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

[Maxim Levitsky <mlevitsk@redhat.com>]

On Tue, 2021-05-04 at 10:17 -0700, Sean Christopherson 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@vger.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/svm/svm.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index a7271f31df47..8f2b184270c0 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1100,7 +1100,9 @@ static u64 svm_write_l1_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
>  	return svm->vmcb->control.tsc_offset;
>  }
>  
> -static void svm_check_invpcid(struct vcpu_svm *svm)
> +/* Evaluate instruction intercepts that depend on guest CPUID features. */
> +static void svm_recalc_instruction_intercepts(struct kvm_vcpu *vcpu,
> +					      struct vcpu_svm *svm)
>  {
>  	/*
>  	 * Intercept INVPCID if shadow paging is enabled to sync/free shadow
> @@ -1113,6 +1115,13 @@ static void svm_check_invpcid(struct vcpu_svm *svm)
>  		else
>  			svm_clr_intercept(svm, INTERCEPT_INVPCID);
>  	}
> +
> +	if (kvm_cpu_cap_has(X86_FEATURE_RDTSCP)) {
> +		if (guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
> +			svm_clr_intercept(svm, INTERCEPT_RDTSCP);
> +		else
> +			svm_set_intercept(svm, INTERCEPT_RDTSCP);
> +	}
>  }
>  
>  static void init_vmcb(struct kvm_vcpu *vcpu)
> @@ -1248,7 +1257,7 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
>  		svm_clr_intercept(svm, INTERCEPT_PAUSE);
>  	}
>  
> -	svm_check_invpcid(svm);
> +	svm_recalc_instruction_intercepts(vcpu, svm);
>  
>  	/*
>  	 * If the host supports V_SPEC_CTRL then disable the interception
> @@ -3084,6 +3093,7 @@ static int (*const svm_exit_handlers[])(struct kvm_vcpu *vcpu) = {
>  	[SVM_EXIT_STGI]				= stgi_interception,
>  	[SVM_EXIT_CLGI]				= clgi_interception,
>  	[SVM_EXIT_SKINIT]			= skinit_interception,
> +	[SVM_EXIT_RDTSCP]			= kvm_handle_invalid_op,
>  	[SVM_EXIT_WBINVD]                       = kvm_emulate_wbinvd,
>  	[SVM_EXIT_MONITOR]			= kvm_emulate_monitor,
>  	[SVM_EXIT_MWAIT]			= kvm_emulate_mwait,
> @@ -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);
>  
>  	/* For sev guests, the memory encryption bit is not reserved in CR3.  */
>  	if (sev_guest(vcpu->kvm)) {
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
	Maxim Levitsky