netdev - [PATCH net] net_sched: hfsc: Address reentrant enqueue adding class to eltree twice

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Message-ID: <20250521205351.1395206-1-pctammela@mojatatu.com>
Date: Wed, 21 May 2025 17:53:51 -0300
From: Pedro Tammela <pctammela@...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,
	horms@...nel.org,
	Pedro Tammela <pctammela@...atatu.com>,
	Savy <savy@...t3mfailure.io>,
	Will <will@...lsroot.io>
Subject: [PATCH net] net_sched: hfsc: Address reentrant enqueue adding class to eltree twice

Savy says:
    "We are writing to report that this recent patch
    (141d34391abbb315d68556b7c67ad97885407547) [1]
    can be bypassed, and a UAF can still occur when HFSC is utilized with
    NETEM.

    The patch only checks the cl->cl_nactive field to determine whether
    it is the first insertion or not [2], but this field is only
    incremented by init_vf [3].

    By using HFSC_RSC (which uses init_ed) [4], it is possible to bypass the
    check and insert the class twice in the eltree.
    Under normal conditions, this would lead to an infinite loop in
    hfsc_dequeue for the reasons we already explained in this report [5].

    However, if TBF is added as root qdisc and it is configured with a
    very low rate,
    it can be utilized to prevent packets from being dequeued.
    This behavior can be exploited to perform subsequent insertions in the
    HFSC eltree and cause a UAF."

To fix both the UAF and the inifinite loop, with netem as an hfsc child,
check explicitly in hfsc_enqueue whether the class is already in the eltree
whenever the HFSC_RSC flag is set.

[1] https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=141d34391abbb315d68556b7c67ad97885407547
[2] https://elixir.bootlin.com/linux/v6.15-rc5/source/net/sched/sch_hfsc.c#L1572
[3] https://elixir.bootlin.com/linux/v6.15-rc5/source/net/sched/sch_hfsc.c#L677
[4] https://elixir.bootlin.com/linux/v6.15-rc5/source/net/sched/sch_hfsc.c#L1574
[5] https://lore.kernel.org/netdev/8DuRWwfqjoRDLDmBMlIfbrsZg9Gx50DHJc1ilxsEBNe2D6NMoigR_eIRIG0LOjMc3r10nUUZtArXx4oZBIdUfZQrwjcQhdinnMis_0G7VEk=@willsroot.io/T/#u

Fixes: 37d9cf1a3ce3 ("sched: Fix detection of empty queues in child qdiscs")
Reported-by: Savy <savy@...t3mfailure.io>
Reported-by: Will <will@...lsroot.io>
Signed-off-by: Pedro Tammela <pctammela@...atatu.com>
---
 net/sched/sch_hfsc.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/sched/sch_hfsc.c b/net/sched/sch_hfsc.c
index cb8c525ea..1c4067bec 100644
--- a/net/sched/sch_hfsc.c
+++ b/net/sched/sch_hfsc.c
@@ -175,6 +175,11 @@ struct hfsc_sched {
 
 #define	HT_INFINITY	0xffffffffffffffffULL	/* infinite time value */
 
+static bool cl_in_el_or_vttree(struct hfsc_class *cl)
+{
+	return ((cl->cl_flags & HFSC_FSC) && cl->cl_nactive) ||
+		((cl->cl_flags & HFSC_RSC) && !RB_EMPTY_NODE(&cl->el_node));
+}
 
 /*
  * eligible tree holds backlogged classes being sorted by their eligible times.
@@ -1089,6 +1094,7 @@ hfsc_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
 		qdisc_purge_queue(parent->qdisc);
 	hfsc_adjust_levels(parent);
 	sch_tree_unlock(sch);
+	RB_CLEAR_NODE(&cl->el_node);
 
 	qdisc_class_hash_grow(sch, &q->clhash);
 
@@ -1569,7 +1575,7 @@ hfsc_enqueue(struct sk_buff *skb, struct Qdisc *sch, struct sk_buff **to_free)
 		return err;
 	}
 
-	if (first && !cl->cl_nactive) {
+	if (first && !cl_in_el_or_vttree(cl)) {
 		if (cl->cl_flags & HFSC_RSC)
 			init_ed(cl, len);
 		if (cl->cl_flags & HFSC_FSC)
-- 
2.43.0