GHSA-G3XR-84MJ-FVX6
Vulnerability from github – Published: 2025-12-04 15:30 – Updated: 2026-07-30 06:32In 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) 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) 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": [],
"aliases": [
"CVE-2025-40214"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2025-12-04T13:15:48Z",
"severity": "HIGH"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\naf_unix: Initialise scc_index in unix_add_edge().\n\nQuang Le reported that the AF_UNIX GC could garbage-collect a\nreceive queue of an alive in-flight socket, with a nice repro.\n\nThe repro consists of three stages.\n\n 1)\n 1-a. Create a single cyclic reference with many sockets\n 1-b. close() all sockets\n 1-c. Trigger GC\n\n 2)\n 2-a. Pass sk-A to an embryo sk-B\n 2-b. Pass sk-X to sk-X\n 2-c. Trigger GC\n\n 3)\n 3-a. accept() the embryo sk-B\n 3-b. Pass sk-B to sk-C\n 3-c. close() the in-flight sk-A\n 3-d. Trigger GC\n\nAs of 2-c, sk-A and sk-X are linked to unix_unvisited_vertices,\nand unix_walk_scc() groups them into two different SCCs:\n\n unix_sk(sk-A)-\u003evertex-\u003escc_index = 2 (UNIX_VERTEX_INDEX_START)\n unix_sk(sk-X)-\u003evertex-\u003escc_index = 3\n\nOnce GC completes, unix_graph_grouped is set to true.\nAlso, unix_graph_maybe_cyclic is set to true due to sk-X\u0027s\ncyclic self-reference, which makes close() trigger GC.\n\nAt 3-b, unix_add_edge() allocates unix_sk(sk-B)-\u003evertex and\nlinks it to unix_unvisited_vertices.\n\nunix_update_graph() is called at 3-a. and 3-b., but neither\nunix_graph_grouped nor unix_graph_maybe_cyclic is changed\nbecause both sk-B\u0027s listener and sk-C are not in-flight.\n\n3-c decrements sk-A\u0027s file refcnt to 1.\n\nSince unix_graph_grouped is true at 3-d, unix_walk_scc_fast()\nis finally called and iterates 3 sockets sk-A, sk-B, and sk-X:\n\n sk-A -\u003e sk-B (-\u003e sk-C)\n sk-X -\u003e sk-X\n\nThis is totally fine. All of them are not yet close()d and\nshould be grouped into different SCCs.\n\nHowever, unix_vertex_dead() misjudges that sk-A and sk-B are\nin the same SCC and sk-A is dead.\n\n unix_sk(sk-A)-\u003escc_index == unix_sk(sk-B)-\u003escc_index \u003c-- Wrong!\n \u0026\u0026\n sk-A\u0027s file refcnt == unix_sk(sk-A)-\u003evertex-\u003eout_degree\n ^-- 1 in-flight count for sk-B\n -\u003e sk-A is dead !?\n\nThe problem is that unix_add_edge() does not initialise scc_index.\n\nStage 1) is used for heap spraying, making a newly allocated\nvertex have vertex-\u003escc_index == 2 (UNIX_VERTEX_INDEX_START)\nset by unix_walk_scc() at 1-c.\n\nLet\u0027s track the max SCC index from the previous unix_walk_scc()\ncall and assign the max + 1 to a new vertex\u0027s scc_index.\n\nThis way, we can continue to avoid Tarjan\u0027s algorithm while\npreventing misjudgments.",
"id": "GHSA-g3xr-84mj-fvx6",
"modified": "2026-07-30T06:32:18Z",
"published": "2025-12-04T15:30:32Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2025-40214"
},
{
"type": "WEB",
"url": "https://cert-portal.siemens.com/productcert/html/ssa-253495.html"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/1aa7e40ee850c9053e769957ce6541173891204d"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/20003fbb9174121b27bd1da6ebe61542ac4c327d"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/4cd8d755c7d4f515dd9abf483316aca2f1b7b0f3"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/60e6489f8e3b086bd1130ad4450a2c112e863791"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/db81ad20fd8aef7cc7d536c52ee5ea4c1f979128"
},
{
"type": "WEB",
"url": "https://mohandacherir.github.io/Qdiv7/posts/unix_new_gc"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
"type": "CVSS_V3"
}
]
}
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