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@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
---
 arch/x86/kvm/vmx/vmx.c | 43 +++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 42 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index ff2296fa7d39..24e921c4e7e3 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -159,7 +159,7 @@ module_param(allow_smaller_maxphyaddr, bool, S_IRUGO);
 
 /*
  * List of MSRs that can be directly passed to the guest.
- * In addition to these x2apic and PT MSRs are handled specially.
+ * In addition to these x2apic/PT/CET MSRs are handled specially.
  */
 static u32 vmx_possible_passthrough_msrs[MAX_POSSIBLE_PASSTHROUGH_MSRS] = {
 	MSR_IA32_SPEC_CTRL,
@@ -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;
 	}
 
 	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)
-- 
2.43.0

On Sun, Feb 18, 2024, Yang Weijiang wrote:
> @@ -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;

Hmm, I find this is unnecessarily confusing and brittle.  E.g. in the unlikely
event more CET stuff comes along, this lurking return could cause problems.

Why not handle S_CET and U_CET in a single common path?  IMO, this is less error
prone, and more clearly captures the relationship between S/U_CET, SHSTK, and IBT.
Updating MSR intercepts is not a hot path, so the overhead of checking guest CPUID
multiple times should be a non-issue.  And eventually KVM should effectively cache
all of those lookups, i.e. the cost will be negilible.

	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_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 (kvm_cpu_cap_has(X86_FEATURE_SHSTK) ||
	    kvm_cpu_cap_has(X86_FEATURE_IBT)) {
		incpt = !guest_cpuid_has(vcpu, X86_FEATURE_IBT) &&
			!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);
	}

On 5/2/2024 7:07 AM, Sean Christopherson wrote:
> On Sun, Feb 18, 2024, Yang Weijiang wrote:
>> @@ -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;
> Hmm, I find this is unnecessarily confusing and brittle.  E.g. in the unlikely
> event more CET stuff comes along, this lurking return could cause problems.
>
> Why not handle S_CET and U_CET in a single common path?  IMO, this is less error
> prone, and more clearly captures the relationship between S/U_CET, SHSTK, and IBT.
> Updating MSR intercepts is not a hot path, so the overhead of checking guest CPUID
> multiple times should be a non-issue.  And eventually KVM should effectively cache
> all of those lookups, i.e. the cost will be negilible.
>
> 	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_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 (kvm_cpu_cap_has(X86_FEATURE_SHSTK) ||
> 	    kvm_cpu_cap_has(X86_FEATURE_IBT)) {
> 		incpt = !guest_cpuid_has(vcpu, X86_FEATURE_IBT) &&
> 			!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);
> 	}

It looks fine to me, will apply it, thanks!