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net/sched: Abort __tc_modify_qdisc if parent class does not exist

Published Jul 25, 2025

Description

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 products

Remediation

No remediation recorded yet.

Metrics

Weaknesses (1)

CWE-476

NULL Pointer Dereference

References (16)

Change history (0)

No recorded changes yet.

Sources
CVE.org / MITRE
Status PUBLISHED
Assigner Linux
Published Jul 25, 2025
Updated Jul 14, 2026
Reserved Apr 16, 2025
Link CVE-2025-38457
CISA Vulnrichment
Updated n/a
NVD
Status Modified
Modified Jun 17, 2026
Link CVE-2025-38457
Red Hat
Severity Moderate
Public date Jul 25, 2025
Links CVE-2025-38457 - Bugzilla