Re: [PATCH 03/15] KVM: SVM: Inject #UD on RDTSCP when it should be disabled in the guest

[Jim Mattson]

On Tue, May 4, 2021 at 10:17 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> Intercept RDTSCP to inject #UD if RDTSC is disabled in the guest.
>
> Note, SVM does not support intercepting RDPID.  Unlike VMX's
> ENABLE_RDTSCP control, RDTSCP interception does not apply to RDPID.  This
> is a benign virtualization hole as the host kernel (incorrectly) sets
> MSR_TSC_AUX if RDTSCP is supported, and KVM loads the guest's MSR_TSC_AUX
> into hardware if RDTSCP is supported in the host, i.e. KVM will not leak
> the host's MSR_TSC_AUX to the guest.
>
> But, when the kernel bug is fixed, KVM will start leaking the host's
> MSR_TSC_AUX if RDPID is supported in hardware, but RDTSCP isn't available
> for whatever reason.  This leak will be remedied in a future commit.
>
> Fixes: 46896c73c1a4 ("KVM: svm: add support for RDTSCP")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
...
> @@ -4007,8 +4017,7 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>         svm->nrips_enabled = kvm_cpu_cap_has(X86_FEATURE_NRIPS) &&
>                              guest_cpuid_has(vcpu, X86_FEATURE_NRIPS);
>
> -       /* Check again if INVPCID interception if required */
> -       svm_check_invpcid(svm);
> +       svm_recalc_instruction_intercepts(vcpu, svm);

Does the right thing happen here if the vCPU is in guest mode when
userspace decides to toggle the CPUID.80000001H:EDX.RDTSCP bit on or
off?