CVE-2024-50100: USB: gadget: dummy-hcd: Fix "task hung" problem

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

USB: gadget: dummy-hcd: Fix "task hung" problem

The syzbot fuzzer has been encountering "task hung" problems ever since the dummy-hcd driver was changed to use hrtimers instead of regular timers. It turns out that the problems are caused by a subtle difference between the timerpending() and hrtimeractive() APIs.

The changeover blindly replaced the first by the second. However, timerpending() returns True when the timer is queued but not when its callback is running, whereas hrtimeractive() returns True when the hrtimer is queued or its callback is running. This difference occasionally caused dummyurbenqueue() to think that the callback routine had not yet started when in fact it was almost finished. As a result the hrtimer was not restarted, which made it impossible for the driver to dequeue later the URB that was just enqueued. This caused usbkillurb() to hang, and things got worse from there.

Since hrtimers have no API for telling when they are queued and the callback isn't running, the driver must keep track of this for itself. That's what this patch does, adding a new "timerpending" flag and setting or clearing it at the appropriate times.