Fix segfault for certain invalid WebSocket input.

The WebSocket code would allocate, on the stack, a string large enough
to hold a key provided by the client, and the WEBSOCKET_GUID. If the key
is NULL, this causes a segfault. If the key is too large, it could
overflow the stack.

This patch checks the key for NULL and checks the length of the key to
avoid stack smashing nastiness.

(closes issue ASTERISK-21825)
Reported by: Alfred Farrugia
Tested by: Alfred Farrugia, David M. Lee
Patches:
    issueA21825_check_if_key_is_sent.patch uploaded by Walter Doekes (license 5674)
........

Merged revisions 391560 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@391561 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_http_websocket.c

@@ -577,9 +577,18 @@ int ast_websocket_uri_cb(struct ast_tcptls_session_instance *ser, const struct a
 		/* Version 7 defined in specification http://tools.ietf.org/html/draft-ietf-hybi-thewebsocketprotocol-07 */
 		/* Version 8 defined in specification http://tools.ietf.org/html/draft-ietf-hybi-thewebsocketprotocol-10 */
 		/* Version 13 defined in specification http://tools.ietf.org/html/rfc6455 */
-		char combined[strlen(key) + strlen(WEBSOCKET_GUID) + 1], base64[64];
+		char *combined, base64[64];
+		unsigned combined_length;
 		uint8_t sha[20];
 
+		combined_length = (key ? strlen(key) : 0) + strlen(WEBSOCKET_GUID) + 1;
+		if (!key || combined_length > 8192) { /* no stack overflows please */
+			fputs("HTTP/1.1 400 Bad Request\r\n"
+			      "Sec-WebSocket-Version: 7, 8, 13\r\n\r\n", ser->f);
+			ao2_ref(protocol_handler, -1);
+			return 0;
+		}
+
 		if (!(session = ao2_alloc(sizeof(*session), session_destroy_fn))) {
 			ast_log(LOG_WARNING, "WebSocket connection from '%s' could not be accepted\n",
 				ast_sockaddr_stringify(&ser->remote_address));
@@ -589,7 +598,8 @@ int ast_websocket_uri_cb(struct ast_tcptls_session_instance *ser, const struct a
 			return 0;
 		}
 
-		snprintf(combined, sizeof(combined), "%s%s", key, WEBSOCKET_GUID);
+		combined = ast_alloca(combined_length);
+		snprintf(combined, combined_length, "%s%s", key, WEBSOCKET_GUID);
 		ast_sha1_hash_uint(sha, combined);
 		ast_base64encode(base64, (const unsigned char*)sha, 20, sizeof(base64));