Re: [PATCH v6 06/14] x86: Add early SHA support for Secure Launch early measurements

[Ard Biesheuvel]

On Fri, 12 May 2023 at 13:04, Matthew Garrett <mjg59@xxxxxxxxxxxxx> wrote:
>
> On Tue, May 09, 2023 at 06:21:44PM -0700, Eric Biggers wrote:
>
> > SHA-1 is insecure.  Why are you still using SHA-1?  Don't TPMs support SHA-2
> > now?
>
> TXT is supported on some TPM 1.2 systems as well. TPM 2 systems are also
> at the whim of the firmware in terms of whether the SHA-2 banks are
> enabled. But even if the SHA-2 banks are enabled, if you suddenly stop
> extending the SHA-1 banks, a malicious actor can later turn up and
> extend whatever they want into them and present a SHA-1-only
> attestation. Ideally whatever is handling that attestation should know
> whether or not to expect an attestation with SHA-2, but the easiest way
> to maintain security is to always extend all banks.
>

Wouldn't it make more sense to measure some terminating event into the
SHA-1 banks instead?