Re: [PATCH] KVM: SEV: Disable KVM_CAP_VM_COPY_ENC_CONTEXT_FROM for SEV-ES

[Peter Gonda]

On Tue, Sep 14, 2021 at 12:41 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> -stable, for giggles
>
> On Tue, Sep 14, 2021, Peter Gonda wrote:
> > On Tue, Sep 14, 2021 at 11:32 AM Sean Christopherson <seanjc@xxxxxxxxxx> 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.

I do not think so. You cannot call KVM_SEV_LAUNCH_UPDATE_VMSA on the
mirror because svm_mem_enc_op() blocks calls from the mirror. So
either you have to update vmsa from the mirror or have the original VM
read through its mirror's vCPUs when calling
KVM_SEV_LAUNCH_UPDATE_VMSA. Not sure which way is better but I don't
see a way to do this without updating KVM.

>
> > > 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@xxxxxxxxxxxxxxx
>
> > 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.

Ack. I'll send out a V2 with updated changelog after we've settled on
the first issue.