Re: [PATCH v13 11/12] x509, pkcs7: Limit crypto combinations that may be used for module signing

Ignat Korchagin
2026-01-20T18:31:16Z

Hi David,

On Tue, Jan 20, 2026 at 2:52 PM David Howells <dhowells@redhat.com> wrote:
>
> Limit the set of crypto combinations that may be used for module signing as
> no indication of hash algorithm used for signing is added to the hash of
> the data, so in theory a data blob hashed with a different algorithm can be
> substituted provided it has the same hash output.
>
> This also rejects the use of less secure algorithms.
>
> 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/public_key.c | 55 +++++++++++++++++++++++++++--
> 1 file changed, 53 insertions(+), 2 deletions(-)
>
> diff --git a/crypto/asymmetric_keys/public_key.c b/crypto/asymmetric_keys/public_key.c
> index 13a5616becaa..90b98e1a952d 100644
> --- a/crypto/asymmetric_keys/public_key.c
> +++ b/crypto/asymmetric_keys/public_key.c
> @@ -24,6 +24,52 @@ MODULE_DESCRIPTION("In-software asymmetric public-key subtype");
> MODULE_AUTHOR("Red Hat, Inc.");
> MODULE_LICENSE("GPL");
>
> +struct public_key_restriction {
> + const char *pkey_algo; /* Signing algorithm (e.g. "rsa") */
> + const char *pkey_enc; /* Signature encoding (e.g. "pkcs1") */
> + const char *hash_algo; /* Content hash algorithm (e.g. "sha256") */
> +};
> +
> +static const struct public_key_restriction public_key_restrictions[] = {
> + /* algo encoding hash */
> + { "rsa", "pkcs1", "sha256" },
> + { "rsa", "pkcs1", "sha384" },
> + { "rsa", "pkcs1", "sha512" },
> + { "rsa", "emsa-pss", "sha512" },

Don't we want to allow sha256 for emsa-pss?

> + { "ecdsa", "x962", "sha256" },
> + { "ecdsa", "x962", "sha384" },
> + { "ecdsa", "x962", "sha512" },
> + { "ecrdsa", "raw", "sha256" },
> + { "ecrdsa", "raw", "sha384" },
> + { "ecrdsa", "raw", "sha512" },
> + { "mldsa44", "raw", "sha512" },
> + { "mldsa65", "raw", "sha512" },
> + { "mldsa87", "raw", "sha512" },
> + /* ML-DSA may also do its own hashing over the entire message. */
> + { "mldsa44", "raw", "-" },
> + { "mldsa65", "raw", "-" },
> + { "mldsa87", "raw", "-" },
> +};
> +
> +/*
> + * Determine if a particular key/hash combination is allowed.
> + */
> +static int is_public_key_sig_allowed(const struct public_key_signature *sig)
> +{
> + for (int i = 0; i < ARRAY_SIZE(public_key_restrictions); i++) {
> + if (strcmp(public_key_restrictions[i].pkey_algo, sig->pkey_algo) != 0)
> + continue;
> + if (strcmp(public_key_restrictions[i].pkey_enc, sig->encoding) != 0)
> + continue;
> + if (strcmp(public_key_restrictions[i].hash_algo, sig->hash_algo) != 0)
> + continue;
> + return 0;
> + }
> + pr_warn_once("Public key signature combo (%s,%s,%s) rejected\n",
> + sig->pkey_algo, sig->encoding, sig->hash_algo);
> + return -EKEYREJECTED;
> +}
> +
> /*
> * Provide a part of a description of the key for /proc/keys.
> */
> @@ -391,12 +437,17 @@ int public_key_verify_signature(const struct public_key *pkey,
> bool issig;
> int ret;
>
> - pr_devel("==>%s()\n", __func__);
> -
> BUG_ON(!pkey);
> BUG_ON(!sig);
> BUG_ON(!sig->s);
>
> + ret = is_public_key_sig_allowed(sig);
> + if (ret < 0)
> + return ret;
> +
> + pr_devel("==>%s(%s,%s,%s)\n",
> + __func__, sig->pkey_algo, sig->encoding, sig->hash_algo);
> +
> /*
> * If the signature specifies a public key algorithm, it *must* match
> * the key's actual public key algorithm.
>

Ignat