commit 03b64fa6c12140793aaad7a25df906ad70182bda
author Hanno Becker <hanno.becker@arm.com> Tue Jun 11 14:39:38 2019 +0100
committer Hanno Becker <hanno.becker@arm.com> Tue Jun 25 08:42:20 2019 +0100
tree 445fcce35302fb9c9f60fc0714c685c9226b0df4
parent aabbb582eb90754390a399b15cc56aebbf97c1c0

Rearrange ExtendedMasterSecret parsing logic

`mbedtls_ssl_handshake_params::extended_ms` holds the state of the
ExtendedMasterSecret extension in the current handshake. Initially
set to 'disabled' for both client and server,
- the client sets it to 'enabled' as soon as it finds the ExtendedMS
  extension in the `ServerHello` and it has advertised that extension
  in its ClientHello,
- the server sets it to 'enabled' as soon as it finds the ExtendedMS
  extension in the `ClientHello` and is willing to advertise is in its
  `ServerHello`.

This commit slightly restructures this logic in prepraration for the
removal of `mbedtls_ssl_handshake_params::extended_ms` in case both
the use and the enforcement of the ExtendedMasterSecret extension have
been fixed at compile-time. Namely, in this case there is no need for
the `extended_ms` field in the handshake structure, as the ExtendedMS
must be in use if the handshake progresses beyond the Hello stage.

Paving the way for the removal of mbedtls_ssl_handshake_params::extended_ms
this commit introduces a temporary variable tracking the presence of the
ExtendedMS extension in the ClientHello/ServerHello messages, leaving
the derivation of `extended_ms` (and potential failure) to the end of
the parsing routine.

library/ssl_cli.c

diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 238eeb1..257a517 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -1341,9 +1341,6 @@
     }
 
     ((void) buf);
-
-    ssl->handshake->extended_ms = MBEDTLS_SSL_EXTENDED_MS_ENABLED;
-
     return( 0 );
 }
 #endif /* MBEDTLS_SSL_EXTENDED_MASTER_SECRET */
@@ -1604,6 +1601,9 @@
 #if defined(MBEDTLS_SSL_RENEGOTIATION)
     int renegotiation_info_seen = 0;
 #endif
+#if defined(MBEDTLS_SSL_EXTENDED_MASTER_SECRET)
+    int extended_ms_seen = 0;
+#endif
     int handshake_failure = 0;
     const mbedtls_ssl_ciphersuite_t *suite_info;
 
@@ -1984,6 +1984,7 @@
             {
                 return( ret );
             }
+            extended_ms_seen = 1;
 
             break;
 #endif /* MBEDTLS_SSL_EXTENDED_MASTER_SECRET */
@@ -2092,14 +2093,19 @@
      */
 #if defined(MBEDTLS_SSL_EXTENDED_MASTER_SECRET)
     if( mbedtls_ssl_conf_get_ems( ssl->conf ) ==
-          MBEDTLS_SSL_EXTENDED_MS_ENABLED &&
-        mbedtls_ssl_conf_get_ems_enforced( ssl->conf ) ==
-          MBEDTLS_SSL_EXTENDED_MS_ENFORCE_ENABLED &&
-        ssl->handshake->extended_ms == MBEDTLS_SSL_EXTENDED_MS_DISABLED)
+        MBEDTLS_SSL_EXTENDED_MS_ENABLED )
     {
-        MBEDTLS_SSL_DEBUG_MSG( 1, ( "Peer not offering extended master "
+        if( extended_ms_seen )
+        {
+            ssl->handshake->extended_ms = MBEDTLS_SSL_EXTENDED_MS_ENABLED;
+        }
+        else if( mbedtls_ssl_conf_get_ems_enforced( ssl->conf ) ==
+                 MBEDTLS_SSL_EXTENDED_MS_ENFORCE_ENABLED )
+        {
+            MBEDTLS_SSL_DEBUG_MSG( 1, ( "Peer not offering extended master "
                                     "secret, while it is enforced") );
-        handshake_failure = 1;
+            handshake_failure = 1;
+        }
     }
 #endif /* MBEDTLS_SSL_EXTENDED_MASTER_SECRET */