CVE-2025-38553

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

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

netemenqueue'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 netemdequeue, 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 netemenqueue recursion depth like in actmirred 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 netemskbcb 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/8DuRWwfqjoRDLDmBMlIfbrsZg9Gx50DHJc1ilxsEBNe2D6NMoigReIRIG0LOjMc3r10nUUZtArXx4oZBIdUfZQrwjcQhdinnMis0G7VEk=@willsroot.io/ [2] https://lwn.net/Articles/719297/