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 timer_pending() and hrtimer_active() APIs.

The changeover blindly replaced the first by the second. However,

timer_pending() returns True when the timer is queued but not when its

callback is running, whereas hrtimer_active() returns True when the

hrtimer is queued _or_ its callback is running. This difference

occasionally caused dummy_urb_enqueue() 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

usb_kill_urb() 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 "timer_pending" flag and

setting or clearing it at the appropriate times.