CVE-2025-38553

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

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

netem_enqueues 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 packets 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/

Affected Software

References

• https://git.kernel.org/stable/c/09317dfb681ac5a96fc69bea0c54441cf91b8270
• https://git.kernel.org/stable/c/103c4e27ec9f5fe53022e46e976abf52c7221baf
• https://git.kernel.org/stable/c/250f8796006c0f2bc638ce545f601d49ae8d528b
• https://git.kernel.org/stable/c/325f5ec67cc0a77f2d0d453445b9857f1cd06c76
• https://git.kernel.org/stable/c/795cb393e38977aa991e70a9363da0ee734b2114
• https://git.kernel.org/stable/c/ec8e0e3d7adef940cdf9475e2352c0680189d14e