CVE-2024-58057: idpf: convert workqueues to unbound

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

idpf: convert workqueues to unbound

When a workqueue is created with WQUNBOUND, its work items are served by special worker-pools, whose host workers are not bound to any specific CPU. In the default configuration (i.e. when queuedelayedwork and friends do not specify which CPU to run the work item on), WQUNBOUND allows the work item to be executed on any CPU in the same node of the CPU it was enqueued on. While this solution potentially sacrifices locality, it avoids contention with other processes that might dominate the CPU time of the processor the work item was scheduled on.

This is not just a theoretical problem: in a particular scenario misconfigured process was hogging most of the time from CPU0, leaving less than 0.5% of its CPU time to the kworker. The IDPF workqueues that were using the kworker on CPU0 suffered large completion delays as a result, causing performance degradation, timeouts and eventual system crash.

I have also run a manual test to gauge the performance improvement. The test consists of an antagonist process (./stress --cpu 2) consuming as much of CPU 0 as possible. This process is run under taskset 01 to bind it to CPU0, and its priority is changed with chrt -pQ 9900 10000 ${pid} and renice -n -20 ${pid} after start.

Then, the IDPF driver is forced to prefer CPU0 by editing all calls to queuedelayedwork, moddelayedwork, etc... to use CPU 0.

Finally, ktraces for the workqueue events are collected.

Without the current patch, the antagonist process can force arbitrary delays between workqueuequeuework and workqueueexecutestart, that in my tests were as high as 30ms. With the current patch applied, the workqueue can be migrated to another unloaded CPU in the same node, and, keeping everything else equal, the maximum delay I could see was 6us.