Re: [PATCH 9/8] KVM: x86: limit MSR_IA32_SPEC_CTRL access based on CPUID availability

From: Liran Alon
Date: Tue Jan 09 2018 - 10:58:32 EST



----- pbonzini@xxxxxxxxxx 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@xxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
> 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@xxxxxxxxxx>

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