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

Andy Lutomirski
2021-11-16T05:00:59Z

On Mon, Nov 15, 2021, at 10:41 AM, Marc Orr wrote:
> 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.

I don’t understand the point you’re trying to make. If the host _kernel_has a bug that allows a guest to trigger invalid host memory access, this is bad. We want to know about it and fix it, abcs the security folks want to minimize the chance that such a bug exists.

If host _userspace_ such a bug, the kernel should not crash if it’s exploited.