Re: [RFC PATCH 00/13] Add support for Mirror VM.

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* James Bottomley (jejb@linux.ibm.com) wrote:
> On Thu, 2021-08-19 at 15:28 +0100, Dr. David Alan Gilbert wrote:
> > * James Bottomley (jejb@linux.ibm.com) wrote:
> > > On Thu, 2021-08-19 at 09:22 +0100, Dr. David Alan Gilbert wrote:
> [...]
> > > > I think it really does have to cope with migration to a new
> > > > version of host.
> > > 
> > > Well, you're thinking of OVMF as belonging to the host because of
> > > the way it is supplied, but think about the way it works in
> > > practice now, forgetting about confidential computing: OVMF is RAM
> > > resident in ordinary guests, so when you migrate them, the whole of
> > > OVMF (or at least what's left at runtime) goes with the migration,
> > > thus it's not possible to change the guest OVMF by migration.  The
> > > above is really just an extension of that principle, the only
> > > difference for confidential computing being you have to have an
> > > image of the current OVMF ROM in the target to seed migration.
> > > 
> > > Technically, the problem is we can't overwrite running code and
> > > once the guest is re-sited to the target, the OVMF there has to
> > > match exactly what was on the source for the RT to still
> > > function.   Once the migration has run, the OVMF on the target must
> > > be identical to what was on the source (including internally
> > > allocated OVMF memory), and if we can't copy the MH code, we have
> > > to rely on the target image providing this identical code and we
> > > copy the rest.
> > 
> > I'm OK with the OVMF now being part of the guest image, and having to
> > exist on both; it's a bit delicate though unless we have a way to
> > check it (is there an attest of the destination happening here?)
> 
> There will be in the final version.  The attestations of the source and
> target, being the hash of the OVMF (with the registers in the -ES
> case), should be the same (modulo any firmware updates to the PSP,
> whose firmware version is also hashed) to guarantee the OVMF is the
> same on both sides.  We'll definitely take an action to get QEMU to
> verify this ... made a lot easier now we have signed attestations ...

Hmm; I'm not sure you're allowed to have QEMU verify that - we don't
trust it; you need to have either the firmware say it's OK to migrate
to the destination (using the existing PSP mechanism) or get the source
MH to verify a quote from the destination.

[Somewhere along the line, if you're not using the PSP, I think you also
need to check the guest policy to check it is allowed to migrate].

Dave

> James
> 
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK