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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Fri, 12 May 2023 13:18:45 +0200
Message-ID: <CAMj1kXE8m5jCH3vW54ys=dE2-Vf_gnnueR6_g4Rq-LSJ5BqRjA@mail.gmail.com>
Subject: Re: [PATCH v6 06/14] x86: Add early SHA support for Secure Launch
 early measurements
To:     Matthew Garrett <mjg59@srcf.ucam.org>
Cc:     Eric Biggers <ebiggers@kernel.org>,
        Ross Philipson <ross.philipson@oracle.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        linux-integrity@vger.kernel.org, linux-doc@vger.kernel.org,
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        dpsmith@apertussolutions.com, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, hpa@zytor.com, James.Bottomley@hansenpartnership.com,
        luto@amacapital.net, nivedita@alum.mit.edu,
        kanth.ghatraju@oracle.com, trenchboot-devel@googlegroups.com
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On Fri, 12 May 2023 at 13:04, Matthew Garrett <mjg59@srcf.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?