net/sched: Restrict conditions for adding duplicating netems to qdisc tree
[ Upstream commit ec8e0e3d7adef940cdf9475e2352c0680189d14e ]
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.
Previous approaches suggested in discussions in chronological order:
1) Track duplication status or ttl in the sk_buff struct. Considered
too specific a use case to extend such a struct, though this would
be a resilient fix and address other previous and potential future
DOS bugs like the one described in loopy fun [2].
2) Restrict netem_enqueue recursion depth like in act_mirred with a
per cpu variable. However, netem_dequeue can call enqueue on its
child, and the depth restriction could be bypassed if the child is a
netem.
3) Use the same approach as in 2, but add metadata in netem_skb_cb
to handle the netem_dequeue case and track a packet's involvement
in duplication. This is an overly complex approach, and Jamal
notes that the skb cb can be overwritten to circumvent this
safeguard.
4) Prevent the addition of a netem to a qdisc tree if its ancestral
path contains a netem. However, filters and actions can cause a
packet to change paths when re-enqueued to the root from netem
duplication, leading us to the current solution: prevent a
duplicating netem from inhabiting the same tree as other netems.
[1] https://lore.kernel.org/netdev/8DuRWwfqjoRDLDmBMlIfbrsZg9Gx50DHJc1ilxsEBNe2D6NMoigR_eIRIG0LOjMc3r10nUUZtArXx4oZBIdUfZQrwjcQhdinnMis_0G7VEk=@willsroot.io/
[2] https://lwn.net/Articles/719297/
BUG=b/440033990
TEST=presubmit
RELEASE_NOTE=Fixed CVE-2025-38553 in the Linux kernel.
cos-patch: security-moderate
Fixes: 0afb51e72855 ("[PKT_SCHED]: netem: reinsert for duplication")
Reported-by: William Liu <will@willsroot.io>
Reported-by: Savino Dicanosa <savy@syst3mfailure.io>
Change-Id: Iada55106f6e979c57952e201de173b767f3259cb
Signed-off-by: William Liu <will@willsroot.io>
Signed-off-by: Savino Dicanosa <savy@syst3mfailure.io>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20250708164141.875402-1-will@willsroot.io
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/111035
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Chenglong Tang <chenglongtang@google.com>
diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index cb38e58..2613353 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -962,6 +962,41 @@ static int parse_attr(struct nlattr *tb[], int maxtype, struct nlattr *nla,
return 0;
}
+static const struct Qdisc_class_ops netem_class_ops;
+
+static int check_netem_in_tree(struct Qdisc *sch, bool duplicates,
+ 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 (duplicates ||
+ ((struct netem_sched_data *)qdisc_priv(root))->duplicate)
+ 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 (duplicates ||
+ ((struct netem_sched_data *)qdisc_priv(q))->duplicate)
+ 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)
@@ -1020,6 +1055,11 @@ 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, extack);
+ if (ret)
+ goto unlock;
+
q->duplicate = qopt->duplicate;
/* for compatibility with earlier versions.
