Mitigate UAF crashes due to wl_client_destroy reentrancy

There are situations in which a call into wl_client_destroy() can
result in a reentrant call into wl_client_destroy() - which
results in UAF / double free crashes.

For example, this can occur in the following scenario.

1. Server receives a message notifying it that a client has
   disconnected (WL_EVENT_HANGUP [1])

2. This beings client destruction with a call to wl_client_destroy()

3. wl_client_destroy() kicks off callbacks as client-associated
   resources are cleaned up and their destructors and destruction
   signals are invoked.

4. These callbacks eventually lead to an explicit call to
   wl_display_flush_clients() as the server attempts to flush
   events to other connected clients.

5. Since the client has already begun destruction, when it is
   reached in the iteration the flush fails wl_client_destroy()
   is called again [2].

This patch guards against this reentrant condition by removing
the client from the display's client list when wl_client_destroy()
is first called. This prevents access / iteration over the client
after wl_client_destroy() is called.

In the example above, wl_display_flush_clients() will pass over
the client currently undergoing destruction and the reentrant
call is avoided.

[1] 8f499bf404/src/wayland-server.c (L342)

[2] 8f499bf404/src/wayland-server.c (L1512)

Signed-off-by: Thomas Lukaszewicz [thomaslukaszewicz@gmail.com](mailto:thomaslukaszewicz@gmail.com)

src/wayland-server.c

@@ -933,6 +933,18 @@ wl_client_get_destroy_late_listener(struct wl_client *client,
 WL_EXPORT void
 wl_client_destroy(struct wl_client *client)
 {
+
+	/* wl_client_destroy() should not be called twice for the same client. */
+	if (wl_list_empty(&client->link)) {
+		client->error = 1;
+		wl_log("wl_client_destroy: encountered re-entrant client destruction.\n");
+		return;
+	}
+
+	wl_list_remove(&client->link);
+	/* Keep the client link safe to inspect. */
+	wl_list_init(&client->link);
+
 	wl_priv_signal_final_emit(&client->destroy_signal, client);
 
 	wl_client_flush(client);
@@ -943,7 +955,6 @@ wl_client_destroy(struct wl_client *client)
 
 	wl_priv_signal_final_emit(&client->destroy_late_signal, client);
 
-	wl_list_remove(&client->link);
 	wl_list_remove(&client->resource_created_signal.listener_list);
 
 	if (client->data_dtor)

tests/client-test.c

@@ -158,3 +158,50 @@ TEST(client_destroy_listener)
 	wl_display_destroy(display);
 }
 
+static void
+client_destroy_remove_link_notify(struct wl_listener *l, void *data)
+{
+	struct wl_client *client = data;
+	struct client_destroy_listener *listener =
+		wl_container_of(l, listener, listener);
+
+	/* The client destruction signal should not be emitted more than once. */
+	assert(!listener->done);
+	listener->done = true;
+
+	/* The client should have been removed from the display's list. */
+	assert(wl_list_empty(wl_client_get_link(client)));
+}
+
+/*
+ * Tests that wl_client_destroy() will remove the client from the display's
+ * client list to prevent client access during destruction.
+ */
+TEST(client_destroy_removes_link)
+{
+	struct wl_display *display;
+	struct wl_client *client;
+	struct client_destroy_listener destroy_listener;
+	int s[2];
+
+	assert(socketpair(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0, s) == 0);
+	display = wl_display_create();
+	assert(display);
+	client = wl_client_create(display, s[0]);
+	assert(client);
+
+	destroy_listener.listener.notify = client_destroy_remove_link_notify;
+	destroy_listener.done = false;
+	wl_client_add_destroy_listener(client, &destroy_listener.listener);
+
+	assert(wl_client_get_destroy_listener(client,
+		client_destroy_remove_link_notify) == &destroy_listener.listener);
+
+	wl_client_destroy(client);
+	assert(destroy_listener.done);
+
+	close(s[0]);
+	close(s[1]);
+
+	wl_display_destroy(display);
+}