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

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Date:   Tue, 14 Sep 2021 11:58:37 -0600
From:   Peter Gonda <pgonda@...gle.com>
To:     Sean Christopherson <seanjc@...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, stable@...r.kernel.org
Subject: Re: [PATCH] KVM: SEV: Disable KVM_CAP_VM_COPY_ENC_CONTEXT_FROM for SEV-ES

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.

>
> 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.

>
> > Fixes: 54526d1fd593 ("KVM: x86: Support KVM VMs sharing SEV context")
>
> Cc: stable@...r.kernel.org

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