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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