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>
static void _perf_event_disable(struct perf_event *event)
{
	struct perf_event_context *ctx = event->ctx;
	struct task_struct *task = ctx->task;

	if (!task) {
		/*
		 * Disable the event on the cpu that it's on
		 */
		cpu_function_call(event->cpu, __perf_event_disable, event);
		return;
	}

retry:
	if (!task_function_call(task, __perf_event_disable, event))
		return;

	raw_spin_lock_irq(&ctx->lock);
	/*
	 * If the event is still active, we need to retry the cross-call.
	 */
	if (event->state == PERF_EVENT_STATE_ACTIVE) {
		raw_spin_unlock_irq(&ctx->lock);
		/*
		 * Reload the task pointer, it might have been changed by
		 * a concurrent perf_event_context_sched_out().
		 */
		task = ctx->task;
		goto retry;
	}

	/*
	 * Since we have the lock this context can't be scheduled
	 * in, so we can change the state safely.
	 */
	if (event->state == PERF_EVENT_STATE_INACTIVE) {
		update_group_times(event);
		event->state = PERF_EVENT_STATE_OFF;
	}
	raw_spin_unlock_irq(&ctx->lock);
}

static void _perf_event_disable(struct perf_event *event)
{
	struct perf_event_context *ctx = event->ctx;
	struct task_struct *task = ctx->task;

	if (!task) {
		/*
		 * Disable the event on the cpu that it's on
		 */
		cpu_function_call(event->cpu, __perf_event_disable, event);
		return;
	}

retry:
	if (!task_function_call(task, __perf_event_disable, event))
		return;

	raw_spin_lock_irq(&ctx->lock);
	/*
	 * If the event is still active, we need to retry the cross-call.
	 */
	if (event->state == PERF_EVENT_STATE_ACTIVE) {
		raw_spin_unlock_irq(&ctx->lock);
		/*
		 * Reload the task pointer, it might have been changed by
		 * a concurrent perf_event_context_sched_out().
		 */
		task = ctx->task;
		goto retry;
	}

	/*
	 * Since we have the lock this context can't be scheduled
	 * in, so we can change the state safely.
	 */
	if (event->state == PERF_EVENT_STATE_INACTIVE) {
		update_group_times(event);
		event->state = PERF_EVENT_STATE_OFF;
	}
	raw_spin_unlock_irq(&ctx->lock);
}
