inotify: Avoid reporting event with invalid wd

When inotify_freeing_mark() races with inotify_handle_inode_event() it
can happen that inotify_handle_inode_event() sees that i_mark->wd got
already reset to -1 and reports this value to userspace which can
confuse the inotify listener. Avoid the problem by validating that wd is
sensible (and pretend the mark got removed before the event got
generated otherwise).

CC: [e-mail]
Fixes: 7e790dd5fc93 ("inotify: fix error paths in inotify_update_watch")
Message-Id: <[e-mail]>
Reported-by: [e-mail]
Reviewed-by: Amir Goldstein <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>

fs/notify/inotify/inotify_fsnotify.c

···
 	struct fsnotify_event *fsn_event;
 	struct fsnotify_group *group = inode_mark->group;
 	int ret;
-	int len = 0;
+	int len = 0, wd;
 	int alloc_len = sizeof(struct inotify_event_info);
 	struct mem_cgroup *old_memcg;
 
···
 	i_mark = container_of(inode_mark, struct inotify_inode_mark,
 			      fsn_mark);
 
+	/*
+	 * We can be racing with mark being detached. Don't report event with
+	 * invalid wd.
+	 */
+	wd = READ_ONCE(i_mark->wd);
+	if (wd == -1)
+		return 0;
 	/*
 	 * Whoever is interested in the event, pays for the allocation. Do not
 	 * trigger OOM killer in the target monitoring memcg as it may have
···
 	fsn_event = &event->fse;
 	fsnotify_init_event(fsn_event);
 	event->mask = mask;
-	event->wd = i_mark->wd;
+	event->wd = wd;
 	event->sync_cookie = cookie;
 	event->name_len = len;
 	if (len)