Re: [PATCH v16 28/51] KVM: x86: Enable CET virtualization for VMX and advertise to userspace

Binbin Wu
2025-09-22T08:06:08Z

On 9/20/2025 6:32 AM, Sean Christopherson wrote:
> From: Yang Weijiang <weijiang.yang@intel.com>
>
> Add support for the LOAD_CET_STATE VM-Enter and VM-Exit controls, the
> CET XFEATURE bits in XSS, and advertise support for IBT and SHSTK to
> userspace. Explicitly clear IBT and SHSTK onn SVM, as additional work is
> needed to enable CET on SVM, e.g. to context switch S_CET and other state.
>
> Disable KVM CET feature if unrestricted_guest is unsupported/disabled as
> KVM does not support emulating CET, as running without Unrestricted Guest
> can result in KVM emulating large swaths of guest code. While it's highly
> unlikely any guest will trigger emulation while also utilizing IBT or
> SHSTK, there's zero reason to allow CET without Unrestricted Guest as that
> combination should only be possible when explicitly disabling
> unrestricted_guest for testing purposes.
>
> Disable CET if VMX_BASIC[bit56] == 0, i.e. if hardware strictly enforces
> the presence of an Error Code based on exception vector, as attempting to
> inject a #CP with an Error Code (#CP architecturally has an Error Code)
> will fail due to the #CP vector historically not having an Error Code.
>
> Clear S_CET and SSP-related VMCS on "reset" to emulate the architectural
> of CET MSRs and SSP being reset to 0 after RESET, power-up and INIT. Note,
> KVM already clears guest CET state that is managed via XSTATE in
> kvm_xstate_reset().
>
> 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>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Signed-off-by: Chao Gao <chao.gao@intel.com>
> [sean: move some bits to separate patches, massage changelog]
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>

> ---
> 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 ++++--
> 6 files changed, 45 insertions(+), 3 deletions(-)
>
> 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 b5c4cb13630c..b861a88083e1 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -946,6 +946,7 @@ void kvm_set_cpu_caps(void)
> VENDOR_F(WAITPKG),
> F(SGX_LC),
> F(BUS_LOCK_DETECT),
> + X86_64_F(SHSTK),
> );
>
> /*
> @@ -990,6 +991,7 @@ 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 67f4eed01526..73dde1645e46 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -5221,6 +5221,10 @@ 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 59c83888bdc0..02aadb9d730e 100644
> --- a/arch/x86/kvm/vmx/capabilities.h
> +++ b/arch/x86/kvm/vmx/capabilities.h
> @@ -73,6 +73,11 @@ 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_cc(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 a7d9e60b2771..69e35440cee7 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2615,6 +2615,7 @@ 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));
> @@ -4881,6 +4882,14 @@ 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);
> @@ -6348,6 +6357,10 @@ 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));
> @@ -6378,6 +6391,10 @@ 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",
> @@ -7959,7 +7976,6 @@ 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);
>
> @@ -7971,6 +7987,18 @@ 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_cc()) {
> + 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 23d6e89b96f2..af8224e074ee 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 | \