perf: Fix event leak upon exec and file release

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

perf: Fix event leak upon exec and file release

The perf pending task work is never waited upon the matching event

release. In the case of a child event, released via free_event()

directly, this can potentially result in a leaked event, such as in the

following scenario that doesn't even require a weak IRQ work

implementation to trigger:

schedule()

prepare_task_switch()

=======> <NMI>

perf_event_overflow()

event->pending_sigtrap = ...

irq_work_queue(&event->pending_irq)

<======= </NMI>

perf_event_task_sched_out()

event_sched_out()

event->pending_sigtrap = 0;

atomic_long_inc_not_zero(&event->refcount)

task_work_add(&event->pending_task)

finish_lock_switch()

=======> <IRQ>

perf_pending_irq()

//do nothing, rely on pending task work

<======= </IRQ>

begin_new_exec()

perf_event_exit_task()

perf_event_exit_event()

// If is child event

free_event()

WARN(atomic_long_cmpxchg(&event->refcount, 1, 0) != 1)

// event is leaked

Similar scenarios can also happen with perf_event_remove_on_exec() or

simply against concurrent perf_event_release().

Fix this with synchonizing against the possibly remaining pending task

work while freeing the event, just like is done with remaining pending

IRQ work. This means that the pending task callback neither need nor

should hold a reference to the event, preventing it from ever beeing

freed.