat);

Re: [PATCH v12 22/24] KVM: nVMX: Enable CET support for nested guest

[Sean Christopherson]

On Mon, Aug 11, 2025, Chao Gao wrote:
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 618cc6c6425c..f20f205c6560 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -721,6 +721,27 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
>  	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
>  					 MSR_IA32_MPERF, MSR_TYPE_R);
>  
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
> +					 MSR_IA32_U_CET, MSR_TYPE_RW);
> +
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
> +					 MSR_IA32_S_CET, MSR_TYPE_RW);
> +
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
> +					 MSR_IA32_PL0_SSP, MSR_TYPE_RW);
> +
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
> +					 MSR_IA32_PL1_SSP, MSR_TYPE_RW);
> +
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
> +					 MSR_IA32_PL2_SSP, MSR_TYPE_RW);
> +
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
> +					 MSR_IA32_PL3_SSP, MSR_TYPE_RW);
> +
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
> +					 MSR_IA32_INT_SSP_TAB, MSR_TYPE_RW);

As with the "regular" support, I don't think MSR_IA32_INT_SSP_TAB needs to be
passed through (though it's entirely possible I'm missing/forgetting something).

> +
>  	kvm_vcpu_unmap(vcpu, &map);
>  
>  	vmx->nested.force_msr_bitmap_recalc = false;
> @@ -2521,6 +2542,32 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct loaded_vmcs *vmcs0
>  	}
>  }
>  
> +static inline

Do not use "inline" for functions that are visible only to the local compilation
unit.  "inline" is just a hint, and modern compilers are smart enough to inline
functions when appropriate without a hint.

A longer explanation/rant here: https://lore.kernel.org/all/ZAdfX+S323JVWNZC@google.com


> void cet_vmcs_fields_get(struct kvm_vcpu *vcpu, u64 *s_cet,

I don't love the names, as it's not super clear what they are doing.  How about:

  vmcs_{read,write}_cet_state()

I like that vmcs_{read,write} matches the vmcs_{read,write}{l,16,32,64} helpers
(makes it clear they are reading/writing VMCS fields), and the cet_state connects
the dots to VM_EXIT_LOAD_CET_STATE (helps clarify which CET fields are accessed).

[PATCH v12 23/24] KVM: nVMX: Add consistency checks for CR0.WP and CR4.CET

[Chao Gao]

Add consistency checks for CR4.CET and CR0.WP in guest-state or host-state
area in the VMCS12. This ensures that configurations with CR4.CET set and
CR0.WP not set result in VM-entry failure, aligning with architectural
behavior.

Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/kvm/vmx/nested.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index f20f205c6560..47e413e56764 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -3114,6 +3114,9 @@ static int nested_vmx_check_host_state(struct kvm_vcpu *vcpu,
 	    CC(!kvm_vcpu_is_legal_cr3(vcpu, vmcs12->host_cr3)))
 		return -EINVAL;
 
+	if (CC(vmcs12->host_cr4 & X86_CR4_CET && !(vmcs12->host_cr0 & X86_CR0_WP)))
+		return -EINVAL;
+
 	if (CC(is_noncanonical_msr_address(vmcs12->host_ia32_sysenter_esp, vcpu)) ||
 	    CC(is_noncanonical_msr_address(vmcs12->host_ia32_sysenter_eip, vcpu)))
 		return -EINVAL;
@@ -3228,6 +3231,9 @@ static int nested_vmx_check_guest_state(struct kvm_vcpu *vcpu,
 	    CC(!nested_guest_cr4_valid(vcpu, vmcs12->guest_cr4)))
 		return -EINVAL;
 
+	if (CC(vmcs12->guest_cr4 & X86_CR4_CET && !(vmcs12->guest_cr0 & X86_CR0_WP)))
+		return -EINVAL;
+
 	if ((vmcs12->vm_entry_controls & VM_ENTRY_LOAD_DEBUG_CONTROLS) &&
 	    (CC(!kvm_dr7_valid(vmcs12->guest_dr7)) ||
 	     CC(!vmx_is_valid_debugctl(vcpu, vmcs12->guest_ia32_debugctl, false))))
-- 
2.47.1

Re: [PATCH v12 23/24] KVM: nVMX: Add consistency checks for CR0.WP and CR4.CET

[Sean Christopherson]

On Mon, Aug 11, 2025, Chao Gao wrote:
> Add consistency checks for CR4.CET and CR0.WP in guest-state or host-state
> area in the VMCS12. This ensures that configurations with CR4.CET set and
> CR0.WP not set result in VM-entry failure, aligning with architectural
> behavior.

The consistency check patches need to land before KVM lets L1 enable CET for L2.

Re: [PATCH v12 23/24] KVM: nVMX: Add consistency checks for CR0.WP and CR4.CET

[Chao Gao]

On Tue, Aug 19, 2025 at 10:27:44AM -0700, Sean Christopherson wrote:
>On Mon, Aug 11, 2025, Chao Gao wrote:
>> Add consistency checks for CR4.CET and CR0.WP in guest-state or host-state
>> area in the VMCS12. This ensures that configurations with CR4.CET set and
>> CR0.WP not set result in VM-entry failure, aligning with architectural
>> behavior.
>
>The consistency check patches need to land before KVM lets L1 enable CET for L2.

ok. I will separate the hunk below from the previous patch and place it at the
end of this series.

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 51d69f368689..2c8ee37debb2 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -7123,7 +7123,7 @@ static void nested_vmx_setup_exit_ctls(struct vmcs_config *vmcs_conf,
		VM_EXIT_HOST_ADDR_SPACE_SIZE |
 #endif
		VM_EXIT_LOAD_IA32_PAT | VM_EXIT_SAVE_IA32_PAT |
-		VM_EXIT_CLEAR_BNDCFGS;
+		VM_EXIT_CLEAR_BNDCFGS | VM_EXIT_LOAD_CET_STATE;
	msrs->exit_ctls_high |=
		VM_EXIT_ALWAYSON_WITHOUT_TRUE_MSR |
		VM_EXIT_LOAD_IA32_EFER | VM_EXIT_SAVE_IA32_EFER |
@@ -7145,7 +7145,8 @@ static void nested_vmx_setup_entry_ctls(struct vmcs_config *vmcs_conf,
 #ifdef CONFIG_X86_64
		VM_ENTRY_IA32E_MODE |
 #endif
-		VM_ENTRY_LOAD_IA32_PAT | VM_ENTRY_LOAD_BNDCFGS;
+		VM_ENTRY_LOAD_IA32_PAT | VM_ENTRY_LOAD_BNDCFGS |
+		VM_ENTRY_LOAD_CET_STATE;
	msrs->entry_ctls_high |=
		(VM_ENTRY_ALWAYSON_WITHOUT_TRUE_MSR | VM_ENTRY_LOAD_IA32_EFER |
		 VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL);

[PATCH v12 24/24] KVM: nVMX: Add consistency checks for CET states

[Chao Gao]

Introduce consistency checks for CET states during nested VM-entry.

A VMCS contains both guest and host CET states, each comprising the
IA32_S_CET MSR, SSP, and IA32_INTERRUPT_SSP_TABLE_ADDR MSR. Various
checks are applied to CET states during VM-entry as documented in SDM
Vol3 Chapter "VM ENTRIES". Implement all these checks during nested
VM-entry to emulate the architectural behavior.

In summary, there are three kinds of checks on guest/host CET states
during VM-entry:

A. Checks applied to both guest states and host states:

 * The IA32_S_CET field must not set any reserved bits; bits 10 (SUPPRESS)
   and 11 (TRACKER) cannot both be set.
 * SSP should not have bits 1:0 set.
 * The IA32_INTERRUPT_SSP_TABLE_ADDR field must be canonical.

B. Checks applied to host states only

 * IA32_S_CET MSR and SSP must be canonical if the CPU enters 64-bit mode
   after VM-exit. Otherwise, IA32_S_CET and SSP must have their higher 32
   bits cleared.

C. Checks applied to guest states only:

 * IA32_S_CET MSR and SSP are not required to be canonical (i.e., 63:N-1
   are identical, where N is the CPU's maximum linear-address width). But,
   bits 63:N of SSP must be identical.

Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/kvm/vmx/nested.c | 47 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 47e413e56764..7c88fedc27c7 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -3104,6 +3104,17 @@ static bool is_l1_noncanonical_address_on_vmexit(u64 la, struct vmcs12 *vmcs12)
 	return !__is_canonical_address(la, l1_address_bits_on_exit);
 }
 
+static bool is_valid_cet_state(struct kvm_vcpu *vcpu, u64 s_cet, u64 ssp, u64 ssp_tbl)
+{
+	if (!is_cet_msr_valid(vcpu, s_cet) || !IS_ALIGNED(ssp, 4))
+		return false;
+
+	if (is_noncanonical_msr_address(ssp_tbl, vcpu))
+		return false;
+
+	return true;
+}
+
 static int nested_vmx_check_host_state(struct kvm_vcpu *vcpu,
 				       struct vmcs12 *vmcs12)
 {
@@ -3173,6 +3184,26 @@ static int nested_vmx_check_host_state(struct kvm_vcpu *vcpu,
 			return -EINVAL;
 	}
 
