linux-kernel - Re: [PATCH] KVM: SEV: Disable KVM_CAP_VM_COPY_ENC_CONTEXT

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Message-ID: <YUDsy4W0/FeIEJDr@google.com>
Date:   Tue, 14 Sep 2021 18:41:15 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peter Gonda <pgonda@...gle.com>
Cc:     kvm list <kvm@...r.kernel.org>, Marc Orr <marcorr@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Nathan Tempelman <natet@...gle.com>,
        Brijesh Singh <brijesh.singh@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: SEV: Disable KVM_CAP_VM_COPY_ENC_CONTEXT_FROM for
 SEV-ES

-stable, for giggles

On Tue, Sep 14, 2021, Peter Gonda wrote:
> On Tue, Sep 14, 2021 at 11:32 AM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > On Tue, Sep 14, 2021, Peter Gonda wrote:
> > > Copying an ASID into new vCPUs will not work for SEV-ES since the vCPUs
> > > VMSAs need to be setup and measured before SEV_LAUNCH_FINISH. Return an
> > > error if a users tries to KVM_CAP_VM_COPY_ENC_CONTEXT_FROM from an
> > > SEV-ES guest.
> >
> > What happens if userspace does KVM_CAP_VM_COPY_ENC_CONTEXT_FROM before the source
> > has created vCPUs, i.e. before it has done SEV_LAUNCH_FINISH?
> 
> That's not enough. If you wanted to be able to mirror SEV-ES you'd
> also need to call LAUNCH_UPDATE_VMSA on the mirror's vCPUs before
> SEV_LAUNCH_FINISH. That is do-able but I was writing a small change to
> fix this bug. If mirroring of SEV-ES is wanted it's a much bigger
> change.

Is it doable without KVM updates?  If so, then outright rejection may not be the
correct behavior.

> > Might be worth noting that the destination cannot be an SEV guest, and therefore
> > can't be an SEV-ES guest either.
> 
> sev_guest() implies sev_es_guest() so I think this case is covered.

Yes, I was suggesting calling that out in the changelog so that readers/reviewers
don't worry about that case.

> > Cc: stable@...r.kernel.org

> Oops. I'll update in the V2 if needed. Added to this thread for now.

FWIW, you don't actually need to Cc stable, just including it in the changelog is
sufficient as the script automagic will pick it up when it hits Linus' tree.