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Date:   Fri, 12 May 2023 12:28:47 +0100
From:   Matthew Garrett <mjg59@srcf.ucam.org>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Eric Biggers <ebiggers@kernel.org>,
        Ross Philipson <ross.philipson@oracle.com>,
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        kanth.ghatraju@oracle.com, trenchboot-devel@googlegroups.com
Subject: Re: [PATCH v6 06/14] x86: Add early SHA support for Secure Launch
 early measurements
Message-ID: <20230512112847.GF14461@srcf.ucam.org>
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On Fri, May 12, 2023 at 01:18:45PM +0200, Ard Biesheuvel wrote:
> 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?

Unless we assert that SHA-1 events are unsupported, it seems a bit odd 
to force a policy on people who have both banks enabled. People with 
mixed fleets are potentially going to be dealing with SHA-1 measurements 
for a while yet, and while there's obviously a security benefit in using 
SHA-2 instead it'd be irritating to have to maintain two attestation 
policies.