tools/xenstore: drop watch event messages exceeding maximum size

By setting a watch with a very large tag it is possible to trick
xenstored to send watch event messages exceeding the maximum allowed
payload size. This might in turn lead to a crash of xenstored as the
resulting error can cause dereferencing a NULL pointer in case there
is no active request being handled by the guest the watch event is
being sent to.

Fix that by just dropping such watch events. Additionally modify the
error handling to test the pointer to be not NULL before dereferencing
it.

This is XSA-324.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>

tools/xenstore/xenstored_core.c

@@ -680,6 +680,9 @@ void send_reply(struct connection *conn, enum xsd_sockmsg_type type,
 	/* Replies reuse the request buffer, events need a new one. */
 	if (type != XS_WATCH_EVENT) {
 		bdata = conn->in;
+		/* Drop asynchronous responses, e.g. errors for watch events. */
+		if (!bdata)
+			return;
 		bdata->inhdr = true;
 		bdata->used = 0;
 		conn->in = NULL;

tools/xenstore/xenstored_watch.c

@@ -92,6 +92,10 @@ static void add_event(struct connection *conn,
 	}
 
 	len = strlen(name) + 1 + strlen(watch->token) + 1;
+	/* Don't try to send over-long events. */
+	if (len > XENSTORE_PAYLOAD_MAX)
+		return;
+
 	data = talloc_array(ctx, char, len);
 	if (!data)
 		return;