netdev - [PATCH net] net/sched: sch_qfq: do not free existing class in qfq_change

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Message-ID: <20260112175656.17605-1-edumazet@google.com>
Date: Mon, 12 Jan 2026 17:56:56 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, Jamal Hadi Salim <jhs@...atatu.com>, 
	Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com, Eric Dumazet <edumazet@...gle.com>, 
	syzbot+07f3f38f723c335f106d@...kaller.appspotmail.com
Subject: [PATCH net] net/sched: sch_qfq: do not free existing class in qfq_change_class()

Fixes qfq_change_class() error case.

cl->qdisc and cl should only be freed if a new class and qdisc
were allocated, or we risk various UAF.

Fixes: 462dbc9101ac ("pkt_sched: QFQ Plus: fair-queueing service at DRR cost")
Reported-by: syzbot+07f3f38f723c335f106d@...kaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6965351d.050a0220.eaf7.00c5.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/sched/sch_qfq.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/sched/sch_qfq.c b/net/sched/sch_qfq.c
index f4013b547438ffe1bdc8ba519971a1681df4700b..9d59090bbe934ad56ab08a59708aab375aa77cf0 100644
--- a/net/sched/sch_qfq.c
+++ b/net/sched/sch_qfq.c
@@ -529,8 +529,10 @@ static int qfq_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
 	return 0;
 
 destroy_class:
-	qdisc_put(cl->qdisc);
-	kfree(cl);
+	if (!existing) {
+		qdisc_put(cl->qdisc);
+		kfree(cl);
+	}
 	return err;
 }
 
-- 
2.52.0.457.g6b5491de43-goog