[PATCH net] net/sched: taprio: fix use-after-free in advance_sched() on schedule switch

[Vinicius Costa Gomes]

In advance_sched(), when should_change_schedules() returns true,
switch_schedules() is called to promote the admin schedule to oper.
switch_schedules() queues the old oper schedule for RCU freeing via
call_rcu(), but 'next' still points into an entry of the old oper
schedule. The subsequent 'next->end_time = end_time' and
rcu_assign_pointer(q->current_entry, next) are use-after-free.

Fix this by selecting 'next' from the new oper schedule immediately
after switch_schedules(), and using its pre-calculated end_time.
setup_first_end_time() sets the first entry's end_time to
base_time + interval when the schedule is installed, so the value
is already correct.

The deleted 'end_time = sched_base_time(admin)' assignment was also
harmful independently: it would overwrite the new first entry's
pre-calculated end_time with just base_time.

Fixes: a3d43c0d56f1 ("taprio: Add support adding an admin schedule")
Reported-by: Junxi Qian <qjx1298677004@xxxxxxxxx>
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@xxxxxxxxx>
---
 net/sched/sch_taprio.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index f721c03514f6..0316f2dee06a 100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c
@@ -972,11 +972,12 @@ static enum hrtimer_restart advance_sched(struct hrtimer *timer)
 	}
 
 	if (should_change_schedules(admin, oper, end_time)) {
-		/* Set things so the next time this runs, the new
-		 * schedule runs.
-		 */
-		end_time = sched_base_time(admin);
 		switch_schedules(q, &admin, &oper);
+		/* After changing schedules, the next entry is the first one
+		 * in the new schedule, with a pre-calculated end_time.
+		 */
+		next = list_first_entry(&oper->entries, struct sched_entry, list);
+		end_time = next->end_time;
 	}
 
 	next->end_time = end_time;

---
base-commit: 02f72964395911e7a09bb2ea2fe6f79eda4ea2c2
change-id: 20260410-taprio-user-after-free-fix-net-fbd67f66512b

Best regards,
--  
Vinicius