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

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On 5/30/24 7:16 PM, Eric Biggers wrote:
On Thu, May 30, 2024 at 06:03:18PM -0700, Ross Philipson wrote:
From: "Daniel P. Smith" <dpsmith@xxxxxxxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ross Philipson <ross.philipson@xxxxxxxxxx>

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?

Yes those are fair points. We will address them and indicate we prefer SHA-256 or better.


/*
  * 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.

Ack, will address.

Thank you


- Eric





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