[PATCH] Fix use-after-free in io_wqe_inc_running() due to wq already being free'd

[Jordy Zomer]

My syzkaller instance reported a use-after-free bug in io_wqe_inc_running.
I tried fixing this by checking if wq isn't NULL in io_wqe_worker.
If it does; return an -EFAULT. This because create_io_worker() will clean-up 
the worker if there's an error.

If you want I could send you the syzkaller reproducer and crash-logs :)

Best Regards,

Jordy Zomer

Signed-off-by: Jordy Zomer <jordy@pwning.systems>
---
 fs/io-wq.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/io-wq.c b/fs/io-wq.c
index 0ae9ecadf295..9ed92d88a088 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -482,6 +482,10 @@ static int io_wqe_worker(void *data)
        char buf[TASK_COMM_LEN];
 
        worker->flags |= (IO_WORKER_F_UP | IO_WORKER_F_RUNNING);
+
+       if (wq == NULL)
+               return -EFAULT;
+
        io_wqe_inc_running(worker);
 
        sprintf(buf, "iou-wrk-%d", wq->task_pid);
-- 
2.25.1