Vulnerability CVE-2024-43869
Published: 2024-08-21

Description:
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.

 References:
https://git.kernel.org/stable/c/9ad46f1fef421d43cdab3a7d1744b2f43b54dae0
https://git.kernel.org/stable/c/ed2c202dac55423a52d7e2290f2888bf08b8ee99
https://git.kernel.org/stable/c/104e258a004037bc7dba9f6085c71dad6af57ad4
https://git.kernel.org/stable/c/f34d8307a73a18de5320fcc6f40403146d061891
https://git.kernel.org/stable/c/3a5465418f5fd970e86a86c7f4075be262682840
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