In the Linux kernel, the following vulnerability has been resolved:
io-wq: order the exit bit against worker creation task work
io_wq_exit_workers() cancels task work for queued worker creation with task_work_cancel_match(). That has a plain load of task->task_works.
io_queue_worker_create() adds the new entry and tests IO_WQ_BIT_EXIT to cancel it if the workqueue is already on its way out.
That must be ordered. task_work_add() has a full barrier via cmpxchg(). But io_wq_exit_start() sets the bit with set_bit() which doesn't have any memory ordering.
So afaict, on weakly ordered architectures the exiting task may load task_works before the store of the bit is visible. The other side tests the bit before the store is visible as well. So the entry remains queued with worker_refs and the exiting task waits on worker_done indefinitely.
If the task still has rings then io_uring_del_tctx_node() provides the barrier via test_and_set_bit() in io_wq_set_exit_on_idle(). When it has closed all rings though that barrier is gone.
Add the barrier after the set_bit().
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