This is a note to let you know that I've just added the patch titled
fs/pipe: Fix lockdep false-positive in watchqueue pipe_write()
to the 5.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
fs-pipe-fix-lockdep-false-positive-in-watchqueue-pip.patch
and it can be found in the queue-5.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit be7cd770fe928dd49c14b3afbbe4c298d8a6e27a
Author: Jann Horn <jannh@xxxxxxxxxx>
Date: Fri Nov 24 16:08:22 2023 +0100
fs/pipe: Fix lockdep false-positive in watchqueue pipe_write()
[ Upstream commit 055ca83559912f2cfd91c9441427bac4caf3c74e ]
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>
Link: https://lore.kernel.org/r/20231124150822.2121798-1-jannh@xxxxxxxxxx
Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/fs/pipe.c b/fs/pipe.c
index a8b8ef2dae7b7..0a8095070b70e 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -423,6 +423,18 @@ 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;
@@ -435,11 +447,6 @@ 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.
