Re: [PATCH v5 11/19] KVM:VMX: Emulate read and write to CET MSRs

Sean Christopherson
2023-08-04T21:28:33Z

On Fri, Aug 04, 2023, Chao Gao wrote:
> On Thu, Aug 03, 2023 at 12:27:24AM -0400, Yang Weijiang wrote:
> >Add emulation interface for CET MSR read and write.
> >The emulation code is split into common part and vendor specific
> >part, the former resides in x86.c to benefic different x86 CPU
> >vendors, the latter for VMX is implemented in this patch.
> >
> >Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> >---
> > arch/x86/kvm/vmx/vmx.c | 27 +++++++++++
> > arch/x86/kvm/x86.c | 104 +++++++++++++++++++++++++++++++++++++----
> > arch/x86/kvm/x86.h | 18 +++++++
> > 3 files changed, 141 insertions(+), 8 deletions(-)
> >
> >diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> >index 6aa76124e81e..ccf750e79608 100644
> >--- a/arch/x86/kvm/vmx/vmx.c
> >+++ b/arch/x86/kvm/vmx/vmx.c
> >@@ -2095,6 +2095,18 @@ static 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:
> >+ case MSR_KVM_GUEST_SSP:
> >+ case MSR_IA32_INT_SSP_TAB:
> >+ if (kvm_get_msr_common(vcpu, msr_info))
> >+ return 1;
> >+ if (msr_info->index == MSR_KVM_GUEST_SSP)
> >+ msr_info->data = vmcs_readl(GUEST_SSP);
> >+ else if (msr_info->index == MSR_IA32_S_CET)
> >+ msr_info->data = vmcs_readl(GUEST_S_CET);
> >+ else if (msr_info->index == MSR_IA32_INT_SSP_TAB)
> >+ msr_info->data = vmcs_readl(GUEST_INTR_SSP_TABLE);
>
> This if-else-if suggests that they are focibly grouped together to just
> share the call of kvm_get_msr_common(). For readability, I think it is better
> to handle them separately.
>
> e.g.,
> case MSR_IA32_S_CET:
> if (kvm_get_msr_common(vcpu, msr_info))
> return 1;
> msr_info->data = vmcs_readl(GUEST_S_CET);
> break;
>
> case MSR_KVM_GUEST_SSP:
> if (kvm_get_msr_common(vcpu, msr_info))
> return 1;
> msr_info->data = vmcs_readl(GUEST_SSP);
> break;

Actually, we can do even better. We have an existing framework for these types
of prechecks, I just completely forgot about it :-( (my "look at PAT" was a bad
suggestion).

Handle the checks in __kvm_set_msr() and __kvm_get_msr(), i.e. *before* calling
into vendor code. Then vendor code doesn't need to make weird callbacks.

> > int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > {
> > u32 msr = msr_info->index;
> >@@ -3981,6 +4014,45 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > vcpu->arch.guest_fpu.xfd_err = data;
> > break;
> > #endif
> >+#define CET_EXCLUSIVE_BITS (CET_SUPPRESS | CET_WAIT_ENDBR)
> >+#define CET_CTRL_RESERVED_BITS GENMASK(9, 6)

Please use a single namespace for these #defines, e.g. CET_CTRL_* or maybe
CET_US_* for everything.

> >+#define CET_SHSTK_MASK_BITS GENMASK(1, 0)
> >+#define CET_IBT_MASK_BITS (GENMASK_ULL(5, 2) | \
> >+ GENMASK_ULL(63, 10))
> >+#define CET_LEG_BITMAP_BASE(data) ((data) >> 12)

Bah, stupid SDM. Please spell out "LEGACY", I though "LEG" was short for "LEGAL"
since this looks a lot like a page shift, i.e. getting a pfn.

> >+static bool kvm_cet_is_msr_accessible(struct kvm_vcpu *vcpu,
> >+ struct msr_data *msr)
> >+{
> >+ if (is_shadow_stack_msr(msr->index)) {
> >+ if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK))
> >+ return false;
> >+
> >+ if (msr->index == MSR_KVM_GUEST_SSP)
> >+ return msr->host_initiated;
> >+
> >+ return msr->host_initiated ||
> >+ guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
> >+ }
> >+
> >+ if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK) &&
> >+ !kvm_cpu_cap_has(X86_FEATURE_IBT))
> >+ return false;
> >+
> >+ return msr->host_initiated ||
> >+ guest_cpuid_has(vcpu, X86_FEATURE_IBT) ||
> >+ guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);

Similar to my suggestsion for XSS, I think we drop the waiver for host_initiated
accesses, i.e. require the feature to be enabled and exposed to the guest, even
for the host.

