events: Reuse value read using READ_ONCE instead of re-reading it

In perf_event_addr_filters_apply, the task associated with
the event (event->ctx->task) is read using READ_ONCE at the beginning
of the function, checked, and then re-read from event->ctx->task,
voiding all guarantees of the checks. Reuse the value that was read by
READ_ONCE to ensure the consistency of the task struct throughout the
function.

Fixes: 375637b ("perf/core: Introduce address range filtering")
Signed-off-by: Baptiste Lepers <baptiste.lepers@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210906015310.12802-1-baptiste.lepers@gmail.com

kernel/events/core.c

@@ -10193,7 +10193,7 @@ static void perf_event_addr_filters_apply(struct perf_event *event)
 		return;
 
 	if (ifh->nr_file_filters) {
-		mm = get_task_mm(event->ctx->task);
+		mm = get_task_mm(task);
 		if (!mm)
 			goto restart;