io_uring/io-wq: only free worker if it was allocated for creation

commit e6db6f93 upstream.

We have two types of task_work based creation, one is using an existing
worker to setup a new one (eg when going to sleep and we have no free
workers), and the other is allocating a new worker. Only the latter
should be freed when we cancel task_work creation for a new worker.

Fixes: af82425c ("io_uring/io-wq: free worker if task_work creation is canceled")
Reported-by:  <syzbot+d56ec896af3637bdb7e4@syzkaller.appspotmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

io_uring/io-wq.c

@@ -1217,7 +1217,12 @@ static void io_wq_cancel_tw_create(struct io_wq *wq)
 
 		worker = container_of(cb, struct io_worker, create_work);
 		io_worker_cancel_cb(worker);
-		kfree(worker);
+		/*
+		 * Only the worker continuation helper has worker allocated and
+		 * hence needs freeing.
+		 */
+		if (cb->func == create_worker_cont)
+			kfree(worker);
 	}
 }