Make CBC decoding constant time.
This patch makes the decoding of SSLv3 and TLS CBC records constant
time. Without this, a timing side-channel can be used to build a padding
oracle and mount Vaudenay's attack.
This patch also disables the stitched AESNI+SHA mode pending a similar
fix to that code.
In order to be easy to backport, this change is implemented in ssl/,
rather than as a generic AEAD mode. In the future this should be changed
around so that HMAC isn't in ssl/, but crypto/ as FIPS expects.
(cherry picked from commit e130841bccfc0bb9da254dc84e23bc6a1c78a64e)
diff --git a/ssl/s3_pkt.c b/ssl/s3_pkt.c
index 9246ff2..b2e5dc1 100644
--- a/ssl/s3_pkt.c
+++ b/ssl/s3_pkt.c
@@ -290,11 +290,9 @@
unsigned char *p;
unsigned char md[EVP_MAX_MD_SIZE];
short version;
- int mac_size;
+ unsigned mac_size;
int clear=0;
size_t extra;
- int decryption_failed_or_bad_record_mac = 0;
- unsigned char *mac = NULL;
rr= &(s->s3->rrec);
sess=s->session;
@@ -405,17 +403,10 @@
rr->data=rr->input;
enc_err = s->method->ssl3_enc->enc(s,0);
- if (enc_err <= 0)
+ if (enc_err == 0)
{
- if (enc_err == 0)
- /* SSLerr() and ssl3_send_alert() have been called */
- goto err;
-
- /* Otherwise enc_err == -1, which indicates bad padding
- * (rec->length has not been changed in this case).
- * To minimize information leaked via timing, we will perform
- * the MAC computation anyway. */
- decryption_failed_or_bad_record_mac = 1;
+ /* SSLerr() and ssl3_send_alert() have been called */
+ goto err;
}
#ifdef TLS_DEBUG
@@ -433,45 +424,54 @@
if (!clear)
{
/* !clear => s->read_hash != NULL => mac_size != -1 */
+ unsigned char *mac = NULL;
+ unsigned char mac_tmp[EVP_MAX_MD_SIZE];
mac_size=EVP_MD_CTX_size(s->read_hash);
- OPENSSL_assert(mac_size >= 0);
+ OPENSSL_assert(mac_size <= EVP_MAX_MD_SIZE);
- if (rr->length > SSL3_RT_MAX_COMPRESSED_LENGTH+extra+mac_size)
+ /* orig_len is the length of the record before any padding was
+ * removed. This is public information, as is the MAC in use,
+ * therefore we can safely process the record in a different
+ * amount of time if it's too short to possibly contain a MAC.
+ */
+ if (rr->orig_len < mac_size ||
+ /* CBC records must have a padding length byte too. */
+ (EVP_CIPHER_CTX_mode(s->enc_read_ctx) == EVP_CIPH_CBC_MODE &&
+ rr->orig_len < mac_size+1))
{
-#if 0 /* OK only for stream ciphers (then rr->length is visible from ciphertext anyway) */
- al=SSL_AD_RECORD_OVERFLOW;
- SSLerr(SSL_F_SSL3_GET_RECORD,SSL_R_PRE_MAC_LENGTH_TOO_LONG);
- goto f_err;
-#else
- decryption_failed_or_bad_record_mac = 1;
-#endif
- }
- /* check the MAC for rr->input (it's in mac_size bytes at the tail) */
- if (rr->length >= (unsigned int)mac_size)
- {
- rr->length -= mac_size;
- mac = &rr->data[rr->length];
- }
- else
- {
- /* record (minus padding) is too short to contain a MAC */
-#if 0 /* OK only for stream ciphers */
al=SSL_AD_DECODE_ERROR;
SSLerr(SSL_F_SSL3_GET_RECORD,SSL_R_LENGTH_TOO_SHORT);
goto f_err;
-#else
- decryption_failed_or_bad_record_mac = 1;
- rr->length = 0;
-#endif
}
- i=s->method->ssl3_enc->mac(s,md,0);
- if (i < 0 || mac == NULL || CRYPTO_memcmp(md, mac, (size_t)mac_size) != 0)
+
+ if (EVP_CIPHER_CTX_mode(s->enc_read_ctx) == EVP_CIPH_CBC_MODE)
{
- decryption_failed_or_bad_record_mac = 1;
+ /* We update the length so that the TLS header bytes
+ * can be constructed correctly but we need to extract
+ * the MAC in constant time from within the record,
+ * without leaking the contents of the padding bytes.
+ * */
+ mac = mac_tmp;
+ ssl3_cbc_copy_mac(mac_tmp, rr, mac_size);
+ rr->length -= mac_size;
}
+ else
+ {
+ /* In this case there's no padding, so |rec->orig_len|
+ * equals |rec->length| and we checked that there's
+ * enough bytes for |mac_size| above. */
+ rr->length -= mac_size;
+ mac = &rr->data[rr->length];
+ }
+
+ i=s->method->ssl3_enc->mac(s,md,0 /* not send */);
+ if (i < 0 || mac == NULL || CRYPTO_memcmp(md, mac, (size_t)mac_size) != 0)
+ enc_err = -1;
+ if (rr->length > SSL3_RT_MAX_COMPRESSED_LENGTH+extra+mac_size)
+ enc_err = -1;
}
- if (decryption_failed_or_bad_record_mac)
+ if (enc_err < 0)
{
/* A separate 'decryption_failed' alert was introduced with TLS 1.0,
* SSL 3.0 only has 'bad_record_mac'. But unless a decryption
