commit f1ed5951e353ea4e5173cef5bd533cb9c9e52440
author Ronald Cron <ronald.cron@arm.com> Thu Mar 24 14:15:28 2022 +0100
committer Ronald Cron <ronald.cron@arm.com> Mon Mar 28 12:16:13 2022 +0200
tree bdd5468834389a1574d68754bcbf5ece690cc069
parent c2e2876e0e5f50d4851d818ca866829af2822ac2

ssl_srv.c: Mark ETM as disabled if cipher is not CBC

Encrypt-Then-Mac (ETM) is supported in Mbed TLS
server for TLS version geater than SSLv3 and only
for the CBC cipher mode thus make it
clear in the SSL context.

The previous code was ok as long as the check of
the ETM status was done only in the case of the CBC
cipher mode but fragile as #5573 revealed.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>

library/ssl_srv.c

diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 1a63173..8d5e39f 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -2354,12 +2354,8 @@
     const mbedtls_ssl_ciphersuite_t *suite = NULL;
     const mbedtls_cipher_info_t *cipher = NULL;
 
-    if( ssl->session_negotiate->encrypt_then_mac == MBEDTLS_SSL_ETM_DISABLED ||
-        ssl->minor_ver == MBEDTLS_SSL_MINOR_VERSION_0 )
-    {
-        *olen = 0;
-        return;
-    }
+    if( ssl->minor_ver == MBEDTLS_SSL_MINOR_VERSION_0 )
+        ssl->session_negotiate->encrypt_then_mac = MBEDTLS_SSL_ETM_DISABLED;
 
     /*
      * RFC 7366: "If a server receives an encrypt-then-MAC request extension
@@ -2372,6 +2368,11 @@
         ( cipher = mbedtls_cipher_info_from_type( suite->cipher ) ) == NULL ||
         cipher->mode != MBEDTLS_MODE_CBC )
     {
+        ssl->session_negotiate->encrypt_then_mac = MBEDTLS_SSL_ETM_DISABLED;
+    }
+
+    if( ssl->session_negotiate->encrypt_then_mac == MBEDTLS_SSL_ETM_DISABLED )
+    {
         *olen = 0;
         return;
     }