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Date:   Fri, 12 May 2023 12:04:55 +0100
From:   Matthew Garrett <mjg59@srcf.ucam.org>
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     Ross Philipson <ross.philipson@oracle.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
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        trenchboot-devel@googlegroups.com
Subject: Re: [PATCH v6 06/14] x86: Add early SHA support for Secure Launch
 early measurements
Message-ID: <20230512110455.GD14461@srcf.ucam.org>
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 <20230504145023.835096-7-ross.philipson@oracle.com>
 <20230510012144.GA1851@quark.localdomain>
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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.