CVE-2025-38553

Published: August 20, 2025Last modified: August 20, 2025

Description

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/

Status

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/ad340a4b4adb855b18b3666f26ad65c8968e2deb
• https://git.kernel.org/stable/c/cab2809944989889f88a1a8b5cff1c78460c72cb
• https://git.kernel.org/stable/c/ec8e0e3d7adef940cdf9475e2352c0680189d14e
• https://git.kernel.org/stable/c/f088b6ebe8797a3f948d2cae47f34bfb45cc6522