[PATCH 5.14 005/432] io_uring: fix io_try_cancel_userdata race for iowq

Greg Kroah-Hartman
2021-09-16T17:21:31Z

From: Pavel Begunkov <asml.silence@gmail.com>

commit dadebc350da2bef62593b1df007a6e0b90baf42a upstream.

WARNING: CPU: 1 PID: 5870 at fs/io_uring.c:5975 io_try_cancel_userdata+0x30f/0x540 fs/io_uring.c:5975
CPU: 0 PID: 5870 Comm: iou-wrk-5860 Not tainted 5.14.0-rc6-next-20210820-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:io_try_cancel_userdata+0x30f/0x540 fs/io_uring.c:5975
Call Trace:
io_async_cancel fs/io_uring.c:6014 [inline]
io_issue_sqe+0x22d5/0x65a0 fs/io_uring.c:6407
io_wq_submit_work+0x1dc/0x300 fs/io_uring.c:6511
io_worker_handle_work+0xa45/0x1840 fs/io-wq.c:533
io_wqe_worker+0x2cc/0xbb0 fs/io-wq.c:582
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

io_try_cancel_userdata() can be called from io_async_cancel() executing
in the io-wq context, so the warning fires, which is there to alert
anyone accessing task->io_uring->io_wq in a racy way. However,
io_wq_put_and_exit() always first waits for all threads to complete,
so the only detail left is to zero tctx->io_wq after the context is
removed.

note: one little assumption is that when IO_WQ_WORK_CANCEL, the executor
won't touch ->io_wq, because io_wq_destroy() might cancel left pending
requests in such a way.

Cc: stable@vger.kernel.org
Reported-by: syzbot+b0c9d1588ae92866515f@syzkaller.appspotmail.com
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/dfdd37a80cfa9ffd3e59538929c99cdd55d8699e.1629721757.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/io_uring.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -6310,6 +6310,7 @@ static void io_wq_submit_work(struct io_
if (timeout)
io_queue_linked_timeout(timeout);

+ /* either cancelled or io-wq is dying, so don't touch tctx->iowq */
if (work->flags & IO_WQ_WORK_CANCEL)
ret = -ECANCELED;

@@ -9137,8 +9138,8 @@ static void io_uring_clean_tctx(struct i
* Must be after io_uring_del_task_file() (removes nodes under
* uring_lock) to avoid race with io_uring_try_cancel_iowq().
*/
- tctx->io_wq = NULL;
io_wq_put_and_exit(wq);
+ tctx->io_wq = NULL;
}
}