Re: [PATCH] KVM: x86: reinstate vendor-agnostic check on SPEC_CTRL cpuid bits

[Denis V. Lunev]

On 12/3/20 6:15 PM, Paolo Bonzini wrote:
> Until commit e7c587da1252 ("x86/speculation: Use synthetic bits for IBRS/IBPB/STIBP",
> 2018-05-17), KVM was testing both Intel and AMD CPUID bits before allowing the
> guest to write MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD.  Testing only Intel bits
> on VMX processors, or only AMD bits on SVM processors, fails if the guests are
> created with the "opposite" vendor as the host.
>
> While at it, also tweak the host CPU check to use the vendor-agnostic feature bit
> X86_FEATURE_IBPB, since we only care about the availability of the MSR on the host
> here and not about specific CPUID bits.
>
> Fixes: e7c587da1252 ("x86/speculation: Use synthetic bits for IBRS/IBPB/STIBP")
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Denis V. Lunev <den@xxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/svm.c |  3 ++-
>  arch/x86/kvm/vmx/vmx.c | 10 +++++++---
>  2 files changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 62390fbc9233..0b4aa60b2754 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -2686,12 +2686,13 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>  		break;
>  	case MSR_IA32_PRED_CMD:
>  		if (!msr->host_initiated &&
> +		    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL) &&
>  		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBPB))
>  			return 1;
>  
>  		if (data & ~PRED_CMD_IBPB)
>  			return 1;
> -		if (!boot_cpu_has(X86_FEATURE_AMD_IBPB))
> +		if (!boot_cpu_has(X86_FEATURE_IBPB))
>  			return 1;
>  		if (!data)
>  			break;
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index c3441e7e5a87..b74d2105ced7 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2028,7 +2028,10 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		break;
>  	case MSR_IA32_SPEC_CTRL:
>  		if (!msr_info->host_initiated &&
> -		    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
> +                    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL) &&
> +                    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_STIBP) &&
> +                    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBRS) &&
> +                    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_SSBD))
>  			return 1;
>  
>  		if (kvm_spec_ctrl_test_value(data))
> @@ -2063,12 +2066,13 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		goto find_uret_msr;
>  	case MSR_IA32_PRED_CMD:
>  		if (!msr_info->host_initiated &&
> -		    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
> +		    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL) &&
> +		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBPB))
>  			return 1;
>  
>  		if (data & ~PRED_CMD_IBPB)
>  			return 1;
> -		if (!boot_cpu_has(X86_FEATURE_SPEC_CTRL))
> +		if (!boot_cpu_has(X86_FEATURE_IBPB))
>  			return 1;
>  		if (!data)
>  			break;
Reviewed-by: Denis V. Lunev <den@xxxxxxxxxx>