Zeroize local MAC variables

Zeroize local MAC variables used for CBC+HMAC cipher suites. In encryption,
this is just good hygiene but probably not needed for security since the
data protected by the MAC that could leak is about to be transmitted anyway.
In DTLS decryption, this could be a security issue since an adversary could
learn the MAC of data that they were trying to inject. At least with
encrypt-then-MAC, the adversary could then easily inject a datagram with
a corrected packet. TLS would still be safe since the receiver would close
the connection after the bad MAC.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

2 files changed, 23 insertions, 2 deletions

diff --git a/ChangeLog.d/ssl-mac-zeroize.txt b/ChangeLog.d/ssl-mac-zeroize.txt
new file mode 100644
index 0000000..b49c7ac
--- /dev/null
+++ b/ChangeLog.d/ssl-mac-zeroize.txt
@@ -0,0 +1,5 @@
+Security
+   * Zeroize intermediate variables used to calculate the MAC in CBC cipher
+     suites. This hardens the library in case stack memory leaks through a
+     memory disclosure vulnerabilty, which could formerly have allowed a
+     man-in-the-middle to inject fake ciphertext into a DTLS connection.
diff --git a/library/ssl_msg.c b/library/ssl_msg.c
index 928d6fc..ce0fd4d 100644
--- a/library/ssl_msg.c
+++ b/library/ssl_msg.c
@@ -717,6 +717,7 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
             ssl_mac( &transform->md_ctx_enc, transform->mac_enc,
                      data, rec->data_len, rec->ctr, rec->type, mac );
             memcpy( data + rec->data_len, mac, transform->maclen );
+            mbedtls_platform_zeroize( mac, transform->maclen );
         }
         else
 #endif
@@ -737,6 +738,7 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
             mbedtls_md_hmac_reset( &transform->md_ctx_enc );
 
             memcpy( data + rec->data_len, mac, transform->maclen );
+            mbedtls_platform_zeroize( mac, transform->maclen );
         }
         else
 #endif
@@ -1021,6 +1023,7 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
             rec->data_len += transform->maclen;
             post_avail -= transform->maclen;
             auth_done++;
+            mbedtls_platform_zeroize( mac, transform->maclen );
         }
 #endif /* MBEDTLS_SSL_ENCRYPT_THEN_MAC */
     }
@@ -1305,13 +1308,20 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl,
                                    transform->maclen );
 
             /* Compare expected MAC with MAC at the end of the record. */
+            ret = 0;
             if( mbedtls_ct_memcmp( data + rec->data_len, mac_expect,
                                               transform->maclen ) != 0 )
             {
                 MBEDTLS_SSL_DEBUG_MSG( 1, ( "message mac does not match" ) );
-                return( MBEDTLS_ERR_SSL_INVALID_MAC );
+                ret = MBEDTLS_ERR_SSL_INVALID_MAC;
+                goto hmac_failed_etm_enabled;
             }
             auth_done++;
+
+        hmac_failed_etm_enabled:
+            mbedtls_platform_zeroize( mac_expect, transform->maclen );
+            if( ret != 0 )
+                return( ret );
         }
 #endif /* MBEDTLS_SSL_ENCRYPT_THEN_MAC */
 
@@ -1562,7 +1572,7 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl,
             if( ret != 0 )
             {
                 MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ct_hmac", ret );
-                return( ret );
+                goto hmac_failed_etm_disabled;
             }
 
             mbedtls_ct_memcpy_offset( mac_peer, data,
@@ -1592,6 +1602,12 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl,
             correct = 0;
         }
         auth_done++;
+
+    hmac_failed_etm_disabled:
+        mbedtls_platform_zeroize( mac_peer, transform->maclen );
+        mbedtls_platform_zeroize( mac_expect, transform->maclen );
+        if( ret != 0 )
+            return( ret );
     }
 
     /*