CVE-2025-40214

Summary

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-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.

Affected Software

Weaknesses

ADP Enrichment

Additional References

• https://cert-portal.siemens.com/productcert/html/ssa-253495.html

References

• https://git.kernel.org/stable/c/20003fbb9174121b27bd1da6ebe61542ac4c327d
• https://git.kernel.org/stable/c/4cd8d755c7d4f515dd9abf483316aca2f1b7b0f3
• https://git.kernel.org/stable/c/db81ad20fd8aef7cc7d536c52ee5ea4c1f979128
• https://git.kernel.org/stable/c/1aa7e40ee850c9053e769957ce6541173891204d
• https://git.kernel.org/stable/c/60e6489f8e3b086bd1130ad4450a2c112e863791
• https://mohandacherir.github.io/Qdiv7/posts/unix_new_gc/

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