CVE-2022-49607

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

perf/core: Fix data race between perf_event_set_output() and perf_mmap_close()

Yang Jihing reported a race between perf_event_set_output() and perf_mmap_close():

CPU1					CPU2

perf_mmap_close(e2)
  if (atomic_dec_and_test(&e2->rb->mmap_count)) // 1 - > 0
    detach_rest = true

					ioctl(e1, IOC_SET_OUTPUT, e2)
					  perf_event_set_output(e1, e2)

  ...
  list_for_each_entry_rcu(e, &e2->rb->event_list, rb_entry)
    ring_buffer_attach(e, NULL);
    // e1 isn't yet added and
    // therefore not detached

					    ring_buffer_attach(e1, e2->rb)
					      list_add_rcu(&e1->rb_entry,
							   &e2->rb->event_list)

After this; e1 is attached to an unmapped rb and a subsequent perf_mmap() will loop forever more:

again:
	mutex_lock(&e->mmap_mutex);
	if (event->rb) {
		...
		if (!atomic_inc_not_zero(&e->rb->mmap_count)) {
			...
			mutex_unlock(&e->mmap_mutex);
			goto again;
		}
	}

The loop in perf_mmap_close() holds e2->mmap_mutex, while the attach in perf_event_set_output() holds e1->mmap_mutex. As such there is no serialization to avoid this race.

Change perf_event_set_output() to take both e1->mmap_mutex and e2->mmap_mutex to alleviate that problem. Additionally, have the loop in perf_mmap() detach the rb directly, this avoids having to wait for the concurrent perf_mmap_close() to get around to doing it to make progress.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.