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
