2013-07-12 03:12:43

by Chun-Yi Lee

Subject: [PATCH] asymmetric keys: explicitly add the leading zero byte to encoded message

From: Chun-Yi Lee <[email protected]>

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 <[email protected]>
Cc: Josh Boyer <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Herbert Xu <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: David Howells <[email protected]>
Signed-off-by: Chun-Yi Lee <[email protected]>
---
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

2013-07-16 02:36:27

by joeyli

Subject: Re: [PATCH] asymmetric keys: explicitly add the leading zero byte to encoded message

Hi all experts,

Does there have any suggestions or comments for this patch to asymmetric
keys?


Thanks a lot!
Joey Lee

於 五，2013-07-12 於 11:11 +0800，Lee, Chun-Yi 提到：
> From: Chun-Yi Lee <[email protected]>
>
> 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 <[email protected]>
> Cc: Josh Boyer <[email protected]>
> Cc: Randy Dunlap <[email protected]>
> Cc: Herbert Xu <[email protected]>
> Cc: "David S. Miller" <[email protected]>
> Cc: David Howells <[email protected]>
> Signed-off-by: Chun-Yi Lee <[email protected]>
> ---
> 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);
>