CVE-2026-23413: clsact: Fix use-after-free in init/destroy rollback asymmetry
In the Linux kernel, the following vulnerability has been resolved:
clsact: Fix use-after-free in init/destroy rollback asymmetry
Fix a use-after-free in the clsact qdisc upon init/destroy rollback asymmetry. The latter is achieved by first fully initializing a clsact instance, and then in a second step having a replacement failure for the new clsact qdisc instance. clsactinit() initializes ingress first and then takes care of the egress part. This can fail midway, for example, via tcfblockgetext(). Upon failure, the kernel will trigger the clsactdestroy() callback.
Commit 1cb6f0bae504 ("bpf: Fix too early release of tcxentry") details the way how the transition is happening. If tcfblockgetext on the q->ingressblock ends up failing, we took the tcxminiqinc reference count on the ingress side, but not yet on the egress side. clsactdestroy() tests whether the {ingress,egress}entry was non-NULL. However, even in midway failure on the replacement, both are in fact non-NULL with a valid egressentry from the previous clsact instance.
What we really need to test for is whether the qdisc instance-specific ingress or egress side previously got initialized. This adds a small helper for checking the miniq initialization called miniqdiscpairinited, and utilizes that upon clsactdestroy() in order to fix the use-after-free scenario. Convert the ingressdestroy() side as well so both are consistent to each other.
Affected Software
Remediation
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Patch 1cb6f0bae504
Event History
Frequently Asked Questions
What access does an attacker need to exploit this issue?
The supplied CVSS vector indicates local access, low attack complexity, and low privileges required. No user interaction is required.
What conditions trigger the vulnerable path?
The issue occurs during clsact qdisc initialization and destruction when initialization fails partway through replacement. The described failure point is tcf_block_get_ext() on the ingress block, after an ingress reference is acquired but before the egress side is initialized for the new instance.
What is the potential impact?
The issue is a use-after-free in the Linux kernel. The provided severity vector rates confidentiality, integrity, and availability impacts as high.
Which software is identified as affected?
The affected software listed is the Linux kernel, including debian/linux. The provided data does not identify specific affected or fixed version numbers.