[tip: perf/urgent] events: Reuse value read using READ_ONCE instead of re-reading it

[tip-bot2 for Baptiste Lepers]

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID:     b89a05b21f46150ac10a962aa50109250b56b03b
Gitweb:        https://git.kernel.org/tip/b89a05b21f46150ac10a962aa50109250b56b03b
Author:        Baptiste Lepers <baptiste.lepers@xxxxxxxxx>
AuthorDate:    Mon, 06 Sep 2021 11:53:10 +10:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 15 Sep 2021 17:49:06 +02:00

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: 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
---
 kernel/events/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 744e872..0c000cb 100644
--- a/kernel/events/core.c
+++ b/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;