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

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.