Re: [PATCH 0/2] KVM: arm/arm64: Add VCPU workarounds firmware register

[Marc Zyngier <marc.zyngier@arm.com>]

On Tue, 22 Jan 2019 13:56:34 +0000,
Dave Martin <Dave.Martin@arm.com> wrote:
> 
> On Tue, Jan 22, 2019 at 11:11:09AM +0000, Marc Zyngier wrote:
> > On Tue, 22 Jan 2019 10:17:00 +0000,
> > Dave Martin <Dave.Martin@arm.com> wrote:
> > > 
> > > On Mon, Jan 07, 2019 at 12:05:35PM +0000, Andre Przywara wrote:
> > > > Workarounds for Spectre variant 2 or 4 vulnerabilities require some help
> > > > from the firmware, so KVM implements an interface to provide that for
> > > > guests. When such a guest is migrated, we want to make sure we don't
> > > > loose the protection the guest relies on.
> > > > 
> > > > This introduces two new firmware registers in KVM's GET/SET_ONE_REG
> > > > interface, so userland can save the level of protection implemented by
> > > > the hypervisor and used by the guest. Upon restoring these registers,
> > > > we make sure we don't downgrade and reject any values that would mean
> > > > weaker protection.
> > > 
> > > Just trolling here, but could we treat these as immutable, like the ID
> > > registers?  
> > > 
> > > We don't support migration between nodes that are "too different" in any
> > > case, so I wonder if adding complex logic to compare vulnerabilities and
> > > workarounds is liable to create more problems than it solves...
> > 
> > And that's exactly the case we're trying to avoid. Two instances of
> > the same HW. One with firmware mitigations, one without. Migrating in
> > one direction is perfectly safe, migrating in the other isn't.
> > 
> > It is not about migrating to different HW at all.
> 
> So this is a realistic scenario when deploying a firmware update across
> a cluter that has homogeneous hardware -- there will temporarly be
> different firmware versions running on different nodes?

Case in point: I have on my desk two AMD Seattle systems. One with an
ancient firmware that doesn't mitigate anything, and one that has all
the mitigations applied (and correctly advertised). I can migrate
stuff back and forth, and that's really bad.

What people do in their data centre is none of my business,
really. What concerns me is that there is a potential for something
bad to happen without people noticing. And it is KVM's job to do the
right thing in this case.

> My concern is really "will the checking be too buggy / untested in
> practice to be justified by the use case".

Not doing anything is not going to make the current situation "less
buggy". We have all the stuff we need to test this. We can even
artificially create the various scenarios on a model.

> I'll take a closer look at the checking logic.

Thanks,

	M.

-- 
Jazz is not dead, it just smell funny.