> >diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> >index c69fc027f5ec..3b79d6db2f83 100644
> >--- a/arch/x86/kvm/x86.h
> >+++ b/arch/x86/kvm/x86.h
> >@@ -552,4 +552,22 @@ int kvm_sev_es_string_io(struct kvm_vcpu *vcpu, unsigned int size,
> > unsigned int port, void *data, unsigned int count,
> > int in);
> >
> >+/*
> >+ * Guest xstate MSRs have been loaded in __msr_io(), disable preemption before
> >+ * access the MSRs to avoid MSR content corruption.
> >+ */
>
> I think it is better to describe what the function does prior to jumping into
> details like where guest FPU is loaded.
>
> /*
> * 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_xsave_msr(struct msr_data *msr_info)
> >+{
> >+ kvm_fpu_get();
> >+ rdmsrl(msr_info->index, msr_info->data);
> >+ kvm_fpu_put();
> >+}
> >+
> >+static inline void kvm_set_xsave_msr(struct msr_data *msr_info)
> >+{
> >+ kvm_fpu_get();
> >+ wrmsrl(msr_info->index, msr_info->data);
> >+ kvm_fpu_put();
> >+}
>
> Can you rename functions to kvm_get/set_xstate_msr() to align with the comment
> and patch 6? And if there is no user outside x86.c, you can just put these two
> functions right after the is_xstate_msr() added in patch 6.

+1. These should also assert that (a) guest FPU state is loaded and (b) the MSR
is passed through to the guest. I might be ok dropping (b) if both VMX and SVM
passthrough all MSRs if they're exposed to the guest, i.e. not lazily passed
through.

Sans any changes to kvm_{g,s}et_xsave_msr(), I think this? (completely untested)

---
arch/x86/kvm/vmx/vmx.c | 34 +++-------
arch/x86/kvm/x86.c | 151 +++++++++++++++--------------------------
2 files changed, 64 insertions(+), 121 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 491039aeb61b..1211eb469d06 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2100,16 +2100,13 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
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_GUEST_SSP:
+ msr_info->data = vmcs_readl(GUEST_SSP);
+ break;
case MSR_IA32_INT_SSP_TAB:
- if (kvm_get_msr_common(vcpu, msr_info))
- return 1;
- if (msr_info->index == MSR_KVM_GUEST_SSP)
- msr_info->data = vmcs_readl(GUEST_SSP);
- else if (msr_info->index == MSR_IA32_S_CET)
- msr_info->data = vmcs_readl(GUEST_S_CET);
- else if (msr_info->index == MSR_IA32_INT_SSP_TAB)
- msr_info->data = vmcs_readl(GUEST_INTR_SSP_TABLE);
+ msr_info->data = vmcs_readl(GUEST_INTR_SSP_TABLE);
break;
case MSR_IA32_DEBUGCTLMSR:
msr_info->data = vmcs_read64(GUEST_IA32_DEBUGCTL);
@@ -2432,25 +2429,14 @@ static 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_PL0_SSP ... MSR_IA32_PL2_SSP:
- if (kvm_set_msr_common(vcpu, msr_info))
- return 1;
- if (data) {
- vmx_disable_write_intercept_sss_msr(vcpu);
- wrmsrl(msr_index, data);
- }
- break;
case MSR_IA32_S_CET:
+ vmcs_writel(GUEST_S_CET, data);
+ break;
case MSR_KVM_GUEST_SSP:
+ vmcs_writel(GUEST_SSP, data);
+ break;
case MSR_IA32_INT_SSP_TAB:
- if (kvm_set_msr_common(vcpu, msr_info))
- return 1;
- if (msr_index == MSR_KVM_GUEST_SSP)
- vmcs_writel(GUEST_SSP, data);
- else if (msr_index == MSR_IA32_S_CET)
- vmcs_writel(GUEST_S_CET, data);
- else if (msr_index == MSR_IA32_INT_SSP_TAB)
- vmcs_writel(GUEST_INTR_SSP_TABLE, data);
+ vmcs_writel(GUEST_INTR_SSP_TABLE, data);
break;
case MSR_IA32_PERF_CAPABILITIES:
if (data && !vcpu_to_pmu(vcpu)->version)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7385fc25a987..75e6de7c9268 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1838,6 +1838,11 @@ bool kvm_msr_allowed(struct kvm_vcpu *vcpu, u32 index, u32 type)
}
EXPORT_SYMBOL_GPL(kvm_msr_allowed);

