Re: [PATCH 15/15] KVM: x86: Hide RDTSCP and RDPID if MSR_TSC_AUX probing failed

[Maxim Levitsky]

On Tue, 2021-05-04 at 10:17 -0700, Sean Christopherson wrote:
> If probing MSR_TSC_AUX failed, hide RDTSCP and RDPID, and WARN if either
> feature was reported as supported.  In theory, such a scenario should
> never happen as both Intel and AMD state that MSR_TSC_AUX is available if
> RDTSCP or RDPID is supported.  But, KVM injects #GP on MSR_TSC_AUX
> accesses if probing failed, faults on WRMSR(MSR_TSC_AUX) may be fatal to
> the guest (because they happen during early CPU bringup), and KVM itself
> has effectively misreported RDPID support in the past.
> 
> Note, this also has the happy side effect of omitting MSR_TSC_AUX from
> the list of MSRs that are exposed to userspace if probing the MSR fails.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/cpuid.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index c96f79c9fff2..bf0f74ce4974 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -567,6 +567,21 @@ void kvm_set_cpu_caps(void)
>  		F(ACE2) | F(ACE2_EN) | F(PHE) | F(PHE_EN) |
>  		F(PMM) | F(PMM_EN)
>  	);
> +
> +	/*
> +	 * Hide RDTSCP and RDPID if either feature is reported as supported but
> +	 * probing MSR_TSC_AUX failed.  This is purely a sanity check and
> +	 * should never happen, but the guest will likely crash if RDTSCP or
> +	 * RDPID is misreported, and KVM has botched MSR_TSC_AUX emulation in
> +	 * the past, e.g. the sanity check may fire if this instance of KVM is
> +	 * running as L1 on top of an older, broken KVM.
> +	 */
> +	if (WARN_ON((kvm_cpu_cap_has(X86_FEATURE_RDTSCP) ||
> +		     kvm_cpu_cap_has(X86_FEATURE_RDPID)) &&
> +		     !kvm_is_supported_user_return_msr(MSR_TSC_AUX))) {
> +		kvm_cpu_cap_clear(X86_FEATURE_RDTSCP);
> +		kvm_cpu_cap_clear(X86_FEATURE_RDPID);
> +	}
>  }
>  EXPORT_SYMBOL_GPL(kvm_set_cpu_caps);
>  
Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky