linux-kernel - Re: [PATCH MANUALSEL 5.16 5/8] KVM: SVM: Don't kill SEV guest if SMAP erratum triggers in usermode

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Message-ID: <042c4562-3eb8-f141-a363-67ec1def8e2c@redhat.com>
Date:   Thu, 10 Feb 2022 17:33:40 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Liam Merwick <liam.merwick@...cle.com>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, kvm@...r.kernel.org
Subject: Re: [PATCH MANUALSEL 5.16 5/8] KVM: SVM: Don't kill SEV guest if SMAP
 erratum triggers in usermode

On 2/9/22 19:56, Sasha Levin wrote:
> From: Sean Christopherson <seanjc@...gle.com>
> 
> [ Upstream commit cdf85e0c5dc766fc7fc779466280e454a6d04f87 ]
> 
> 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.

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

Paolo

> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> Reviewed-by: Liam Merwick <liam.merwick@...cle.com>
> Message-Id: <20220120010719.711476-10-seanjc@...gle.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Sasha Levin <sashal@...nel.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 3efada37272c0..d6a4acaa65742 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4530,7 +4530,21 @@ static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, void *insn, int i
>   	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);
>   	}
>   
>   	return false;