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

[ross . philipson]

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