Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support

Marc Orr
2021-11-14T07:54:16Z

On Sat, Nov 13, 2021 at 10:35 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Fri, Nov 12, 2021, Marc Orr wrote:
> > > > > If *it* is the host kernel, then you probably shouldn't do that -
> > > > > otherwise you just killed the host kernel on which all those guests are
> > > > > running.
> > > >
> > > > I agree, it seems better to terminate the single guest with an issue.
> > > > Rather than killing the host (and therefore all guests). So I'd
> > > > suggest even in this case we do the 'convert to shared' approach or
> > > > just outright terminate the guest.
> > > >
> > > > Are there already examples in KVM of a KVM bug in servicing a VM's
> > > > request results in a BUG/panic/oops? That seems not ideal ever.
> > >
> > > Plenty of examples. kvm_spurious_fault() is the obvious one. Any NULL pointer
> > > deref will lead to a BUG, etc... And it's not just KVM, e.g. it's possible, if
> > > unlikely, for the core kernel to run into guest private memory (e.g. if the kernel
> > > botches an RMP change), and if that happens there's no guarantee that the kernel
> > > can recover.
> > >
> > > I fully agree that ideally KVM would have a better sense of self-preservation,
> > > but IMO that's an orthogonal discussion.
> >
> > I don't think we should treat the possibility of crashing the host
> > with live VMs nonchalantly. It's a big deal. Doing so has big
> > implications on the probability that any cloud vendor wil bee able to
> > deploy this code to production. And aren't cloud vendors one of the
> > main use cases for all of this confidential compute stuff? I'm
> > honestly surprised that so many people are OK with crashing the host.
>
> I'm not treating it nonchalantly, merely acknowledging that (a) some flavors of kernel
> bugs (or hardware issues!) are inherently fatal to the system, and (b) crashing the
> host may be preferable to continuing on in certain cases, e.g. if continuing on has a
> high probablity of corrupting guest data.

I disagree. Crashing the host -- and _ALL_ of its VMs (including
non-confidential VMs) -- is not preferable to crashing a single SNP
VM. Especially when that SNP VM is guaranteed to detect the memory
corruption and react accordingly.