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

Dr. David Alan Gilbert
2021-11-15T12:31:47Z

* 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?

Still; I wonder if it's best to kill the guest - maybe it's best for
the host to kill the guest and leave behind diagnostics of what
happened; for someone debugging the crash, it's going to be less useful
to know that page X was wrongly accessed (which is what the guest would
see), and more useful to know that it was the kernel's vhost-... driver
that accessed it.

Dave

> On Fri, Nov 12, 2021, Peter Gonda wrote:
> > Here is an alternative to the current approach: On RMP violation (host
> > or userspace) the page fault handler converts the page from private to
> > shared to allow the write to continue. This pulls from s390’s error
> > handling which does exactly this. See ‘arch_make_page_accessible()’.
>
> Ah, after further reading, s390 does _not_ do implicit private=>shared conversions.
>
> s390's arch_make_page_accessible() is somewhat similar, but it is not a direct
> comparison. IIUC, it exports and integrity protects the data and thus preserves
> the guest's data in an encrypted form, e.g. so that it can be swapped to disk.
> And if the host corrupts the data, attempting to convert it back to secure on a
> subsequent guest access will fail.
>
> The host kernel's handling of the "convert to secure" failures doesn't appear to
> be all that robust, e.g. it looks like there are multiple paths where the error
> is dropped on the floor and the guest is resumed , but IMO soft hanging the guest
> is still better than inducing a fault in the guest, and far better than potentially
> coercing the guest into reading corrupted memory ("spurious" PVALIDATE). And s390's
> behavior is fixable since it's purely a host error handling problem.
>
> To truly make a page shared, s390 requires the guest to call into the ultravisor
> to make a page shared. And on the host side, the host can pin a page as shared
> to prevent the guest from unsharing it while the host is accessing it as a shared
> page.
>
> So, inducing #VC is similar in the sense that a malicious s390 can also DoS itself,
> but is quite different in that (AFAICT) s390 does not create an attack surface where
> a malicious or buggy host userspace can induce faults in the guest, or worst case in
> SNP, exploit a buggy guest into accepting and accessing corrupted data.
>
> It's also different in that s390 doesn't implicitly convert between shared and
> private. Functionally, it doesn't really change the end result because a buggy
> host that writes guest private memory will DoS the guest (by inducing a #VC or
> corrupting exported data), but at least for s390 there's a sane, legitimate use
> case for accessing guest private memory (swap and maybe migration?), whereas for
> SNP, IMO implicitly converting to shared on a host access is straight up wrong.
>
> > Additionally it adds less complexity to the SNP kernel patches, and
> > requires no new ABI.
>
> I disagree, this would require "new" ABI in the sense that it commits KVM to
> supporting SNP without requiring userspace to initiate any and all conversions
> between shared and private. Which in my mind is the big elephant in the room:
> do we want to require new KVM (and kernel?) ABI to allow/force userspace to
> explicitly declare guest private memory for TDX _and_ SNP, or just TDX?
>
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK