CVE-2025-21703: netem: Update sch->q.qlen before qdisc_tree_reduce_backlog()

Severity: high (CVSS 7.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-02-18 · updated 2026-10-15
Weakness: CWE-416

Description

In the Linux kernel, the following vulnerability has been resolved: netem: Update sch->q.qlen before qdisc_tree_reduce_backlog() qdisc_tree_reduce_backlog() notifies parent qdisc only if child qdisc becomes empty, therefore we need to reduce the backlog of the child qdisc before calling it. Otherwise it would miss the opportunity to call cops->qlen_notify(), in the case of DRR, it resulted in UAF since DRR uses ->qlen_notify() to maintain its active list.

Affected products

Linux kernel: from 5.4.288 before 5.4.291; from 5.10.232 before 5.10.235; from 5.15.175 before 5.15.179; from 6.1.121 before 6.1.129; from 6.6.67 before 6.6.78; from 6.12.6 before 6.12.14 — fixed in 5.10.235, 5.15.179, 5.4.291, 6.1.129, 6.12.14, 6.13.3

How to fix

Upgrade to a fixed version: Linux kernel 5.10.235, 5.15.179, 5.4.291, 6.1.129. Check the vendor advisory for mitigations.

References

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