crypto: asymmetric_keys - Adjust signature size calculation for NIST P521

Adjust the calculation of the maximum signature size for support of
NIST P521. While existing curves may prepend a 0 byte to their coordinates
(to make the number positive), NIST P521 will not do this since only the
first bit in the most significant byte is used.

If the encoding of the x & y coordinates requires at least 128 bytes then
an additional byte is needed for the encoding of the length. Take this into
account when calculating the maximum signature size.
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Tested-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/asymmetric_keys/public_key.c

@@ -234,6 +234,7 @@ static int software_key_query(const struct kernel_pkey_params *params,
 	info->key_size = len * 8;
 
 	if (strncmp(pkey->pkey_algo, "ecdsa", 5) == 0) {
+		int slen = len;
 		/*
 		 * ECDSA key sizes are much smaller than RSA, and thus could
 		 * operate on (hashed) inputs that are larger than key size.
@@ -247,8 +248,19 @@ static int software_key_query(const struct kernel_pkey_params *params,
 		 * Verify takes ECDSA-Sig (described in RFC 5480) as input,
 		 * which is actually 2 'key_size'-bit integers encoded in
 		 * ASN.1.  Account for the ASN.1 encoding overhead here.
+		 *
+		 * NIST P192/256/384 may prepend a '0' to a coordinate to
+		 * indicate a positive integer. NIST P521 never needs it.
 		 */
-		info->max_sig_size = 2 * (len + 3) + 2;
+		if (strcmp(pkey->pkey_algo, "ecdsa-nist-p521") != 0)
+			slen += 1;
+		/* Length of encoding the x & y coordinates */
+		slen = 2 * (slen + 2);
+		/*
+		 * If coordinate encoding takes at least 128 bytes then an
+		 * additional byte for length encoding is needed.
+		 */
+		info->max_sig_size = 1 + (slen >= 128) + 1 + slen;
 	} else {
 		info->max_data_size = len;
 		info->max_sig_size = len;