+#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)
+
/*
* Write @data into the MSR specified by @index. Select MSR specific fault
* checks are bypassed if @host_initiated is %true.
@@ -1897,6 +1902,35 @@ 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_can_use(vcpu, X86_FEATURE_SHSTK) &&
+ !guest_can_use(vcpu, X86_FEATURE_IBT))
+ return 1;
+ if (data & CET_US_RESERVED_BITS)
+ return 1;
+ if (!guest_can_use(vcpu, X86_FEATURE_SHSTK) &&
+ (data & CET_US_SHSTK_MASK_BITS))
+ return 1;
+ if (!guest_can_use(vcpu, X86_FEATURE_IBT) &&
+ (data & CET_US_IBT_MASK_BITS))
+ return 1;
+ if (!IS_ALIGNED(CET_US_LEGACY_BITMAP_BASE(data), 4))
+ return 1;
+
+ /* IBT can be suppressed iff the TRACKER isn't WAIT_ENDR. */
+ if ((data & CET_SUPPRESS) && (data & CET_WAIT_ENDBR))
+ return 1;
+ break;
+ case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
+ case MSR_KVM_GUEST_SSP:
+ if (!guest_can_use(vcpu, X86_FEATURE_SHSTK))
+ return 1;
+ if (is_noncanonical_address(data, vcpu))
+ return 1;
+ if (!IS_ALIGNED(data, 4))
+ return 1;
+ break;
}

msr.data = data;
@@ -1940,6 +1974,17 @@ static int __kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data,
!guest_cpuid_has(vcpu, X86_FEATURE_RDPID))
return 1;
break;
+ case MSR_IA32_U_CET:
+ case MSR_IA32_S_CET:
+ if (!guest_can_use(vcpu, X86_FEATURE_IBT) &&
+ !guest_can_use(vcpu, X86_FEATURE_SHSTK))
+ return 1;
+ break;
+ case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
+ case MSR_KVM_GUEST_SSP:
+ if (!guest_can_use(vcpu, X86_FEATURE_SHSTK))
+ return 1;
+ break;
}

msr.index = index;
@@ -3640,47 +3685,6 @@ static bool kvm_is_msr_to_save(u32 msr_index)
return false;
}

-static inline bool is_shadow_stack_msr(u32 msr)
-{
- return msr == MSR_IA32_PL0_SSP ||
- msr == MSR_IA32_PL1_SSP ||
- msr == MSR_IA32_PL2_SSP ||
- msr == MSR_IA32_PL3_SSP ||
- msr == MSR_IA32_INT_SSP_TAB ||
- msr == MSR_KVM_GUEST_SSP;
-}
-
-static bool kvm_cet_is_msr_accessible(struct kvm_vcpu *vcpu,
- struct msr_data *msr)
-{
- if (is_shadow_stack_msr(msr->index)) {
- if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK))
- return false;
-
- /*
- * This MSR is synthesized mainly for userspace access during
- * Live Migration, it also can be accessed in SMM mode by VMM.
- * Guest is not allowed to access this MSR.
- */
- if (msr->index == MSR_KVM_GUEST_SSP) {
- if (IS_ENABLED(CONFIG_X86_64) && is_smm(vcpu))
- return true;
-
- return msr->host_initiated;
- }
-
- return msr->host_initiated ||
- guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
- }
-
- if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK) &&
- !kvm_cpu_cap_has(X86_FEATURE_IBT))
- return false;
-
- return msr->host_initiated ||
- guest_cpuid_has(vcpu, X86_FEATURE_IBT) ||
- guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
-}

