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

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Jan 20, 2026 at 02:50:57PM +0000, David Howells 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" },
> +	{ "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",		"-" },
> +};

Have you read software_key_determine_akcipher()?  It's the place where
the encoding and hash_algo are validated currently.  This commit adds a
second set of slightly different checks alongside the existing ones.
It's unclear whether the existing checks were considered.

Also, the ML-DSA and RSASSA-PSS support is new in this patchset, and
this commit is a fix for it.  Instead of committing buggy code that is
fixed by a later commit, it's preferable to commit correct code in the
first place.

- Eric