CVE-2025-21971: net_sched: Prevent creation of classes with TC_H_ROOT

Severity: medium (CVSS 5.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-04-01 · updated 2026-10-15
Weakness: CWE-835

Description

In the Linux kernel, the following vulnerability has been resolved: net_sched: Prevent creation of classes with TC_H_ROOT The function qdisc_tree_reduce_backlog() uses TC_H_ROOT as a termination condition when traversing up the qdisc tree to update parent backlog counters. However, if a class is created with classid TC_H_ROOT, the traversal terminates prematurely at this class instead of reaching the actual root qdisc, causing parent statistics to be incorrectly maintained. In case of DRR, this could lead to a crash as reported by Mingi Cho. Prevent the creation of any Qdisc class with classid TC_H_ROOT (0xFFFFFFFF) across all qdisc types, as suggested by Jamal.

Affected products

Linux kernel: from 2.6.26 before 5.4.292; from 5.5 before 5.10.236; from 5.11 before 5.15.180; from 5.16 before 6.1.132; from 6.2 before 6.6.84; from 6.7 before 6.12.20 — fixed in 5.10.236, 5.15.180, 5.4.292, 6.1.132, 6.12.20, 6.13.8

How to fix

Upgrade to a fixed version: Linux kernel 5.10.236, 5.15.180, 5.4.292, 6.1.132. Check the vendor advisory for mitigations.

References

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