fanotify: disallow mount/sb marks on kernel internal pseudo fs

Hopefully, nobody is trying to abuse mount/sb marks for watching all
anonymous pipes/inodes.

I cannot think of a good reason to allow this - it looks like an
oversight that dated back to the original fanotify API.

Link: https://lore.kernel.org/linux-fsdevel/20230628101132.kvchg544mczxv2pm@quack3/
Fixes: 0ff21db9 ("fanotify: hooks the fanotify_mark syscall to the vfsmount code")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20230629042044.25723-1-amir73il@gmail.com>

fs/notify/fanotify/fanotify_user.c

@@ -1622,6 +1622,20 @@ static int fanotify_events_supported(struct fsnotify_group *group,
 	    path->mnt->mnt_sb->s_type->fs_flags & FS_DISALLOW_NOTIFY_PERM)
 		return -EINVAL;
 
+	/*
+	 * mount and sb marks are not allowed on kernel internal pseudo fs,
+	 * like pipe_mnt, because that would subscribe to events on all the
+	 * anonynous pipes in the system.
+	 *
+	 * SB_NOUSER covers all of the internal pseudo fs whose objects are not
+	 * exposed to user's mount namespace, but there are other SB_KERNMOUNT
+	 * fs, like nsfs, debugfs, for which the value of allowing sb and mount
+	 * mark is questionable. For now we leave them alone.
+	 */
+	if (mark_type != FAN_MARK_INODE &&
+	    path->mnt->mnt_sb->s_flags & SB_NOUSER)
+		return -EINVAL;
+
 	/*
 	 * We shouldn't have allowed setting dirent events and the directory
 	 * flags FAN_ONDIR and FAN_EVENT_ON_CHILD in mask of non-dir inode,