Fix uninitialised memory access in constant time functions

Fix an issue reported by Coverity whereby some constant time functions called from the ssl decrypt code could potentially access uninitialised memory. Signed-off-by: Paul Elliott <[email protected]>
Mbed-TLS · May 19, 2022 · b830028 · b830028
1 parent bdd6905
commit b830028
Show file tree

Hide file tree

Showing 3 changed files with 15 additions and 2 deletions.
diff --git a/library/constant_time.c b/library/constant_time.c
@@ -489,6 +489,12 @@ int mbedtls_ct_hmac( mbedtls_md_context_t *ctx,
     MD_CHK( mbedtls_md_update( ctx, add_data, add_data_len ) );
     MD_CHK( mbedtls_md_update( ctx, data, min_data_len ) );
 
+    /* Fill the hash buffer in advance with something that is
+     * not a valid hash (barring an attack on the hash and
+     * deliberately-crafted input), in case the caller doesn't
+     * check the return status properly. */
+    memset( output, '!', hash_size );
+
     /* For each possible length, compute the hash up to that point */
     for( offset = min_data_len; offset <= max_data_len; offset++ )
     {

diff --git a/library/constant_time_internal.h b/library/constant_time_internal.h
@@ -221,6 +221,13 @@ void mbedtls_ct_memcpy_if_eq( unsigned char *dest,
  * offset_secret, but only on \p offset_min, \p offset_max and \p len.
  * Functionally equivalent to `memcpy(dst, src + offset_secret, len)`.
  *
+ * \note                This function reads from \p dest, but the value that
+ *                      is read does not influence the result and this
+ *                      function's behavior is well-defined regardless of the
+ *                      contents of the buffers. This may result in false
+ *                      positives from static or dynamic analyzers, especially
+ *                      if \p dest is not initialized.
+ *
  * \param dest          The destination buffer. This must point to a writable
  *                      buffer of at least \p len bytes.
  * \param src           The base of the source buffer. This must point to a

diff --git a/library/ssl_msg.c b/library/ssl_msg.c
@@ -1590,8 +1590,8 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl,
 #if defined(MBEDTLS_SSL_SOME_MODES_USE_MAC)
     if( auth_done == 0 )
     {
-        unsigned char mac_expect[MBEDTLS_SSL_MAC_ADD];
-        unsigned char mac_peer[MBEDTLS_SSL_MAC_ADD];
+        unsigned char mac_expect[MBEDTLS_SSL_MAC_ADD] = { 0 };
+        unsigned char mac_peer[MBEDTLS_SSL_MAC_ADD] = { 0 };
 
         /* If the initial value of padlen was such that
          * data_len < maclen + padlen + 1, then padlen