Re: [PATCH v4 14/20] KVM:VMX: Set up interception for CET MSRs

[Chao Gao]

On Thu, Jul 20, 2023 at 11:03:46PM -0400, Yang Weijiang wrote:
>Pass through CET MSRs when the associated feature is enabled.
>Shadow Stack feature requires all the CET MSRs to make it
>architectural support in guest. IBT feature only depends on
>MSR_IA32_U_CET and MSR_IA32_S_CET to enable both user and
>supervisor IBT.

If a guest supports SHSTK only, KVM has no way to prevent guest from
enabling IBT because the U/S_CET are pass-thru'd. it is a problem.

I am wondering if it is necessary to pass-thru U/S_CET MSRs. Probably
the answer is yes at least for U_CET MSR because the MSR is per-task.

>
>Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
>---
> arch/x86/kvm/vmx/vmx.c | 35 +++++++++++++++++++++++++++++++++++
> 1 file changed, 35 insertions(+)
>
>diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>index b29817ec6f2e..85cb7e748a89 100644
>--- a/arch/x86/kvm/vmx/vmx.c
>+++ b/arch/x86/kvm/vmx/vmx.c
>@@ -709,6 +709,10 @@ 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;
> 	}
> 
> 	r = possible_passthrough_msr_slot(msr) != -ENOENT;
>@@ -7758,6 +7762,34 @@ 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)
>+{
>+	if (guest_can_use(vcpu, X86_FEATURE_SHSTK)) {
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET,
>+					  MSR_TYPE_RW, false);
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET,
>+					  MSR_TYPE_RW, false);
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP,
>+					  MSR_TYPE_RW, false);
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP,
>+					  MSR_TYPE_RW, false);
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP,
>+					  MSR_TYPE_RW, false);
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP,
>+					  MSR_TYPE_RW, false);
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB,
>+					  MSR_TYPE_RW, false);
>+		return;
>+	}
>+
>+	if (guest_can_use(vcpu, X86_FEATURE_IBT)) {
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET,
>+					  MSR_TYPE_RW, false);
>+		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET,
>+					  MSR_TYPE_RW, false);
>+	}

This is incorrect. see

https://lore.kernel.org/all/ZJYzPn7ipYfO0fLZ@xxxxxxxxxx/

>+}
>+
> static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> {
> 	struct vcpu_vmx *vmx = to_vmx(vcpu);
>@@ -7825,6 +7857,9 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> 
> 	/* Refresh #PF interception to account for MAXPHYADDR changes. */
> 	vmx_update_exception_bitmap(vcpu);
>+
>+	if (kvm_is_cet_supported())

Nit: this check is not necessary. here isn't a hot path. and if
kvm_is_cet_supported() is false, guest_can_use(., X86_FEATURE_SHSTK/IBT)
should be false.

>+		vmx_update_intercept_for_cet_msr(vcpu);
> }
> 
> static u64 vmx_get_perf_capabilities(void)
>-- 
>2.27.0
>