Code: static int perf_swevent_add(struct perf_event *event, int flags)
{
	struct swevent_htable *swhash = this_cpu_ptr(&swevent_htable);
	struct hw_perf_event *hwc = &event->hw;
	struct hlist_head *head;

	if (is_sampling_event(event)) {
		hwc->last_period = hwc->sample_period;
		perf_swevent_set_period(event);
	}

 	hwc->state = !(flags & PERF_EF_START);
 
 	head = find_swevent_head(swhash, event);
	if (!head) {
		/*
		 * We can race with cpu hotplug code. Do not
		 * WARN if the cpu just got unplugged.
		 */
		WARN_ON_ONCE(swhash->online);
 		return -EINVAL;
	}
 
 	hlist_add_head_rcu(&event->hlist_entry, head);
 	perf_event_update_userpage(event);

	return 0;
}

Vulnerability Type: DoS +Priv 
CWE ID: CWE-416
Summary: Race condition in kernel/events/core.c in the Linux kernel before 4.4 allows local users to gain privileges or cause a denial of service (use-after-free) by leveraging incorrect handling of an swevent data structure during a CPU unplug operation.
Commit Message: perf: Fix race in swevent hash

There's a race on CPU unplug where we free the swevent hash array
while it can still have events on. This will result in a
use-after-free which is BAD.

Simply do not free the hash array on unplug. This leaves the thing
around and no use-after-free takes place.

When the last swevent dies, we do a for_each_possible_cpu() iteration
anyway to clean these up, at which time we'll free it, so no leakage
will occur.

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Tested-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Ingo Molnar <mingo@kernel.org>