CVE-2025-40214

In the Linux kernel, the following vulnerability has been resolved:

af_unix: Initialise scc_index in unix_add_edge().

Quang Le reported that the AF_UNIX GC could garbage-collect a receive queue of an alive in-flight socket, with a nice repro.

The repro consists of three stages.

1-a. Create a single cyclic reference with many sockets

1-b. close() all sockets
1-c. Trigger GC

2) 2-a. Pass sk-A to an embryo sk-B 2-b. Pass sk-X to sk-X 2-c. Trigger GC

3-a. accept() the embryo sk-B

3-b. Pass sk-B to sk-C
3-c. close() the in-flight sk-A
3-d. Trigger GC

As of 2-c, sk-A and sk-X are linked to unix_unvisited_vertices, and unix_walk_scc() groups them into two different SCCs:

unix_sk(sk-A)->vertex->scc_index = 2 (UNIX_VERTEX_INDEX_START) unix_sk(sk-X)->vertex->scc_index = 3

Once GC completes, unix_graph_grouped is set to true. Also, unix_graph_maybe_cyclic is set to true due to sk-X's cyclic self-reference, which makes close() trigger GC.

At 3-b, unix_add_edge() allocates unix_sk(sk-B)->vertex and links it to unix_unvisited_vertices.

unix_update_graph() is called at 3-a. and 3-b., but neither unix_graph_grouped nor unix_graph_maybe_cyclic is changed because both sk-B's listener and sk-C are not in-flight.

3-c decrements sk-A's file refcnt to 1.

Since unix_graph_grouped is true at 3-d, unix_walk_scc_fast() is finally called and iterates 3 sockets sk-A, sk-B, and sk-X:

sk-A -> sk-B (-> sk-C) sk-X -> sk-X

This is totally fine. All of them are not yet close()d and should be grouped into different SCCs.

However, unix_vertex_dead() misjudges that sk-A and sk-B are in the same SCC and sk-A is dead.

unix_sk(sk-A)->scc_index == unix_sk(sk-B)->scc_index <-- Wrong! && sk-A's file refcnt == unix_sk(sk-A)->vertex->out_degree ^-- 1 in-flight count for sk-B -> sk-A is dead !?

The problem is that unix_add_edge() does not initialise scc_index.

Stage 1) is used for heap spraying, making a newly allocated vertex have vertex->scc_index == 2 (UNIX_VERTEX_INDEX_START) set by unix_walk_scc() at 1-c.

Let's track the max SCC index from the previous unix_walk_scc() call and assign the max + 1 to a new vertex's scc_index.

This way, we can continue to avoid Tarjan's algorithm while preventing misjudgments.

This vulnerability is rated Moderate rather than Important because successful exploitation requires local user execution and a complex sequence of descriptor passing and heap grooming to trigger premature socket reclamation. In standard Red Hat Enterprise Linux environments, an attacker requires local access to manipulate AF_UNIX socket cycles, limiting the immediate risk primarily to kernel denial of service. Additionally, the flaw is confined to modern kernel revisions and does not affect enterprise versions running older kernel branches such as RHEL 8 and 9.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.