linux-kernel - Re: [PATCH V2] KVM: SEV: Update SEV-ES shutdown intercepts with more metadata

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Date:   Wed, 6 Sep 2023 14:28:58 -0600
From:   Peter Gonda <pgonda@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Tom Lendacky <thomas.lendacky@....com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] KVM: SEV: Update SEV-ES shutdown intercepts with more metadata

On Wed, Sep 6, 2023 at 2:26 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Wed, Sep 06, 2023, Tom Lendacky wrote:
> > On 9/6/23 15:11, Sean Christopherson wrote:
> > > On Wed, Sep 06, 2023, Tom Lendacky wrote:
> > > > On 9/6/23 10:14, Peter Gonda wrote:
> > > > > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > > > > index 956726d867aa..cecf6a528c9b 100644
> > > > > --- a/arch/x86/kvm/svm/svm.c
> > > > > +++ b/arch/x86/kvm/svm/svm.c
> > > > > @@ -2131,12 +2131,14 @@ static int shutdown_interception(struct kvm_vcpu *vcpu)
> > > > >          * The VM save area has already been encrypted so it
> > > > >          * cannot be reinitialized - just terminate.
> > > > >          */
> > > > > -       if (sev_es_guest(vcpu->kvm))
> > > > > -               return -EINVAL;
> > > > > +       if (sev_es_guest(vcpu->kvm)) {
> > > > > +               kvm_run->exit_reason = KVM_EXIT_SHUTDOWN;
> > > > > +               return 0;
> > > > > +       }
> > > >
> > > > Just a nit... feel free to ignore, but, since KVM_EXIT_SHUTDOWN is also set
> > > > at the end of the function and I don't think kvm_vcpu_reset() clears the
> > > > value from kvm_run, you could just set kvm_run->exit_reason on entry and
> > > > just return 0 early for an SEV-ES guest.
> > >
> > > kvm_run is writable by userspace though, so KVM can't rely on kvm_run->exit_reason
> > > for correctness.
> > >
> > > And IIUC, the VMSA is also toast, i.e. doing anything other than marking the VM
> > > dead is futile, no?
> >
> > I was just saying that "kvm_run->exit_reason = KVM_EXIT_SHUTDOWN;" is in the
> > shutdown_interception() function twice now (at both exit points of the
> > function) and can probably just be moved to the top of the function and be
> > common for both exit points, now, right?
> >
> > I'm not saying to get rid of it, just set it sooner.
>
> Ah, I thought you were saying bail early from kvm_vcpu_reset().  I agree that not
> having completely split logic would be ideal.  What about this?
>
>         /*
>          * VMCB is undefined after a SHUTDOWN intercept.  INIT the vCPU to put
>          * the VMCB in a known good state.  Unfortuately, KVM doesn't have
>          * KVM_MP_STATE_SHUTDOWN and can't add it without potentially breaking
>          * userspace.  At a platform view, INIT is acceptable behavior as
>          * there exist bare metal platforms that automatically INIT the CPU
>          * in response to shutdown.
>          *
>          * The VM save area for SEV-ES guests has already been encrypted so it
>          * cannot be reinitialized, i.e. synthesizing INIT is futile.
>          */
>         if (!sev_es_guest(vcpu->kvm)) {
>                 clear_page(svm->vmcb);
>                 kvm_vcpu_reset(vcpu, true);
>         }
>
>         kvm_run->exit_reason = KVM_EXIT_SHUTDOWN;
>         return 0;

Looks better to me. Thanks!