CVE-2025-39721: crypto: qat - flush misc workqueue during device shutdown

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

crypto: qat - flush misc workqueue during device shutdown

Repeated loading and unloading of a device specific QAT driver, for example qat4xxx, in a tight loop can lead to a crash due to a use-after-free scenario. This occurs when a power management (PM) interrupt triggers just before the device-specific driver (e.g., qat4xxx.ko) is unloaded, while the core driver (intelqat.ko) remains loaded.

Since the driver uses a shared workqueue (qatmiscwq) across all devices and owned by intelqat.ko, a deferred routine from the device-specific driver may still be pending in the queue. If this routine executes after the driver is unloaded, it can dereference freed memory, resulting in a page fault and kernel crash like the following:

BUG: unable to handle page fault for address: ffa000002e50a01c #PF: supervisor read access in kernel mode RIP: 0010:pmbhhandler+0x1d2/0x250 [intelqat] Call Trace: pmbhhandler+0x1d2/0x250 [intelqat] processonework+0x171/0x340 workerthread+0x277/0x3a0 kthread+0xf0/0x120 retfromfork+0x2d/0x50

To prevent this, flush the misc workqueue during device shutdown to ensure that all pending work items are completed before the driver is unloaded.

Note: This approach may slightly increase shutdown latency if the workqueue contains jobs from other devices, but it ensures correctness and stability.