Re: [PATCH MANUALSEL 5.10 5/6] KVM: SVM: Don't kill SEV guest if SMAP erratum triggers in usermode

[Paolo Bonzini <pbonzini@redhat.com>]

On 2/9/22 19:57, Sasha Levin wrote:
> From: Sean Christopherson <seanjc@google.com>
> 
> [ Upstream commit cdf85e0c5dc766fc7fc779466280e454a6d04f87 ]

Acked-by: Paolo Bonzini <pbonzini@redhat.com>

Paolo

> 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@google.com>
> Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
> Message-Id: <20220120010719.711476-10-seanjc@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
>   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 fa543c355fbdb..d515c8e68314c 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4155,7 +4155,21 @@ static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, void *insn, int i
>   			return true;
>   
>   		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);
>   	}
>   
>   	return false;