Re: [PATCH 07/14] KVM: x86: SVM: add warning for CVE-2021-3656

[Paolo Bonzini]

On 14/09/21 17:48, Maxim Levitsky wrote:
> Just in case, add a warning ensuring that on guest entry,
> either both VMLOAD and VMSAVE intercept is enabled or
> vVMLOAD/VMSAVE is enabled.
>
> Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> ---
>   arch/x86/kvm/svm/svm.c | 6 ++++++
>   1 file changed, 6 insertions(+)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 861ac9f74331..deeebd05f682 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3784,6 +3784,12 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
>   
>   	WARN_ON_ONCE(kvm_apicv_activated(vcpu->kvm) != kvm_vcpu_apicv_active(vcpu));
>   
> +	/* Check that CVE-2021-3656 can't happen again */
> +	if (!svm_is_intercept(svm, INTERCEPT_VMSAVE) ||
> +	    !svm_is_intercept(svm, INTERCEPT_VMSAVE))
> +		WARN_ON(!(svm->vmcb->control.virt_ext &
> +			  VIRTUAL_VMLOAD_VMSAVE_ENABLE_MASK));
> +
>   	sync_lapic_to_cr8(vcpu);
>   
>   	if (unlikely(svm->asid != svm->vmcb->control.asid)) {

While it's nice to be "proactive", this does adds some extra work. 
Maybe it should be under CONFIG_DEBUG_KERNEL.  It could be useful to 
make it into its own function so we can add similar intercept invariants 
in the same place.

Paolo