perf: Fix event leak upon exit

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

perf: Fix event leak upon exit

When a task is scheduled out, pending sigtrap deliveries are deferred

to the target task upon resume to userspace via task_work.

However failures while adding an event's callback to the task_work

engine are ignored. And since the last call for events exit happen

after task work is eventually closed, there is a small window during

which pending sigtrap can be queued though ignored, leaking the event

refcount addition such as in the following scenario:

TASK A

-----

do_exit()

exit_task_work(tsk);

<IRQ>

perf_event_overflow()

event->pending_sigtrap = pending_id;

irq_work_queue(&event->pending_irq);

</IRQ>

=========> PREEMPTION: TASK A -> TASK B

event_sched_out()

event->pending_sigtrap = 0;

atomic_long_inc_not_zero(&event->refcount)

// FAILS: task work has exited

task_work_add(&event->pending_task)

[...]

<IRQ WORK>

perf_pending_irq()

// early return: event->oncpu = -1

</IRQ WORK>

[...]

=========> TASK B -> TASK A

perf_event_exit_task(tsk)

perf_event_exit_event()

free_event()

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

// leak event due to unexpected refcount == 2

As a result the event is never released while the task exits.

Fix this with appropriate task_work_add()'s error handling.