5.4-stable review patch. If anyone has any objections, please let me know.
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From: William Liu will@willsroot.io
[ 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/8DuRWwfqjoRDLDmBMlIfbrsZg9Gx50DHJc1ilxsEBNe2D... [2] https://lwn.net/Articles/719297/
Fixes: 0afb51e72855 ("[PKT_SCHED]: netem: reinsert for duplication") Reported-by: William Liu will@willsroot.io Reported-by: Savino Dicanosa savy@syst3mfailure.io 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 --- net/sched/sch_netem.c | 40 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 40 insertions(+)
diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c index 27bd18c74e85..cdf2df194d24 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) @@ -1023,6 +1058,11 @@ 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.