Messages in this thread | | | Date | Wed, 20 Oct 2021 20:08:39 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH v6 08/42] x86/sev-es: initialize sev_status/features within #VC handler |
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On Wed, Oct 20, 2021 at 11:10:23AM -0500, Michael Roth wrote: > > 1. Code checks SME/SEV support leaf. HV lies and says there's none. So > > guest doesn't boot encrypted. Oh well, not a big deal, the cloud vendor > > won't be able to give confidentiality to its users => users go away or > > do unencrypted like now. > > > > Problem is solved by political and economical pressure. > > > > 2. Check SEV and SME bit. HV lies here. Oh well, same as the above. > > I'd be worried about the possibility that, through some additional exploits > or failures in the attestation flow,
Well, that puts forward an important question: how do you verify *reliably* that this is an SNP guest?
- attestation?
- CPUID?
- anything else?
I don't see this written down anywhere. Because this assumption will guide the design in the kernel.
> a guest owner was tricked into booting unencrypted on a compromised > host and exposing their secrets. Their attestation process might even > do some additional CPUID sanity checks, which would at the point > be via the SNP CPUID table and look legitimate, unaware that the > kernel didn't actually use the SNP CPUID table until after 0x8000001F > was parsed (if we were to only initialize it after/as-part-of > sme_enable()).
So what happens with that guest owner later?
How is she to notice that she booted unencrypted?
> Fortunately in this scenario I think the guest kernel actually would fail to > boot due to the SNP hardware unconditionally treating code/page tables as > encrypted pages. I tested some of these scenarios just to check, but not > all, and I still don't feel confident enough about it to say that there's > not some way to exploit this by someone who is more clever/persistant than > me.
All this design needs to be preceded with: "We protect against cases A, B and C and not against D, E, etc."
So that it is clear to all parties involved what we're working with and what we're protecting against and what we're *not* protecting against.
End of mail 2, more later.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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