CVE-2021-47577 — io-wq: check for wq exit after adding new worker task_work

Affected: Linux

In the Linux kernel, the following vulnerability has been resolved:

io-wq: check for wq exit after adding new worker task_work

We check IO_WQ_BIT_EXIT before attempting to create a new worker, and wq exit cancels pending work if we have any. But it's possible to have a race between the two, where creation checks exit finding it not set, but we're in the process of exiting. The exit side will cancel pending creation task_work, but there's a gap where we add task_work after we've canceled existing creations at exit time.

Fix this by checking the EXIT bit post adding the creation task_work. If it's set, run the same cancelation that exit does.

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CVE-2021-47577 — io-wq: check for wq exit after adding new worker task_work