Enhance documentation of ssl_write_hostname_ext, adapt ChangeLog.

Add a reference to the relevant RFC, adapt ChangeLog.

ChangeLog

@@ -194,6 +194,8 @@ Security
      team. #569 CVE-2017-2784
 
 Bugfix
+   * Fix memory leak in mbedtls_ssl_set_hostname() when called multiple times.
+     Found by jethrogb, #836.
    * Fix output certificate verification flags set by x509_crt_verify_top() when
      traversing a chain of trusted CA. The issue would cause both flags,
      MBEDTLS_X509_BADCERT_NOT_TRUSTED and MBEDTLS_X509_BADCERT_EXPIRED, to be

include/mbedtls/ssl.h

@@ -1804,13 +1804,12 @@ void mbedtls_ssl_conf_sig_hashes( mbedtls_ssl_config *conf,
  * \note           Maximum hostname length MBEDTLS_SSL_MAX_HOST_NAME_LEN.
  *
  * \return         0 if successful, MBEDTLS_ERR_SSL_ALLOC_FAILED on 
- *                 allocation failure, MBEDTLS_ERR_BAD_INPUT_DATA on 
+ *                 allocation failure, MBEDTLS_ERR_SSL_BAD_INPUT_DATA on 
  *                 too long input hostname.
  *
- * \post           Hostname set to the one provided on success (cleared
+ *                 Hostname set to the one provided on success (cleared
  *                 when NULL). On allocation failure hostname is cleared. 
  *                 On too long input failure, old hostname is unchanged.
- *
  */
 int mbedtls_ssl_set_hostname( mbedtls_ssl_context *ssl, const char *hostname );
 #endif /* MBEDTLS_X509_CRT_PARSE_C */

library/ssl_cli.c

@@ -80,6 +80,13 @@ static void ssl_write_hostname_ext( mbedtls_ssl_context *ssl,
     }
 
     /*
+     * Sect. 3, RFC 6066 (TLS Extensions Definitions)
+     *
+     * In order to provide any of the server names, clients MAY include an
+     * extension of type "server_name" in the (extended) client hello. The
+     * "extension_data" field of this extension SHALL contain
+     * "ServerNameList" where:
+     *
      * struct {
      *     NameType name_type;
      *     select (name_type) {
@@ -96,6 +103,7 @@ static void ssl_write_hostname_ext( mbedtls_ssl_context *ssl,
      * struct {
      *     ServerName server_name_list<1..2^16-1>
      * } ServerNameList;
+     *
      */
     *p++ = (unsigned char)( ( MBEDTLS_TLS_EXT_SERVERNAME >> 8 ) & 0xFF );
     *p++ = (unsigned char)( ( MBEDTLS_TLS_EXT_SERVERNAME      ) & 0xFF );

library/ssl_tls.c

@@ -6218,7 +6218,7 @@ int mbedtls_ssl_set_hostname( mbedtls_ssl_context *ssl, const char *hostname )
 
     return( 0 );
 }
-#endif
+#endif /* MBEDTLS_X509_CRT_PARSE_C */
 
 #if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
 void mbedtls_ssl_conf_sni( mbedtls_ssl_config *conf,