diff mbox

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

Message ID 1377169317-5959-8-git-send-email-jlee@suse.com (mailing list archive)
State RFC, archived
Headers show

Commit Message

Chun-Yi Lee Aug. 22, 2013, 11:01 a.m. UTC 
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(-)

Comments

Pavel Machek Aug. 25, 2013, 4:13 p.m. UTC | #1 
On Thu 2013-08-22 19:01:46, Lee, Chun-Yi wrote:
> 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>

> -	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);

Spot a crash waiting to happen.
									Pavel
diff mbox

Patch

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);