net_sched: ets: fix a race in ets_qdisc_change()

jira VULN-71723
cve CVE-2025-38107
commit-author Eric Dumazet <edumazet@google.com>
commit d92adac

Gerrard Tai reported a race condition in ETS, whenever SFQ perturb timer
fires at the wrong time.

The race is as follows:

CPU 0                                 CPU 1
[1]: lock root
[2]: qdisc_tree_flush_backlog()
[3]: unlock root
 |
 |                                    [5]: lock root
 |                                    [6]: rehash
 |                                    [7]: qdisc_tree_reduce_backlog()
 |
[4]: qdisc_put()

This can be abused to underflow a parent's qlen.

Calling qdisc_purge_queue() instead of qdisc_tree_flush_backlog()
should fix the race, because all packets will be purged from the qdisc
before releasing the lock.

Fixes: b05972f ("net: sched: tbf: don't call qdisc_put() while holding tree lock")
	Reported-by: Gerrard Tai <gerrard.tai@starlabs.sg>
	Suggested-by: Gerrard Tai <gerrard.tai@starlabs.sg>
	Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250611111515.1983366-5-edumazet@google.com
	Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit d92adac)
	Signed-off-by: Roxana Nicolescu <rnicolescu@ciq.com>

Author: roxanan1996

net/sched/sch_ets.c

@@ -672,7 +672,7 @@ static int ets_qdisc_change(struct Qdisc *sch, struct nlattr *opt,
 	for (i = q->nbands; i < oldbands; i++) {
 		if (i >= q->nstrict && q->classes[i].qdisc->q.qlen)
 			list_del_init(&q->classes[i].alist);
-		qdisc_tree_flush_backlog(q->classes[i].qdisc);
+		qdisc_purge_queue(q->classes[i].qdisc);
 	}
 	WRITE_ONCE(q->nstrict, nstrict);
 	memcpy(q->prio2band, priomap, sizeof(priomap));