[PATCH 07/18] asymmetric keys: explicitly add the leading zero byte to encoded message

["Lee, Chun-Yi" <joeyli.kernel@gmail.com>]

Per PKCS1 spec, the EMSA-PKCS1-v1_5 encoded message is leading by 0x00 0x01 in
its first 2 bytes. The leading zero byte is suppressed by MPI so we pass a
pointer to the _preceding_ byte to RSA_verify() in original code, but it has
risk for the byte is not zero because it's not in EM buffer's scope, neither
RSA_verify() nor mpi_get_buffer() didn't take care the leading byte.

To avoid the risk, that's better we explicitly add the leading zero byte to EM
for pass to RSA_verify(). This patch allocate a _EM buffer to capture the
result from RSA_I2OSP(), then set the first byte to zero in EM and copy the
remaining bytes from _EM.

Reviewed-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Lee, Chun-Yi <jlee@suse.com>
---
 crypto/asymmetric_keys/rsa.c |   14 ++++++++++----
 1 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/crypto/asymmetric_keys/rsa.c b/crypto/asymmetric_keys/rsa.c
index e60defe..1fadc7f 100644
--- a/crypto/asymmetric_keys/rsa.c
+++ b/crypto/asymmetric_keys/rsa.c
@@ -401,6 +401,7 @@ static int RSA_verify_signature(const struct public_key *key,
 	/* Variables as per RFC3447 sec 8.2.2 */
 	const u8 *H = sig->digest;
 	u8 *EM = NULL;
+	u8 *_EM = NULL;
 	MPI m = NULL;
 	size_t k;
 
@@ -435,14 +436,19 @@ static int RSA_verify_signature(const struct public_key *key,
 	/* (2c) Convert the message representative (m) to an encoded message
 	 *      (EM) of length k octets.
 	 *
-	 *      NOTE!  The leading zero byte is suppressed by MPI, so we pass a
-	 *      pointer to the _preceding_ byte to RSA_verify()!
+	 *      NOTE!  The leading zero byte is suppressed by MPI, so we add it
+	 *      back to EM before input to RSA_verify()!
 	 */
-	ret = RSA_I2OSP(m, k, &EM);
+	ret = RSA_I2OSP(m, k, &_EM);
 	if (ret < 0)
 		goto error;
 
-	ret = RSA_verify(H, EM - 1, k, sig->digest_size,
+	EM = kmalloc(k, GFP_KERNEL);
+	memset(EM, 0, 1);
+	memcpy(EM + 1, _EM, k-1);
+	kfree(_EM);
+
+	ret = RSA_verify(H, EM, k, sig->digest_size,
 			 RSA_ASN1_templates[sig->pkey_hash_algo].data,
 			 RSA_ASN1_templates[sig->pkey_hash_algo].size);
 
-- 
1.6.4.2