Commit 055ca835 authored by Jann Horn's avatar Jann Horn Committed by Christian Brauner
Browse files

fs/pipe: Fix lockdep false-positive in watchqueue pipe_write() 



When you try to splice between a normal pipe and a notification pipe,
get_pipe_info(..., true) fails, so splice() falls back to treating the
notification pipe like a normal pipe - so we end up in
iter_file_splice_write(), which first locks the input pipe, then calls
vfs_iter_write(), which locks the output pipe.

Lockdep complains about that, because we're taking a pipe lock while
already holding another pipe lock.

I think this probably (?) can't actually lead to deadlocks, since you'd
need another way to nest locking a normal pipe into locking a
watch_queue pipe, but the lockdep annotations don't make that clear.

Bail out earlier in pipe_write() for notification pipes, before taking
the pipe lock.

Reported-and-tested-by: default avatar <syzbot+011e4ea1da6692cf881c@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=011e4ea1da6692cf881c


Fixes: c73be61c ("pipe: Add general notification queue support")
Signed-off-by: default avatarJann Horn <jannh@google.com>
Link: https://lore.kernel.org/r/20231124150822.2121798-1-jannh@google.com


Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent 12c1b632

fs/pipe.c

+12 −5
Original line number Diff line number Diff line
@@ -446,6 +446,18 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from) 
	bool was_empty = false;

	bool wake_next_writer = false;



	/*

	 * Reject writing to watch queue pipes before the point where we lock

	 * the pipe.

	 * Otherwise, lockdep would be unhappy if the caller already has another

	 * pipe locked.

	 * If we had to support locking a normal pipe and a notification pipe at

	 * the same time, we could set up lockdep annotations for that, but

	 * since we don't actually need that, it's simpler to just bail here.

	 */

	if (pipe_has_watch_queue(pipe))

		return -EXDEV;



	/* Null write succeeds. */

	if (unlikely(total_len == 0))

		return 0;
@@ -458,11 +470,6 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from) 
		goto out;

	}



	if (pipe_has_watch_queue(pipe)) {

		ret = -EXDEV;

		goto out;

	}



	/*

	 * If it wasn't empty we try to merge new data into

	 * the last buffer.