On Mon, Nov 15, 2021, Dr. David Alan Gilbert wrote:

* Sean Christopherson (seanjc@google.com) wrote:

quoted

On Fri, Nov 12, 2021, Borislav Petkov wrote:

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On Fri, Nov 12, 2021 at 09:59:46AM -0800, Dave Hansen wrote:

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Or, is there some mechanism that prevent guest-private memory from being
accessed in random host kernel code?

Or random host userspace code...

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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".

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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.