Re: [PATCH v6 08/42] x86/sev-es: initialize sev_status/features within #VC handler

Dr. David Alan Gilbert
2021-10-21T15:56:18Z

* Borislav Petkov (bp@alien8.de) wrote:
> On Wed, Oct 20, 2021 at 11:10:23AM -0500, Michael Roth wrote:
> > At which point we then switch to using the CPUID table? But at that
> > point all the previous CPUID checks, both SEV-related/non-SEV-related,
> > are now possibly not consistent with what's in the CPUID table. Do we
> > then revalidate?
>
> Well, that's a tough question. That's basically the same question as,
> does Linux support heterogeneous cores and can it handle hardware
> features which get enabled after boot. The perfect example is, late
> microcode loading which changes CPUID bits and adds new functionality.
>
> And the answer to that is, well, hard. You need to decide this on a
> case-by-case basis.

I can imagine a malicious hypervisor trying to return different cpuid
answers to different threads or even the same thread at different times.

> But isn't it that the SNP CPUID page will be parsed early enough anyway
> so that kernel proper will see only SNP CPUID info and init properly
> using that?
>
> > Even a non-malicious hypervisor might provide inconsistent values
> > between the two sources due to bugs, or SNP validation suppressing
> > certain feature bits that hypervisor otherwise exposes, etc.
>
> There's also migration, lemme point to a very recent example:
>
> https://lore.kernel.org/r/20211021104744.24126-1-jane.malalane@citrix.com

Ewww.

> which is exactly what you say - a non-malicious HV taking care of its
> migration pool. So how do you handle that?

Well, the spec (AMD 56860 SEV spec) says:

'If firmware encounters a CPUID function that is in the standard or extended ranges, then the
firmware performs a check to ensure that the provided output would not lead to an insecure guest
state'

so I take that 'firmware' to be the PSP; that wording doesn't say that
it checks that the CPUID is identical, just that it 'would not lead to
an insecure guest' - so a hypervisor could hide any 'no longer affected
by' flag for all the CPUs in it's migration pool and the firmware
shouldn't complain; so it should be OK to pessimise.

Dave

> > Now all the code after sme_enable() can potentially take unexpected
> > execution paths, where post-sme_enable() code makes assumptions about
> > pre-sme_enable() checks that may no longer hold true.
>
> So as I said above, if you parse SNP CPUID page early enough, you don't
> have to worry about feature rediscovery. Early enough means, before
> identify_boot_cpu().
>
> --
> Regards/Gruss,
> Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette
>
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK