CVE-2024-43870: 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 taskwork.

However failures while adding an event's callback to the taskwork 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 -----

doexit() exittaskwork(tsk);

<IRQ> perfeventoverflow() event->pendingsigtrap = pendingid; irqworkqueue(&event->pendingirq); </IRQ> =========> PREEMPTION: TASK A -> TASK B eventschedout() event->pendingsigtrap = 0; atomiclongincnotzero(&event->refcount) // FAILS: task work has exited taskworkadd(&event->pendingtask) [...] <IRQ WORK> perfpendingirq() // early return: event->oncpu = -1 </IRQ WORK> [...] =========> TASK B -> TASK A perfeventexittask(tsk) perfeventexitevent() freeevent() WARN(atomiclongcmpxchg(&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 taskworkadd()'s error handling.