Merge pull request #5859 from paul-elliott-arm/fix_ct_uninit_memory_a…

…ccess_2.28

Backport 2.28: Fix uninitialised memory access in constant time functions

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++ )
     {

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

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