int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
{
@@ -4036,46 +4040,9 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
vcpu->arch.guest_fpu.xfd_err = data;
break;
#endif
-#define CET_EXCLUSIVE_BITS (CET_SUPPRESS | CET_WAIT_ENDBR)
-#define CET_CTRL_RESERVED_BITS GENMASK(9, 6)
-#define CET_SHSTK_MASK_BITS GENMASK(1, 0)
-#define CET_IBT_MASK_BITS (GENMASK_ULL(5, 2) | \
- GENMASK_ULL(63, 10))
-#define CET_LEG_BITMAP_BASE(data) ((data) >> 12)
case MSR_IA32_U_CET:
- case MSR_IA32_S_CET:
- if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
- return 1;
- if (!!(data & CET_CTRL_RESERVED_BITS))
- return 1;
- if (!guest_can_use(vcpu, X86_FEATURE_SHSTK) &&
- (data & CET_SHSTK_MASK_BITS))
- return 1;
- if (!guest_can_use(vcpu, X86_FEATURE_IBT) &&
- (data & CET_IBT_MASK_BITS))
- return 1;
- if (!IS_ALIGNED(CET_LEG_BITMAP_BASE(data), 4) ||
- (data & CET_EXCLUSIVE_BITS) == CET_EXCLUSIVE_BITS)
- return 1;
- if (msr == MSR_IA32_U_CET)
- kvm_set_xsave_msr(msr_info);
- break;
- case MSR_KVM_GUEST_SSP:
- case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
- if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
- return 1;
- if (is_noncanonical_address(data, vcpu))
- return 1;
- if (!IS_ALIGNED(data, 4))
- return 1;
- if (msr == MSR_IA32_PL0_SSP || msr == MSR_IA32_PL1_SSP ||
- msr == MSR_IA32_PL2_SSP) {
- vcpu->arch.cet_s_ssp[msr - MSR_IA32_PL0_SSP] = data;
- if (!vcpu->arch.cet_sss_active && data)
- vcpu->arch.cet_sss_active = true;
- } else if (msr == MSR_IA32_PL3_SSP) {
- kvm_set_xsave_msr(msr_info);
- }
+ case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
+ kvm_set_xsave_msr(msr_info);
break;
default:
if (kvm_pmu_is_valid_msr(vcpu, msr))
@@ -4436,17 +4403,8 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
break;
#endif
case MSR_IA32_U_CET:
- case MSR_IA32_S_CET:
- case MSR_KVM_GUEST_SSP:
- case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
- if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
- return 1;
- if (msr == MSR_IA32_PL0_SSP || msr == MSR_IA32_PL1_SSP ||
- msr == MSR_IA32_PL2_SSP) {
- msr_info->data = vcpu->arch.cet_s_ssp[msr - MSR_IA32_PL0_SSP];
- } else if (msr == MSR_IA32_U_CET || msr == MSR_IA32_PL3_SSP) {
- kvm_get_xsave_msr(msr_info);
- }
+ case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
+ kvm_get_xsave_msr(msr_info);
break;
default:
if (kvm_pmu_is_valid_msr(vcpu, msr))
@@ -7330,9 +7288,13 @@ static void kvm_probe_msr_to_save(u32 msr_index)
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_KVM_GUEST_SSP:
case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
- if (!kvm_is_cet_supported())
+ if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK))
return;
break;
default:
@@ -9664,13 +9626,8 @@ static int __kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
kvm_caps.supported_xcr0 = host_xcr0 & KVM_SUPPORTED_XCR0;
}
if (boot_cpu_has(X86_FEATURE_XSAVES)) {
- u32 eax, ebx, ecx, edx;
-
- cpuid_count(0xd, 1, &eax, &ebx, &ecx, &edx);
rdmsrl(MSR_IA32_XSS, host_xss);
kvm_caps.supported_xss = host_xss & KVM_SUPPORTED_XSS;
- if (ecx & XFEATURE_MASK_CET_KERNEL)
- kvm_caps.supported_xss |= XFEATURE_MASK_CET_KERNEL;
}

rdmsrl_safe(MSR_EFER, &host_efer);

base-commit: efb9177acd7a4df5883b844e1ec9c69ef0899c9c
--