fsnotify: drop notification_mutex before destroying event

fsnotify_flush_notify() and fanotify_release() destroy notification
event while holding notification_mutex.

The destruction of fanotify event includes a path_put() call which may
end up calling into a filesystem to delete an inode if we happen to be
the last holders of dentry reference which happens to be the last holder
of inode reference.

That in turn may violate lock ordering for some filesystems since
notification_mutex is also acquired e. g. during write when generating
fanotify event.

Also this is the only thing that forces notification_mutex to be a
sleeping lock.  So drop notification_mutex before destroying a
notification event.

Link: http://lkml.kernel.org/r/1473797711-14111-4-git-send-email-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Cc: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Cc: Eric Paris <eparis@redhat.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/notify/fanotify/fanotify_user.c

@@ -390,9 +390,11 @@ static int fanotify_release(struct inode *ignored, struct file *file)
 	mutex_lock(&group->notification_mutex);
 	while (!fsnotify_notify_queue_is_empty(group)) {
 		fsn_event = fsnotify_remove_first_event(group);
-		if (!(fsn_event->mask & FAN_ALL_PERM_EVENTS))
+		if (!(fsn_event->mask & FAN_ALL_PERM_EVENTS)) {
+			mutex_unlock(&group->notification_mutex);
 			fsnotify_destroy_event(group, fsn_event);
-		else
+			mutex_lock(&group->notification_mutex);
+		} else
 			FANOTIFY_PE(fsn_event)->response = FAN_ALLOW;
 	}
 	mutex_unlock(&group->notification_mutex);

fs/notify/notification.c

@@ -178,7 +178,9 @@ void fsnotify_flush_notify(struct fsnotify_group *group)
 	mutex_lock(&group->notification_mutex);
 	while (!fsnotify_notify_queue_is_empty(group)) {
 		event = fsnotify_remove_first_event(group);
+		mutex_unlock(&group->notification_mutex);
 		fsnotify_destroy_event(group, event);
+		mutex_lock(&group->notification_mutex);
 	}
 	mutex_unlock(&group->notification_mutex);
 }