io_uring/io-wq: clear current->worker_private on exit

A recent fix stopped clearing PF_IO_WORKER from current->flags on exit,
which meant that we can now call inc/dec running on the worker after it
has been removed if it ends up scheduling in/out as part of exit.

If this happens after an RCU grace period has passed, then the struct
pointed to by current->worker_private may have been freed, and we can
now be accessing memory that is freed.

Ensure this doesn't happen by clearing the task worker_private field.
Both io_wq_worker_running() and io_wq_worker_sleeping() check this
field before going any further, and we don't need any accounting etc
done after this worker has exited.

Fixes: fd37b88 ("io_uring/io-wq: don't clear PF_IO_WORKER on exit")
Reported-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

io_uring/io-wq.c

@@ -220,7 +220,12 @@ static void io_worker_exit(struct io_worker *worker)
 	list_del_rcu(&worker->all_list);
 	raw_spin_unlock(&wq->lock);
 	io_wq_dec_running(worker);
-	worker->flags = 0;
+	/*
+	 * this worker is a goner, clear ->worker_private to avoid any
+	 * inc/dec running calls that could happen as part of exit from
+	 * touching 'worker'.
+	 */
+	current->worker_private = NULL;
 
 	kfree_rcu(worker, rcu);
 	io_worker_ref_put(wq);