perf/core: Fix concurrent sys_perf_event_open() vs. 'move_group' race

Di Shen reported a race between two concurrent sys_perf_event_open()
calls where both try and move the same pre-existing software group
into a hardware context.

The problem is exactly that described in commit:

  f63a8daa5812 ("perf: Fix event->ctx locking")

... where, while we wait for a ctx->mutex acquisition, the event->ctx
relation can have changed under us.

That very same commit failed to recognise sys_perf_event_context() as an
external access vector to the events and thereby didn't apply the
established locking rules correctly.

So while one sys_perf_event_open() call is stuck waiting on
mutex_lock_double(), the other (which owns said locks) moves the group
about. So by the time the former sys_perf_event_open() acquires the
locks, the context we've acquired is stale (and possibly dead).

Apply the established locking rules as per perf_event_ctx_lock_nested()
to the mutex_lock_double() for the 'move_group' case. This obviously means
we need to validate state after we acquire the locks.

Reported-by: Di Shen (Keen Lab)
Tested-by: John Dias <joaodias@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Min Chong <mchong@google.com>
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>
Fixes: f63a8daa5812 ("perf: Fix event->ctx locking")
Link: http://lkml.kernel.org/r/20170106131444.GZ3174@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
static void _perf_event_enable(struct perf_event *event)
{
	struct perf_event_context *ctx = event->ctx;

	raw_spin_lock_irq(&ctx->lock);
	if (event->state >= PERF_EVENT_STATE_INACTIVE ||
	    event->state <  PERF_EVENT_STATE_ERROR) {
		raw_spin_unlock_irq(&ctx->lock);
		return;
	}

	/*
	 * If the event is in error state, clear that first.
	 *
	 * That way, if we see the event in error state below, we know that it
	 * has gone back into error state, as distinct from the task having
	 * been scheduled away before the cross-call arrived.
	 */
	if (event->state == PERF_EVENT_STATE_ERROR)
		event->state = PERF_EVENT_STATE_OFF;
	raw_spin_unlock_irq(&ctx->lock);

	event_function_call(event, __perf_event_enable, NULL);
}

static void _perf_event_enable(struct perf_event *event)
{
	struct perf_event_context *ctx = event->ctx;

	raw_spin_lock_irq(&ctx->lock);
	if (event->state >= PERF_EVENT_STATE_INACTIVE ||
	    event->state <  PERF_EVENT_STATE_ERROR) {
		raw_spin_unlock_irq(&ctx->lock);
		return;
	}

	/*
	 * If the event is in error state, clear that first.
	 *
	 * That way, if we see the event in error state below, we know that it
	 * has gone back into error state, as distinct from the task having
	 * been scheduled away before the cross-call arrived.
	 */
	if (event->state == PERF_EVENT_STATE_ERROR)
		event->state = PERF_EVENT_STATE_OFF;
	raw_spin_unlock_irq(&ctx->lock);

	event_function_call(event, __perf_event_enable, NULL);
}
