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Date:   Thu, 21 Oct 2021 18:55:14 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc:     Michael Roth <michael.roth@amd.com>,
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Subject: Re: [PATCH v6 08/42] x86/sev-es: initialize sev_status/features
 within #VC handler
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On Thu, Oct 21, 2021 at 04:56:09PM +0100, Dr. David Alan Gilbert wrote:
> I can imagine a malicious hypervisor trying to return different cpuid
> answers to different threads or even the same thread at different times.

Haha, I guess that will fail not because of SEV* but because of the
kernel not really being able to handle heterogeneous CPUIDs.

> 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.

AFAIU this, I think this would depend on "[t]he policy used by the
firmware to assess CPUID function output can be found in [PPR]."

So if the HV sets the "no longer affected by" flag but the firmware
deems this set flag as insecure, I'm assuming the firmare will clear
it when it returns the CPUID leafs. I guess I need to go find that
policy...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette