linux-kernel - Re: [PATCH 9/8] KVM: x86: limit MSR_IA32_SPEC

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Date:   Tue, 9 Jan 2018 07:58:09 -0800 (PST)
From:   Liran Alon <liran.alon@...cle.com>
To:     <pbonzini@...hat.com>
Cc:     <jmattson@...gle.com>, <x86@...nel.org>, <dwmw@...zon.co.uk>,
        <bp@...en8.de>, <aliguori@...zon.com>, <thomas.lendacky@....com>,
        <rkrcmar@...hat.com>, <linux-kernel@...r.kernel.org>,
        <kvm@...r.kernel.org>
Subject: Re: [PATCH 9/8] KVM: x86: limit MSR_IA32_SPEC_CTRL access based on
 CPUID availability


----- pbonzini@...hat.com wrote:

> MSR_IA32_SPEC_CTRL is not available unless CPU[7,0].EDX[26] is 1.
> Check that against host CPUID or guest CPUID, respectively for
> host-initiated and guest-initiated accesses.
> 
> Suggested-by: Jim Mattson <jmattson@...gle.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
> 	This is for after X86_FEATURE_SPEC_CTRL is added to Linux, but
> 	I still wanted to ack Jim's improvement.
> 
>  arch/x86/kvm/svm.c | 8 ++++++++
>  arch/x86/kvm/vmx.c | 8 ++++++++
>  2 files changed, 16 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 97126c2bd663..3a646580d7c5 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -3648,6 +3648,10 @@ static int svm_get_msr(struct kvm_vcpu *vcpu,
> struct msr_data *msr_info)
>  		msr_info->data = svm->nested.vm_cr_msr;
>  		break;
>  	case MSR_IA32_SPEC_CTRL:
> +		if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
> +	    	    (!msr_info->host_initiated &&
> +       		     !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
> +			return 1;
>  		msr_info->data = svm->spec_ctrl;
>  		break;
>  	case MSR_IA32_UCODE_REV:
> @@ -3806,6 +3810,10 @@ static int svm_set_msr(struct kvm_vcpu *vcpu,
> struct msr_data *msr)
>  		vcpu_unimpl(vcpu, "unimplemented wrmsr: 0x%x data 0x%llx\n", ecx,
> data);
>  		break;
>  	case MSR_IA32_SPEC_CTRL:
> +		if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
> +	    	    (!msr_info->host_initiated &&
> +       		     !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
> +			return 1;
>  		svm->spec_ctrl = data;
>  		break;
>  	case MSR_IA32_APICBASE:
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 49b4a2d61603..42bc7ee293e4 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -3368,6 +3368,10 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu,
> struct msr_data *msr_info)
>  		msr_info->data = guest_read_tsc(vcpu);
>  		break;
>  	case MSR_IA32_SPEC_CTRL:
> +		if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
> +		    (!msr_info->host_initiated &&
> +		     !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
> +			return 1;
>  		msr_info->data = to_vmx(vcpu)->spec_ctrl;
>  		break;
>  	case MSR_IA32_SYSENTER_CS:
> @@ -3510,6 +3514,10 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu,
> struct msr_data *msr_info)
>  		kvm_write_tsc(vcpu, msr_info);
>  		break;
>  	case MSR_IA32_SPEC_CTRL:
> +		if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
> +		    (!msr_info->host_initiated &&
> +		     !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
> +			return 1;
>  		to_vmx(vcpu)->spec_ctrl = data;
>  		break;
>  	case MSR_IA32_CR_PAT:
> -- 
> 1.8.3.1

Reviewed-by: Liran Alon <liran.alon@...cle.com>

The only thing I a bit dislike is that currently these MSRs are always pass-through to guest and therefore
there is no case vmx_set_msr() is called with !msr_info->host_initiated.
Don't you think we should BUG_ON(!msr_info->host_initiated)?

-Liran