Re: [PATCH v13 02/12] pkcs7: Allow the signing algo to calculate the digest itself

Jarkko Sakkinen
2026-01-20T17:53:11Z

On Tue, Jan 20, 2026 at 02:50:48PM +0000, David Howells wrote:
> The ML-DSA public key algorithm really wants to calculate the message
> digest itself, rather than having the digest precalculated and fed to it
> separately as RSA does[*]. The kernel's PKCS#7 parser, however, is
> designed around the latter approach.
>
> [*] ML-DSA does allow for an "external mu", but CMS doesn't yet have that
> standardised.
>
> Fix this by noting in the public_key_signature struct when the signing
> algorithm is going to want this and then, rather than doing the digest of
> the authenticatedAttributes ourselves and overwriting the sig->digest with
> that, replace sig->digest with a copy of the contents of the
> authenticatedAttributes section and adjust the digest length to match.
>
> This will then be fed to the public key algorithm as normal which can do
> what it wants with the data.
>
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Lukas Wunner <lukas@wunner.de>
> cc: Ignat Korchagin <ignat@cloudflare.com>
> cc: Stephan Mueller <smueller@chronox.de>
> cc: Eric Biggers <ebiggers@kernel.org>
> cc: Herbert Xu <herbert@gondor.apana.org.au>
> cc: keyrings@vger.kernel.org
> cc: linux-crypto@vger.kernel.org
> ---
> crypto/asymmetric_keys/pkcs7_parser.c | 4 +--
> crypto/asymmetric_keys/pkcs7_verify.c | 48 ++++++++++++++++++---------
> include/crypto/public_key.h | 1 +
> 3 files changed, 36 insertions(+), 17 deletions(-)
>
> diff --git a/crypto/asymmetric_keys/pkcs7_parser.c b/crypto/asymmetric_keys/pkcs7_parser.c
> index 423d13c47545..3cdbab3b9f50 100644
> --- a/crypto/asymmetric_keys/pkcs7_parser.c
> +++ b/crypto/asymmetric_keys/pkcs7_parser.c
> @@ -599,8 +599,8 @@ int pkcs7_sig_note_set_of_authattrs(void *context, size_t hdrlen,
> }
>
> /* We need to switch the 'CONT 0' to a 'SET OF' when we digest */
> - sinfo->authattrs = value - (hdrlen - 1);
> - sinfo->authattrs_len = vlen + (hdrlen - 1);
> + sinfo->authattrs = value - hdrlen;
> + sinfo->authattrs_len = vlen + hdrlen;
> return 0;
> }
>
> diff --git a/crypto/asymmetric_keys/pkcs7_verify.c b/crypto/asymmetric_keys/pkcs7_verify.c
> index 6d6475e3a9bf..0f9f515b784d 100644
> --- a/crypto/asymmetric_keys/pkcs7_verify.c
> +++ b/crypto/asymmetric_keys/pkcs7_verify.c
> @@ -70,8 +70,6 @@ static int pkcs7_digest(struct pkcs7_message *pkcs7,
> * digest we just calculated.
> */
> if (sinfo->authattrs) {
> - u8 tag;
> -
> if (!sinfo->msgdigest) {
> pr_warn("Sig %u: No messageDigest\n", sinfo->index);
> ret = -EKEYREJECTED;
> @@ -97,20 +95,40 @@ static int pkcs7_digest(struct pkcs7_message *pkcs7,
> * as the contents of the digest instead. Note that we need to
> * convert the attributes from a CONT.0 into a SET before we
> * hash it.
> + *
> + * However, for certain algorithms, such as ML-DSA, the digest
> + * is integrated into the signing algorithm. In such a case,
> + * we copy the authattrs, modifying the tag type, and set that
> + * as the digest.
> */
> - memset(sig->digest, 0, sig->digest_size);
> -
> - ret = crypto_shash_init(desc);
> - if (ret < 0)
> - goto error;
> - tag = ASN1_CONS_BIT | ASN1_SET;
> - ret = crypto_shash_update(desc, &tag, 1);
> - if (ret < 0)
> - goto error;
> - ret = crypto_shash_finup(desc, sinfo->authattrs,
> - sinfo->authattrs_len, sig->digest);
> - if (ret < 0)
> - goto error;
> + if (sig->algo_does_hash) {
> + kfree(sig->digest);
> +
> + ret = -ENOMEM;
> + sig->digest = kmalloc(umax(sinfo->authattrs_len, sig->digest_size),
> + GFP_KERNEL);
> + if (!sig->digest)
> + goto error_no_desc;
> +
> + sig->digest_size = sinfo->authattrs_len;
> + memcpy(sig->digest, sinfo->authattrs, sinfo->authattrs_len);
> + ((u8 *)sig->digest)[0] = ASN1_CONS_BIT | ASN1_SET;
> + ret = 0;
> + } else {
> + u8 tag = ASN1_CONS_BIT | ASN1_SET;
> +
> + ret = crypto_shash_init(desc);
> + if (ret < 0)
> + goto error;
> + ret = crypto_shash_update(desc, &tag, 1);
> + if (ret < 0)
> + goto error;
> + ret = crypto_shash_finup(desc, sinfo->authattrs + 1,
> + sinfo->authattrs_len - 1,
> + sig->digest);
> + if (ret < 0)
> + goto error;
> + }
> pr_devel("AADigest = [%*ph]\n", 8, sig->digest);
> }
>
> diff --git a/include/crypto/public_key.h b/include/crypto/public_key.h
> index 81098e00c08f..e4ec8003a3a4 100644
> --- a/include/crypto/public_key.h
> +++ b/include/crypto/public_key.h
> @@ -46,6 +46,7 @@ struct public_key_signature {
> u8 *digest;
> u32 s_size; /* Number of bytes in signature */
> u32 digest_size; /* Number of bytes in digest */
> + bool algo_does_hash; /* Public key algo does its own hashing */

I'd use the wording you used already in commit message, which
factors more descriptive than what you have here. E.g., name
it "external_digest".

It would be easier to digest this when revisiting the code later...

> const char *pkey_algo;
> const char *hash_algo;
> const char *encoding;
>

Allocation scheme is not the prettiest but I neither have
anything other to offer, so other than the rename request,
I think this is acceptable.

BR, Jarkko