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

[Jann Horn]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=011e4ea1da6692cf881c
Fixes: c73be61cede5 ("pipe: Add general notification queue support")
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
 fs/pipe.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 804a7d789452..226e7f66b590 100644
--- 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.

base-commit: 98b1cc82c4affc16f5598d4fa14b1858671b2263
-- 
2.43.0.rc1.413.gea7ed67945-goog