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

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Date:   Fri, 12 May 2023 13:18:45 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Matthew Garrett <mjg59@...f.ucam.org>
Cc:     Eric Biggers <ebiggers@...nel.org>,
        Ross Philipson <ross.philipson@...cle.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-integrity@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-crypto@...r.kernel.org, iommu@...ts.linux-foundation.org,
        kexec@...ts.infradead.org, linux-efi@...r.kernel.org,
        dpsmith@...rtussolutions.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, James.Bottomley@...senpartnership.com,
        luto@...capital.net, nivedita@...m.mit.edu,
        kanth.ghatraju@...cle.com, trenchboot-devel@...glegroups.com
Subject: Re: [PATCH v6 06/14] x86: Add early SHA support for Secure Launch
 early measurements

On Fri, 12 May 2023 at 13:04, Matthew Garrett <mjg59@...f.ucam.org> 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?