+	if (vmcs12->vm_exit_controls & VM_EXIT_LOAD_CET_STATE) {
+		if (CC(!is_valid_cet_state(vcpu, vmcs12->host_s_cet, vmcs12->host_ssp,
+					   vmcs12->host_ssp_tbl)))
+			return -EINVAL;
+
+		/*
+		 * IA32_S_CET and SSP must be canonical if the host will
+		 * enter 64-bit mode after VM-exit; otherwise, higher
+		 * 32-bits must be all 0s.
+		 */
+		if (ia32e) {
+			if (CC(is_noncanonical_msr_address(vmcs12->host_s_cet, vcpu)) ||
+			    CC(is_noncanonical_msr_address(vmcs12->host_ssp, vcpu)))
+				return -EINVAL;
+		} else {
+			if (CC(vmcs12->host_s_cet >> 32) || CC(vmcs12->host_ssp >> 32))
+				return -EINVAL;
+		}
+	}
+
 	return 0;
 }
 
@@ -3283,6 +3314,22 @@ static int nested_vmx_check_guest_state(struct kvm_vcpu *vcpu,
 	     CC((vmcs12->guest_bndcfgs & MSR_IA32_BNDCFGS_RSVD))))
 		return -EINVAL;
 
+	if (vmcs12->vm_entry_controls & VM_ENTRY_LOAD_CET_STATE) {
+		if (CC(!is_valid_cet_state(vcpu, vmcs12->guest_s_cet, vmcs12->guest_ssp,
+					   vmcs12->guest_ssp_tbl)))
+			return -EINVAL;
+
+		/*
+		 * Guest SSP must have 63:N bits identical, rather than
+		 * be canonical (i.e., 63:N-1 bits identical), where N is
+		 * the CPU's maximum linear-address width. Similar to
+		 * is_noncanonical_msr_address(), use the host's
+		 * linear-address width.
+		 */
+		if (CC(!__is_canonical_address(vmcs12->guest_ssp, max_host_virt_addr_bits() + 1)))
+			return -EINVAL;
+	}
+
 	if (nested_check_guest_non_reg_state(vmcs12))
 		return -EINVAL;
 
-- 
2.47.1

Re: [PATCH v12 00/24] Enable CET Virtualization

[Edgecombe, Rick P]

On Mon, 2025-08-11 at 19:55 -0700, Chao Gao wrote:
> Tests
> ======
> This series has successfully passed the basic CET user shadow stack test
> and kernel IBT test in both L1 and L2 guests. The newly added
> KVM-unit-tests [2] also passed, and its v11 has been tested with the AMD
> CET series by John [3].

I guess there is a bug, but I gave user shadow stack a whirl on L1/L2 and didn't
hit it.

Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

Re: [PATCH v12 00/24] Enable CET Virtualization

[Chao Gao]

On Sat, Aug 16, 2025 at 06:57:00AM +0800, Edgecombe, Rick P wrote:
>On Mon, 2025-08-11 at 19:55 -0700, Chao Gao wrote:
>> Tests
>> ======
>> This series has successfully passed the basic CET user shadow stack test
>> and kernel IBT test in both L1 and L2 guests. The newly added
>> KVM-unit-tests [2] also passed, and its v11 has been tested with the AMD
>> CET series by John [3].
>
>I guess there is a bug, but I gave user shadow stack a whirl on L1/L2 and didn't
>hit it.
>
>Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

Thank you for your testing, Rick.

Re: [PATCH v12 00/24] Enable CET Virtualization

[Xin Li]

