netdev - [PATCH net 1/2] net/sched: Restrict conditions for adding duplicating netems to qdisc tree

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Message-ID: <20250622190344.446090-1-will@willsroot.io>
Date: Sun, 22 Jun 2025 19:05:18 +0000
From: William Liu <will@...lsroot.io>
To: netdev@...r.kernel.org
Cc: jhs@...atatu.com, xiyou.wangcong@...il.com, victor@...atatu.com, pctammela@...atatu.com, pabeni@...hat.com, kuba@...nel.org, stephen@...workplumber.org, dcaratti@...hat.com, savy@...t3mfailure.io, jiri@...nulli.us, davem@...emloft.net, edumazet@...gle.com, horms@...nel.org, William Liu <will@...lsroot.io>
Subject: [PATCH net 1/2] net/sched: Restrict conditions for adding duplicating netems to qdisc tree

netem_enqueue's duplication prevention logic breaks when a netem
resides in a qdisc tree with other netems - this can lead to a
soft lockup and OOM loop in netem_dequeue as seen in [1].
Ensure that a duplicating netem cannot exist in a tree with other
netems.

[1] https://lore.kernel.org/netdev/8DuRWwfqjoRDLDmBMlIfbrsZg9Gx50DHJc1ilxsEBNe2D6NMoigR_eIRIG0LOjMc3r10nUUZtArXx4oZBIdUfZQrwjcQhdinnMis_0G7VEk=@willsroot.io/

Fixes: 0afb51e72855 ("[PKT_SCHED]: netem: reinsert for duplication")
Reported-by: William Liu <will@...lsroot.io>
Reported-by: Savino Dicanosa <savy@...t3mfailure.io>
Signed-off-by: William Liu <will@...lsroot.io>
Signed-off-by: Savino Dicanosa <savy@...t3mfailure.io>
---
 net/sched/sch_netem.c | 45 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index fdd79d3ccd8c..308ce6629d7e 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -973,6 +973,46 @@ static int parse_attr(struct nlattr *tb[], int maxtype, struct nlattr *nla,
 	return 0;
 }
 
+static const struct Qdisc_class_ops netem_class_ops;
+
+static inline bool has_duplication(struct Qdisc *sch)
+{
+	struct netem_sched_data *q = qdisc_priv(sch);
+
+	return q->duplicate != 0;
+}
+
+static int check_netem_in_tree(struct Qdisc *sch, bool only_duplicating,
+			       struct netlink_ext_ack *extack)
+{
+	struct Qdisc *root, *q;
+	unsigned int i;
+
+	root = qdisc_root_sleeping(sch);
+
+	if (sch != root && root->ops->cl_ops == &netem_class_ops) {
+		if (!only_duplicating || has_duplication(root))
+			goto err;
+	}
+
+	if (!qdisc_dev(root))
+		return 0;
+
+	hash_for_each(qdisc_dev(root)->qdisc_hash, i, q, hash) {
+		if (sch != q && q->ops->cl_ops == &netem_class_ops) {
+			if (!only_duplicating || has_duplication(q))
+				goto err;
+		}
+	}
+
+	return 0;
+
+err:
+	NL_SET_ERR_MSG(extack,
+		       "netem: cannot mix duplicating netems with other netems in tree");
+	return -EINVAL;
+}
+
 /* Parse netlink message to set options */
 static int netem_change(struct Qdisc *sch, struct nlattr *opt,
 			struct netlink_ext_ack *extack)
@@ -1031,6 +1071,11 @@ static int netem_change(struct Qdisc *sch, struct nlattr *opt,
 	q->gap = qopt->gap;
 	q->counter = 0;
 	q->loss = qopt->loss;
+
+	ret = check_netem_in_tree(sch, qopt->duplicate == 0, extack);
+	if (ret)
+		goto unlock;
+
 	q->duplicate = qopt->duplicate;
 
 	/* for compatibility with earlier versions.
-- 
2.43.0