---
id: CVE-2025-38457
title: 'net/sched: Abort __tc_modify_qdisc if parent class does not exist'
summary: |-
  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 s…
severity: none
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6 <
    923a276c74e25073ae391e930792ac86a9f77f1e
  - >-
    Linux >= 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6 <
    90436e72c9622c2f70389070088325a3232d339f
  - >-
    Linux >= 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6 <
    25452638f133ac19d75af3f928327d8016952c8e
  - >-
    Linux >= 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6 <
    23c165dde88eac405eebb59051ea1fe139a45803
  - >-
    Linux >= 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6 <
    4c691d1b6b6dbd73f30ed9ee7da05f037b0c49af
  - >-
    Linux >= 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6 <
    8ecd651ef24ab50123692a4e3e25db93cb11602a
  - >-
    Linux >= 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6 <
    e28a383d6485c3bb51dc5953552f76c4dea33eea
  - >-
    Linux >= 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6 <
    ffdde7bf5a439aaa1955ebd581f5c64ab1533963
  - Linux 2.6.20
published: '2025-07-25'
updated: '2026-10-15'
sourceUpdated: '2026-10-15T01:07:17.428Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2025-38457'
references:
  - url: 'https://git.kernel.org/stable/c/923a276c74e25073ae391e930792ac86a9f77f1e'
  - url: 'https://git.kernel.org/stable/c/90436e72c9622c2f70389070088325a3232d339f'
  - url: 'https://git.kernel.org/stable/c/25452638f133ac19d75af3f928327d8016952c8e'
  - url: 'https://git.kernel.org/stable/c/23c165dde88eac405eebb59051ea1fe139a45803'
  - url: 'https://git.kernel.org/stable/c/4c691d1b6b6dbd73f30ed9ee7da05f037b0c49af'
  - url: 'https://git.kernel.org/stable/c/8ecd651ef24ab50123692a4e3e25db93cb11602a'
  - url: 'https://git.kernel.org/stable/c/e28a383d6485c3bb51dc5953552f76c4dea33eea'
  - url: 'https://git.kernel.org/stable/c/ffdde7bf5a439aaa1955ebd581f5c64ab1533963'
tags:
  - cve.org
epss: 0.00196
epssPercentile: 0.0926
ingestedAt: '2026-10-15T05:52:46.088Z'
---

## Overview

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/

## Affected

- `Linux >= 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6 < 923a276c74e25073ae391e930792ac86a9f77f1e`
- `Linux >= 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6 < 90436e72c9622c2f70389070088325a3232d339f`
- `Linux >= 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6 < 25452638f133ac19d75af3f928327d8016952c8e`
- `Linux >= 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6 < 23c165dde88eac405eebb59051ea1fe139a45803`
- `Linux >= 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6 < 4c691d1b6b6dbd73f30ed9ee7da05f037b0c49af`
- `Linux >= 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6 < 8ecd651ef24ab50123692a4e3e25db93cb11602a`
- `Linux >= 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6 < e28a383d6485c3bb51dc5953552f76c4dea33eea`
- `Linux >= 5e50da01d0ce7ef0ba3ed6cfabd62f327da0aca6 < ffdde7bf5a439aaa1955ebd581f5c64ab1533963`
- `Linux 2.6.20`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.