CVE-2024-43869: 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 freeevent() 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() preparetaskswitch() =======> <NMI> perfeventoverflow() event->pendingsigtrap = ... irqworkqueue(&event->pendingirq) <======= </NMI> perfeventtaskschedout() eventschedout() event->pendingsigtrap = 0; atomiclongincnotzero(&event->refcount) taskworkadd(&event->pendingtask) finishlockswitch() =======> <IRQ> perfpendingirq() //do nothing, rely on pending task work <======= </IRQ>

beginnewexec() perfeventexittask() perfeventexitevent() // If is child event freeevent() WARN(atomiclongcmpxchg(&event->refcount, 1, 0) != 1) // event is leaked

Similar scenarios can also happen with perfeventremoveonexec() or simply against concurrent perfeventrelease().

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.