netdev - [PATCH net v2 2/5] net_sched: hfsc: Fix a UAF vulnerability in class with netem as child qdisc

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Message-ID: <20250416102427.3219655-3-victor@mojatatu.com>
Date: Wed, 16 Apr 2025 07:24:24 -0300
From: Victor Nogueira <victor@...atatu.com>
To: netdev@...r.kernel.org
Cc: jhs@...atatu.com,
	xiyou.wangcong@...il.com,
	jiri@...nulli.us,
	davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	toke@...hat.com,
	gerrard.tai@...rlabs.sg,
	pctammela@...atatu.com
Subject: [PATCH net v2 2/5] net_sched: hfsc: Fix a UAF vulnerability in class with netem as child qdisc

As described in Gerrard's report [1], we have a UAF case when an hfsc class
has a netem child qdisc. The crux of the issue is that hfsc is assuming
that checking for cl->qdisc->q.qlen == 0 guarantees that it hasn't inserted
the class in the vttree or eltree (which is not true for the netem
duplicate case).

This patch checks the n_active class variable to make sure that the code
won't insert the class in the vttree or eltree twice, catering for the
reentrant case.

[1] https://lore.kernel.org/netdev/CAHcdcOm+03OD2j6R0=YHKqmy=VgJ8xEOKuP6c7mSgnp-TEJJbw@mail.gmail.com/

Fixes: 37d9cf1a3ce3 ("sched: Fix detection of empty queues in child qdiscs")
Reported-by: Gerrard Tai <gerrard.tai@...rlabs.sg>
Acked-by: Jamal Hadi Salim <jhs@...atatu.com>
Signed-off-by: Victor Nogueira <victor@...atatu.com>
---
 net/sched/sch_hfsc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sched/sch_hfsc.c b/net/sched/sch_hfsc.c
index ce5045eea065..73b0741ffd99 100644
--- a/net/sched/sch_hfsc.c
+++ b/net/sched/sch_hfsc.c
@@ -1564,7 +1564,7 @@ hfsc_enqueue(struct sk_buff *skb, struct Qdisc *sch, struct sk_buff **to_free)
 		return err;
 	}
 
-	if (first) {
+	if (first && !cl->cl_nactive) {
 		if (cl->cl_flags & HFSC_RSC)
 			init_ed(cl, len);
 		if (cl->cl_flags & HFSC_FSC)
-- 
2.34.1