On 8/11/2025 7:55 PM, Chao Gao wrote:
> The FPU support for CET virtualization has already been merged into 6.17-rc1.
> Building on that, this series introduces Intel CET virtualization support for
> KVM.
> 
> Changes in v12:
> 1. collect Tested-by tags from John and Mathias.
> 2. use less verbose names for KVM rdmsr/wrmsr emulation APIs in patch 1/2
>     (Sean/Xin)
> 3. refer to s_cet, ssp, and ssp_table in a consistent order in patch 22
>     (Xin)
> 
> Please note that I didn't include Mathias' patch, which makes CR4.CET
> guest-owned. I expect that patch to be posted separately.
> 
> ---
> Control-flow Enforcement Technology (CET) is a kind of CPU feature used
> to prevent Return/CALL/Jump-Oriented Programming (ROP/COP/JOP) attacks.
> It provides two sub-features(SHSTK,IBT) to defend against ROP/COP/JOP
> style control-flow subversion attacks.
> 
> Shadow Stack (SHSTK):
>    A shadow stack is a second stack used exclusively for control transfer
>    operations. The shadow stack is separate from the data/normal stack and
>    can be enabled individually in user and kernel mode. When shadow stack
>    is enabled, CALL pushes the return address on both the data and shadow
>    stack. RET pops the return address from both stacks and compares them.
>    If the return addresses from the two stacks do not match, the processor
>    generates a #CP.
> 
> Indirect Branch Tracking (IBT):
>    IBT introduces new instruction(ENDBRANCH)to mark valid target addresses
>    of indirect branches (CALL, JMP etc...). If an indirect branch is
>    executed and the next instruction is _not_ an ENDBRANCH, the processor
>    generates a #CP. These instruction behaves as a NOP on platforms that
>    doesn't support CET.
> 
> CET states management
> =====================
> KVM cooperates with host kernel FPU framework to manage guest CET registers.
> With CET supervisor mode state support in this series, KVM can save/restore
> full guest CET xsave-managed states.
> 
> CET user mode and supervisor mode xstates, i.e., MSR_IA32_{U_CET,PL3_SSP}
> and MSR_IA32_PL{0,1,2}, depend on host FPU framework to swap guest and host
> xstates. On VM-Exit, guest CET xstates are saved to guest fpu area and host
> CET xstates are loaded from task/thread context before vCPU returns to
> userspace, vice-versa on VM-Entry. See details in kvm_{load,put}_guest_fpu().
> 
> CET supervisor mode states are grouped into two categories : XSAVE-managed
> and non-XSAVE-managed, the former includes MSR_IA32_PL{0,1,2}_SSP and are
> controlled by CET supervisor mode bit(S_CET bit) in XSS, the later consists
> of MSR_IA32_S_CET and MSR_IA32_INTR_SSP_TBL.
> 
> VMX introduces new VMCS fields, {GUEST|HOST}_{S_CET,SSP,INTR_SSP_TABL}, to
> facilitate guest/host non-XSAVES-managed states. When VMX CET entry/exit load
> bits are set, guest/host MSR_IA32_{S_CET,INTR_SSP_TBL,SSP} are loaded from
> equivalent fields at VM-Exit/Entry. With these new fields, such supervisor
> states require no addtional KVM save/reload actions.
> 
> Tests
> ======
> This series has successfully passed the basic CET user shadow stack test
> and kernel IBT test in both L1 and L2 guests. The newly added
> KVM-unit-tests [2] also passed, and its v11 has been tested with the AMD
> CET series by John [3].
> 
> For your convenience, you can use my WIP QEMU [1] for testing.
> 
> [1]: https://github.com/gaochaointel/qemu-dev qemu-cet
> [2]: https://lore.kernel.org/kvm/20250626073459.12990-1-minipli@grsecurity.net/
> [3]: https://lore.kernel.org/kvm/aH6CH+x5mCDrvtoz@AUSJOHALLEN.amd.com/
> 

I rebased the KVM FRED patch set on top of this KVM CET patch set, and
will send out soon.

Re: [PATCH v12 00/24] Enable CET Virtualization

[Sean Christopherson]

On Mon, 11 Aug 2025 19:55:08 -0700, Chao Gao wrote:
> The FPU support for CET virtualization has already been merged into 6.17-rc1.
> Building on that, this series introduces Intel CET virtualization support for
> KVM.
> 
> Changes in v12:
> 1. collect Tested-by tags from John and Mathias.
> 2. use less verbose names for KVM rdmsr/wrmsr emulation APIs in patch 1/2
>    (Sean/Xin)
> 3. refer to s_cet, ssp, and ssp_table in a consistent order in patch 22
>    (Xin)
> 
> [...]

Applied patches 1-5 to kvm-x86 misc.  I still plan/hope to land CET support
this cycle, but I wanted to land the MSR refactorings in particular in case
other in-flight code is adding users.

[01/24] KVM: x86: Rename kvm_{g,s}et_msr()* to show that they emulate guest accesses
        https://github.com/kvm-x86/linux/commit/d2dcf25a4cf2
[02/24] KVM: x86: Use double-underscore read/write MSR helpers as appropriate
        https://github.com/kvm-x86/linux/commit/db07f3d0eb19
[03/24] KVM: x86: Add kvm_msr_{read,write}() helpers
        https://github.com/kvm-x86/linux/commit/c2aa58b226ab
[04/24] KVM: x86: Manually clear MPX state only on INIT
        https://github.com/kvm-x86/linux/commit/41f6710f99f4
[05/24] KVM: x86: Zero XSTATE components on INIT by iterating over supported features
        https://github.com/kvm-x86/linux/commit/c26675447faf

--
https://github.com/kvm-x86/linux/tree/next