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: <syzbot+011e4ea1da6692cf881c@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=011e4ea1da6692cf881c
Fixes: c73be61cede5 ("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>

diff --git a/fs/pipe.c b/fs/pipe.c
index 804a7d78945217efd3b5394a8a8e6e7d605a937a..226e7f66b5903ea8af0fb8c5647d2b34e9ce90b4 100644 (file)
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -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.