CVE-2026-23413

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. clsact_init() initializes ingress first and then takes care of the egress part. This can fail midway, for example, via tcf_block_get_ext(). Upon failure, the kernel will trigger the clsact_destroy() callback.

Commit 1cb6f0bae504 (bpf: Fix too early release of tcx_entry) details the way how the transition is happening. If tcf_block_get_ext on the q->ingress_block ends up failing, we took the tcx_miniq_inc reference count on the ingress side, but not yet on the egress side. clsact_destroy() 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 egress_entry 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 mini_qdisc_pair_inited, and utilizes that upon clsact_destroy() in order to fix the use-after-free scenario. Convert the ingress_destroy() side as well so both are consistent to each other.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/0509b762bc5e8ea7b8391130730c6d8502fc6e69
• https://git.kernel.org/stable/c/37bef86e5428d59f70a4da82b80f9a8f252fecbe
• https://git.kernel.org/stable/c/4c9af67f99aa3e51b522c54968ab3ac8272be41c
• https://git.kernel.org/stable/c/a0671125d4f55e1e98d9bde8a0b671941987e208
• https://git.kernel.org/stable/c/a73d95b57bf9faebdfed591bcb7ed9292062a84c