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

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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: <syzbot+011e4ea1da6692cf881c@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=011e4ea1da6692cf881c
Fixes: c73be61ced ("pipe: Add general notification queue support")
Signed-off-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/r/20231124150822.2121798-1-jannh@google.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Jann Horn 2023-11-24 16:08:22 +01:00 committed by Christian Brauner
parent 12c1b632d9
commit 055ca83559
1 changed files with 12 additions and 5 deletions
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17
fs/pipe.c
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@ -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.