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Subject[PATCH] asymmetric keys: explicitly add the leading zero byte to encoded message
Date
From: Chun-Yi Lee <jlee@suse.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.

Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Josh Boyer <jwboyer@redhat.com>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Chun-Yi Lee <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 ca1a4f3..7bc99d2 100644
--- a/crypto/asymmetric_keys/rsa.c
+++ b/crypto/asymmetric_keys/rsa.c
@@ -303,6 +303,7 @@ 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;

@@ -337,14 +338,19 @@ 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


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