Re: [PATCH v9 22/27] KVM: VMX: Set up interception for CET MSRs

[Chao Gao]

On Tue, Jan 23, 2024 at 06:41:55PM -0800, Yang Weijiang wrote:
>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 architectual perspective since IBT relies on subset of SHSTK
>relevant MSRs.
>
>Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
>Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Reviewed-by: Chao Gao <chao.gao@xxxxxxxxx>

one nit below,

>---
> arch/x86/kvm/vmx/vmx.c | 41 +++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 41 insertions(+)
>
>diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>index 064a5fe87948..34e91dbbffed 100644
>--- a/arch/x86/kvm/vmx/vmx.c
>+++ b/arch/x86/kvm/vmx/vmx.c
>@@ -692,6 +692,10 @@ static bool is_valid_passthrough_msr(u32 msr)
> 	case MSR_LBR_CORE_TO ... MSR_LBR_CORE_TO + 8:
> 		/* LBR MSRs. These are handled in vmx_update_intercept_for_lbr_msrs() */
> 		return true;
>+	case MSR_IA32_U_CET:
>+	case MSR_IA32_S_CET:
>+	case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
>+		return true;

Please update the comment above vmx_possible_passthrough_msrs[] to indicate CET
MSRs are also handled separately.

> 	}
> 
> 	r = possible_passthrough_msr_slot(msr) != -ENOENT;
>@@ -7767,6 +7771,41 @@ static void update_intel_pt_cfg(struct kvm_vcpu *vcpu)
> 		vmx->pt_desc.ctl_bitmask &= ~(0xfULL << (32 + i * 4));
> }
> 
>+static void vmx_update_intercept_for_cet_msr(struct kvm_vcpu *vcpu)
>+{
>+	bool incpt;
>+
>+	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
>+		incpt = !guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
>+
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET,
>+					  MSR_TYPE_RW, incpt);
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET,
>+					  MSR_TYPE_RW, incpt);
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP,
>+					  MSR_TYPE_RW, incpt);
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP,
>+					  MSR_TYPE_RW, incpt);
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP,
>+					  MSR_TYPE_RW, incpt);
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP,
>+					  MSR_TYPE_RW, incpt);
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB,
>+					  MSR_TYPE_RW, incpt);
>+		if (!incpt)
>+			return;
>+	}
>+
>+	if (kvm_cpu_cap_has(X86_FEATURE_IBT)) {
>+		incpt = !guest_cpuid_has(vcpu, X86_FEATURE_IBT);
>+
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET,
>+					  MSR_TYPE_RW, incpt);
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET,
>+					  MSR_TYPE_RW, incpt);
>+	}
>+}
>+
> static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> {
> 	struct vcpu_vmx *vmx = to_vmx(vcpu);
>@@ -7845,6 +7884,8 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> 
> 	/* Refresh #PF interception to account for MAXPHYADDR changes. */
> 	vmx_update_exception_bitmap(vcpu);
>+
>+	vmx_update_intercept_for_cet_msr(vcpu);
> }
> 
> static u64 vmx_get_perf_capabilities(void)
>-- 
>2.39.3
>