linux-kernel - Re: [PATCH v9 06/19] x86: Add early SHA-1 support for Secure Launch early measurements

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Message-ID: <20240828231350.GA558903@google.com>
Date: Wed, 28 Aug 2024 23:13:50 +0000
From: Eric Biggers <ebiggers@...nel.org>
To: ross.philipson@...cle.com
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-integrity@...r.kernel.org, linux-doc@...r.kernel.org,
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	trenchboot-devel@...glegroups.com
Subject: Re: [PATCH v9 06/19] x86: Add early SHA-1 support for Secure Launch
 early measurements

On Wed, Aug 28, 2024 at 01:14:45PM -0700, ross.philipson@...cle.com wrote:
> On 8/27/24 11:14 AM, 'Eric Biggers' via trenchboot-devel wrote:
> > On Thu, May 30, 2024 at 07:16:56PM -0700, Eric Biggers wrote:
> > > On Thu, May 30, 2024 at 06:03:18PM -0700, Ross Philipson wrote:
> > > > From: "Daniel P. Smith" <dpsmith@...rtussolutions.com>
> > > > 
> > > > For better or worse, Secure Launch needs SHA-1 and SHA-256. The
> > > > choice of hashes used lie with the platform firmware, not with
> > > > software, and is often outside of the users control.
> > > > 
> > > > Even if we'd prefer to use SHA-256-only, if firmware elected to start us
> > > > with the SHA-1 and SHA-256 backs active, we still need SHA-1 to parse
> > > > the TPM event log thus far, and deliberately cap the SHA-1 PCRs in order
> > > > to safely use SHA-256 for everything else.
> > > > 
> > > > The SHA-1 code here has its origins in the code from the main kernel:
> > > > 
> > > > commit c4d5b9ffa31f ("crypto: sha1 - implement base layer for SHA-1")
> > > > 
> > > > A modified version of this code was introduced to the lib/crypto/sha1.c
> > > > to bring it in line with the SHA-256 code and allow it to be pulled into the
> > > > setup kernel in the same manner as SHA-256 is.
> > > > 
> > > > Signed-off-by: Daniel P. Smith <dpsmith@...rtussolutions.com>
> > > > Signed-off-by: Ross Philipson <ross.philipson@...cle.com>
> > > 
> > > Thanks.  This explanation doesn't seem to have made it into the actual code or
> > > documentation.  Can you please get it into a more permanent location?
> > 
> > I see that a new version of the patchset was sent out but this suggestion was
> > not taken.  Are you planning to address it?
> 
> Sorry we sort of overlooked that part of the request. We will take the
> latest commit message, clean it up a little and put it in
> boot/compressed/sha1.c file as a comment. I believe that is what you would
> like us to do.
> 

Do users of this feature need to make a decision about SHA-1?  If so there needs
to be guidance in Documentation/.  A comment in a .c file is not user facing.

- Eric