linux-kernel - Re: [PATCH] KVM: x86: reinstate vendor-agnostic check on SPEC

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Date:   Fri, 4 Dec 2020 09:12:05 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        stable@...r.kernel.org, "Denis V . Lunev" <den@...nvz.org>
Subject: Re: [PATCH] KVM: x86: reinstate vendor-agnostic check on SPEC_CTRL
 cpuid bits

On Thu, Dec 03, 2020, 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@...r.kernel.org
> Reported-by: Denis V. Lunev <den@...nvz.org>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  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)

Doesn't vmx_get_msr() also need to be changed for SPEC_CTRL?  Assuming that's
the case, adding helpers in cpuid.h to detect guest support for SPEC_CTRL (and
maybe for PRED_CMD?) would be helpful.  It'd reduce duplicate code and document
that KVM allows cross-vendor emulation.  The condition for SPEC_CTRL support is
especially long.

>  		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))

Whitespace is messed up.

ERROR: code indent should use tabs where possible
#54: FILE: arch/x86/kvm/vmx/vmx.c:2031:
+                    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL) &&$

WARNING: please, no spaces at the start of a line
#54: FILE: arch/x86/kvm/vmx/vmx.c:2031:
+                    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL) &&$

ERROR: code indent should use tabs where possible
#55: FILE: arch/x86/kvm/vmx/vmx.c:2032:
+                    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_STIBP) &&$

WARNING: please, no spaces at the start of a line
#55: FILE: arch/x86/kvm/vmx/vmx.c:2032:
+                    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_STIBP) &&$

ERROR: code indent should use tabs where possible
#56: FILE: arch/x86/kvm/vmx/vmx.c:2033:
+                    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBRS) &&$

WARNING: please, no spaces at the start of a line
#56: FILE: arch/x86/kvm/vmx/vmx.c:2033:
+                    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBRS) &&$

ERROR: code indent should use tabs where possible
#57: FILE: arch/x86/kvm/vmx/vmx.c:2034:
+                    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_SSBD))$

WARNING: please, no spaces at the start of a line
#57: FILE: arch/x86/kvm/vmx/vmx.c:2034:
+                    !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;
> -- 
> 2.26.2
>