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Message-ID: <20231124150822.2121798-1-jannh@google.com>
Date: Fri, 24 Nov 2023 16:08:22 +0100
From: Jann Horn <jannh@...gle.com>
To: Alexander Viro <viro@...iv.linux.org.uk>,
Christian Brauner <brauner@...nel.org>,
David Howells <dhowells@...hat.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
syzkaller-bugs@...glegroups.com
Subject: [PATCH] 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@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=011e4ea1da6692cf881c
Fixes: c73be61cede5 ("pipe: Add general notification queue support")
Signed-off-by: Jann Horn <jannh@...gle.com>
---
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
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