Re: [PATCH 2/3 v2] kvm: x86: Guest BNDCFGS requires guest MPX support

["Radim Krčmář" <rkrcmar@redhat.com>]

2017-05-24 09:22-0700, Jim Mattson:
> The BNDCFGS MSR should only be exposed to the guest if the guest
> supports MPX. (cf. the TSC_AUX MSR and RDTSCP.)
> 
> Fixes: 0dd376e709975779 ("KVM: x86: add MSR_IA32_BNDCFGS to msrs_to_save")
> Change-Id: I3ad7c01bda616715137ceac878f3fa7e66b6b387
> Signed-off-by: Jim Mattson <jmattson@google.com>
> ---
> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> @@ -144,6 +144,14 @@ static inline bool guest_cpuid_has_rtm(struct kvm_vcpu *vcpu)
>  	return best && (best->ebx & bit(X86_FEATURE_RTM));
>  }
>  
> +static inline bool guest_cpuid_has_mpx(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_cpuid_entry2 *best;
> +
> +	best = kvm_find_cpuid_entry(vcpu, 7, 0);
> +	return best && (best->ebx & bit(X86_FEATURE_MPX));
> +}
> +
>  static inline bool guest_cpuid_has_rdtscp(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_cpuid_entry2 *best;
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> @@ -3195,7 +3195,7 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		msr_info->data = vmcs_readl(GUEST_SYSENTER_ESP);
>  		break;
>  	case MSR_IA32_BNDCFGS:
> -		if (!kvm_mpx_supported())
> +		if (!guest_cpuid_has_mpx(vcpu))
>  			return 1;
>  		msr_info->data = vmcs_read64(GUEST_BNDCFGS);

Userspace can force guest_cpuid_has_mpx() to return true even if the
host does not have MPX (GUEST_BNDCFGS in VMCS), which would allow it to
trigger vmread/vmwrite errors at will.

I think it would make most sense to fail KVM_SET_CPUID that tries to do
that, but checking for host support or silently clearing the bit still
seem better than the host error.

Thanks.