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

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Date: Thu, 30 May 2024 19:16:56 -0700
From: Eric Biggers <ebiggers@...nel.org>
To: Ross Philipson <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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Subject: Re: [PATCH v9 06/19] x86: Add early SHA-1 support for Secure Launch
 early measurements

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?

Also, can you point to where the "deliberately cap the SHA-1 PCRs" thing happens
in the code?

That paragraph is also phrased as a hypothetical, "Even if we'd prefer to use
SHA-256-only".  That implies that you do not, in fact, prefer SHA-256 only.  Is
that the case?  Sure, maybe there are situations where you *have* to use SHA-1,
but why would you not at least *prefer* SHA-256?

> /*
>  * An implementation of SHA-1's compression function.  Don't use in new code!
>  * You shouldn't be using SHA-1, and even if you *have* to use SHA-1, this isn't
>  * the correct way to hash something with SHA-1 (use crypto_shash instead).
>  */
> #define SHA1_DIGEST_WORDS	(SHA1_DIGEST_SIZE / 4)
> #define SHA1_WORKSPACE_WORDS	16
> void sha1_init(__u32 *buf);
> void sha1_transform(__u32 *digest, const char *data, __u32 *W);
>+void sha1(const u8 *data, unsigned int len, u8 *out);

Also, the comment above needs to be updated.

- Eric