net: sched: fix use-after-free in taprio_change()

In 'taprio_change()', 'admin' pointer may become dangling due to sched
switch / removal caused by 'advance_sched()', and critical section
protected by 'q->current_entry_lock' is too small to prevent from such
a scenario (which causes use-after-free detected by KASAN). Fix this
by prefer 'rcu_replace_pointer()' over 'rcu_assign_pointer()' to update
'admin' immediately before an attempt to schedule freeing.

Fixes: a3d43c0 ("taprio: Add support adding an admin schedule")
Reported-by: syzbot+b65e0af58423fc8a73aa@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=b65e0af58423fc8a73aa
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Link: https://patch.msgid.link/20241018051339.418890-1-dmantipov@yandex.ru
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: dmantipov

net/sched/sch_taprio.c

@@ -1965,7 +1965,8 @@ static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
 
 		taprio_start_sched(sch, start, new_admin);
 
-		rcu_assign_pointer(q->admin_sched, new_admin);
+		admin = rcu_replace_pointer(q->admin_sched, new_admin,
+					    lockdep_rtnl_is_held());
 		if (admin)
 			call_rcu(&admin->rcu, taprio_free_sched_cb);