[PATCH 4.14 201/217] events: Reuse value read using READ_ONCE instead of re-reading it

[Greg Kroah-Hartman]

From: Baptiste Lepers <baptiste.lepers@xxxxxxxxx>

commit b89a05b21f46150ac10a962aa50109250b56b03b upstream.

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: 375637bc52495 ("perf/core: Introduce address range filtering")
Signed-off-by: Baptiste Lepers <baptiste.lepers@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20210906015310.12802-1-baptiste.lepers@xxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/events/core.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8435,7 +8435,7 @@ static void perf_event_addr_filters_appl
 	if (!ifh->nr_file_filters)
 		return;
 
-	mm = get_task_mm(event->ctx->task);
+	mm = get_task_mm(task);
 	if (!mm)
 		goto restart;