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

Sean Christopherson
2021-11-13T18:28:25Z

On Fri, Nov 12, 2021, Marc Orr wrote:
> On Fri, Nov 12, 2021 at 4:53 PM Sean Christopherson <seanjc@google.com> wrote:
> > On Fri, Nov 12, 2021, Peter Gonda wrote:
> > > Having a way for userspace to lock pages as shared was an idea I just
> > > proposed the simplest solution to start the conversation.
> >
> > Assuming you meant that to read:
> >
> > Having a way for userspace to lock pages as shared is an alternative idea; I
> > just proposed the simplest solution to start the conversation.
> >
> > The unmapping[*] guest private memory proposal is essentially that, a way for userspace
> > to "lock" the state of a page by requiring all conversions to be initiated by userspace
> > and by providing APIs to associate a pfn 1:1 with a KVM instance, i.e. lock a pfn to
> > a guest.
> >
> > Andy's DMA example brings up a very good point though. If the shared and private
> > variants of a given GPA are _not_ required to point at a single PFN, which is the
> > case in the current unmapping proposal, userspace doesn't need to do any additional
> > juggling to track guest conversions across multiple processes.
> >
> > Any process that's accessing guest (shared!) memory simply does its locking as normal,
> > which as Andy pointed out, is needed for correctness today. If the guest requests a
> > conversion from shared=>private without first ensuring the gfn is unused (by a host
> > "device"), the host will side will continue accessing the old, shared memory, which it
> > locked, while the guest will be doing who knows what. And if the guest provides a GPA
> > that isn't mapped shared in the VMM's address space, it's conceptually no different
> > than if the guest provided a completely bogus GPA, which again needs to be handled today.
> >
> > In other words, if done properly, differentiating private from shared shouldn't be a
> > heavy lift for host userspace.
> >
> > [*] Actually unmapping memory may not be strictly necessary for SNP because a
> > #PF(RMP) is likely just as good as a #PF(!PRESENT) when both are treated as
> > fatal, but the rest of the proposal that allows KVM to understand the stage
> > of a page and exit to userspace accordingly applies.
>
> Thanks for this explanation. When you write "while the guest will be
> doing who knows what":
>
> Isn't that a large weakness of this proposal? To me, it seems better
> for debuggability to corrupt the private memory (i.e., convert the
> page to shared) so the guest can detect the issue via a PVALIDATE
> failure.

The behavior is no different than it is today for regular VMs.

> The main issue I see with corrupting the guest memory is that we may
> not know whether the host is at fault or the guest.

Yes, one issue is that bugs in the host will result in downstream errors in the
guest, as opposed to immediate, synchronous detection in the guest. IMO that is
a significant flaw.

Another issue is that the host kernel, which despite being "untrusted", absolutely
should be acting in the best interests of the guest. Allowing userspace to inject
#VC, e.g. to attempt to attack the guest by triggering a spurious PVALIDATE, means
the kernel is failing miserably on that front.