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

[David Howells <dhowells@redhat.com>]

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",		"-" },
+};
+
+/*
+ * 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.