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Date:   Tue, 18 Jan 2022 18:41:16 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Michael Roth <michael.roth@amd.com>
Cc:     Brijesh Singh <brijesh.singh@amd.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
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        Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ard Biesheuvel <ardb@kernel.org>,
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Subject: Re: [PATCH v8 29/40] x86/compressed/64: add support for SEV-SNP
 CPUID table in #VC handlers
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On Tue, Jan 18, 2022 at 11:20:43AM -0600, Michael Roth wrote:
> The HV fills out the initial contents of the CPUID page, which includes
> the count. SNP/PSP firmware will validate the contents the HV tries to put
> in the initial page, but does not currently enforce that the 'count' field
> is non-zero.

So if the HV sets count to 0, then the PSP can validate all it wants but
you basically don't have a CPUID page. And that's a pretty easy way to
defeat it, if you ask me.

So, if it is too late to change this, I guess the only way out of here
is to terminate the guest on count == 0.

And regardless, what if the HV fakes the count - how do you figure out
what the proper count is? You go and read the whole CPUID page and try
to make sense of what's there, even beyond the "last" function leaf.

> So we can't rely on the 'count' field as an indicator of whether or
> not the CPUID page is active, we need to rely on the presence of the
> ccblob as the true indicator, then treat a non-zero 'count' field as
> an invalid state.

treat a non-zero count field as invalid?

You mean, "a zero count" maybe...

But see above, how do you check whether the HV hasn't "hidden" some
entries by modifying the count field?

Either I'm missing something or this sounds really weird...

-- 
Regards/Gruss,
    Boris.

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