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

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Date:   Wed, 6 Sep 2023 13:11:40 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     Peter Gonda <pgonda@...gle.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 06, 2023, Tom Lendacky wrote:
> On 9/6/23 10:14, Peter Gonda wrote:
> > Currently if an SEV-ES VM shuts down userspace sees KVM_RUN struct with
> 
> s/down userspace/down, userspace/

Heh, yeah, I read that the same way you did.

> > only the INVALID_ARGUMENT. This is a very limited amount of information
> > to debug the situation. Instead KVM can return a
> > KVM_EXIT_SHUTDOWN to alert userspace the VM is shutting down and
> > is not usable any further.
> > 
> > Signed-off-by: Peter Gonda <pgonda@...gle.com>
> > Cc: Paolo Bonzini <pbonzini@...hat.com>
> > Cc: Sean Christopherson <seanjc@...gle.com>
> > Cc: Tom Lendacky <thomas.lendacky@....com>
> > Cc: Joerg Roedel <joro@...tes.org>
> > Cc: Borislav Petkov <bp@...en8.de>
> > Cc: x86@...nel.org
> > Cc: kvm@...r.kernel.org
> > Cc: linux-kernel@...r.kernel.org
> > 
> > ---
> >   arch/x86/kvm/svm/svm.c | 8 +++++---
> >   1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > 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?