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Message-ID: <1ac403a4-6ab5-4943-9cd9-ed3b8edde661@gmail.com>
Date: Thu, 6 Feb 2025 22:28:44 -0600
From: Jacob Bachmeyer <jcb62281@...il.com>
To: trinity pointard <trinity.pointard@...il.com>,
oss-security@...ts.openwall.com
Subject: Re: AMD Microcode Signature Verification Vulnerability
On 2/6/25 17:04, trinity pointard wrote:
>> If an attacker is able to control the hypervisor (necessary to load
>> rogue microcode) and the processor microcode, how can the VM trust that
>> it is actually verifying that attestation and not being sent down a "oh
>> yes it is exactly what you want it to be" garden path?
> Attestations are cryptographically signed by the cpu, and meant to be sent
> elsewhere and verified remotely.
That resolves the issue for the VM owner, but still does not address the
more interesting question: is there a way on current AMD processors to
perform calculations that cannot be upset by tampered microcode? (There
*was* a subset of instructions on the AMD K8 like that.)
> The key used to sign (VCEK) are dependent on
> the microcode version, so it shouldn't be possible to forge new-looking
> signature with old microcodes (i would hope this hold would someone be able to
> decrypt a microcode, though i couldn't find information on that subject).
If you are correct that the /actual signing key/ used depends on the
microcode version, then (logically) the signing key *must* be somewhere
in the microcode. If someone finds a way to decrypt the microcode, for
which all keys required must be *somewhere* in every processor that uses
that microcode, they would clearly be able to extract the attestation
signing key.
I would hope that you are mistaken in that statement that the signing
key depends on the microcode version or that we are both missing
something somewhere.
-- Jacob
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