🛡️ CVE-2025-38553 — kernel

net/sched: Restrict conditions for adding duplicating netems to qdisc tree

In the Linux kernel, the following vulnerability has been resolved:

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.

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/