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

Marc Orr
2021-11-15T18:41:27Z

On Mon, Nov 15, 2021 at 10:26 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Mon, Nov 15, 2021, Dr. David Alan Gilbert wrote:
> > * Sean Christopherson (seanjc@google.com) wrote:
> > > On Fri, Nov 12, 2021, Borislav Petkov wrote:
> > > > On Fri, Nov 12, 2021 at 09:59:46AM -0800, Dave Hansen wrote:
> > > > > Or, is there some mechanism that prevent guest-private memory from being
> > > > > accessed in random host kernel code?
> > >
> > > Or random host userspace code...
> > >
> > > > So I'm currently under the impression that random host->guest accesses
> > > > should not happen if not previously agreed upon by both.
> > >
> > > Key word "should".
> > >
> > > > Because, as explained on IRC, if host touches a private guest page,
> > > > whatever the host does to that page, the next time the guest runs, it'll
> > > > get a #VC where it will see that that page doesn't belong to it anymore
> > > > and then, out of paranoia, it will simply terminate to protect itself.
> > > >
> > > > So cloud providers should have an interest to prevent such random stray
> > > > accesses if they wanna have guests. :)
> > >
> > > Yes, but IMO inducing a fault in the guest because of _host_ bug is wrong.
> >
> > Would it necessarily have been a host bug? A guest telling the host a
> > bad GPA to DMA into would trigger this wouldn't it?
>
> No, because as Andy pointed out, host userspace must already guard against a bad
> GPA, i.e. this is just a variant of the guest telling the host to DMA to a GPA
> that is completely bogus. The shared vs. private behavior just means that when
> host userspace is doing a GPA=>HVA lookup, it needs to incorporate the "shared"
> state of the GPA. If the host goes and DMAs into the completely wrong HVA=>PFN,
> then that is a host bug; that the bug happened to be exploited by a buggy/malicious
> guest doesn't change the fact that the host messed up.

"If the host goes and DMAs into the completely wrong HVA=>PFN, then
that is a host bug; that the bug happened to be exploited by a
buggy/malicious guest doesn't change the fact that the host messed
up."
^^^
Again, I'm flabbergasted that you are arguing that it's OK for a guest
to exploit a host bug to take down host-side processes or the host
itself, either of which could bring down all other VMs on the machine.

I'm going to repeat -- this is not OK! Period.

Again, if the community wants to layer some orchestration scheme
between host userspace, host kernel, and guest, on top of the code to
inject the #VC into the guest, that's fine. This proposal is not
stopping that. In fact, the two approaches are completely orthogonal
and compatible.

But so far I have heard zero reasons why injecting a #VC into the
guest is wrong. Other than just stating that it's wrong.

Again, the guest must be able to detect buggy and malicious host-side
writes to private memory. Or else "confidential computing" doesn't
work. Assuming that's not true is not a valid argument to dismiss
injecting a #VC exception into the guest.