Re: [PATCH 9/9] KVM: SVM: Don't kill SEV guest if SMAP erratum triggers in usermode

[Liam Merwick]

On 20/01/2022 01:07, Sean Christopherson wrote:
> Inject a #GP instead of synthesizing triple fault to try to avoid killing
> the guest if emulation of an SEV guest fails due to encountering the SMAP
> erratum.  The injected #GP may still be fatal to the guest, e.g. if the
> userspace process is providing critical functionality, but KVM should
> make every attempt to keep the guest alive.
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>


Reviewed-by: Liam Merwick <liam.merwick@xxxxxxxxxx>

> ---
>   arch/x86/kvm/svm/svm.c | 16 +++++++++++++++-
>   1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index a4b02a6217fd..88f5bbb0e6a1 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4357,7 +4357,21 @@ static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
>   	is_user = svm_get_cpl(vcpu) == 3;
>   	if (smap && (!smep || is_user)) {
>   		pr_err_ratelimited("KVM: SEV Guest triggered AMD Erratum 1096\n");
> -		kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
> +
> +		/*
> +		 * If the fault occurred in userspace, arbitrarily inject #GP
> +		 * to avoid killing the guest and to hopefully avoid confusing
> +		 * the guest kernel too much, e.g. injecting #PF would not be
> +		 * coherent with respect to the guest's page tables.  Request
> +		 * triple fault if the fault occurred in the kernel as there's
> +		 * no fault that KVM can inject without confusing the guest.
> +		 * In practice, the triple fault is moot as no sane SEV kernel
> +		 * will execute from user memory while also running with SMAP=1.
> +		 */
> +		if (is_user)
> +			kvm_inject_gp(vcpu, 0);
> +		else
> +			kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
>   	}
>   
>   resume_guest: