workqueue: Put the pwq after detaching the rescuer from the pool

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: workqueue: Put the pwq after detaching the rescuer from the pool The commit 68f83057b913("workqueue: Reap workers via kthread_stop() and remove detach_completion") adds code to reap the normal workers but mistakenly does not handle the rescuer and also removes the code waiting for the rescuer in put_unbound_pool(), which caused a use-after-free bug reported by Cheung Wall. To avoid the use-after-free bug, the pool’s reference must be held until the detachment is complete. Therefore, move the code that puts the pwq after detaching the rescuer from the pool.

References

• https://git.kernel.org/stable/c/e7c16028a424dd35be1064a68fa318be4359310f
• https://git.kernel.org/stable/c/835b69c868f53f959d4986bbecd561ba6f38e492
• https://git.kernel.org/stable/c/e76946110137703c16423baf6ee177b751a34b7e