GHSA-CGQ5-RQ9F-QPHV
Vulnerability from github – Published: 2026-09-17 18:31 – Updated: 2026-09-17 18:31In the Linux kernel, the following vulnerability has been resolved:
net/sched: fq_codel: clamp default quantum and mtu
fq_codel_init() sets q->quantum = psched_mtu(qdisc_dev(sch)) without clamping. A device with a huge MTU (e.g. dummy with max_mtu == 0 accepting MTU 2147483634) makes psched_mtu() return 0x80000000, which overflows the signed flow->deficit to INT_MIN in fq_codel_dequeue(), causing an infinite loop and soft lockup. Emulate fq_codel_change() and constrain to [256, FQ_CODEL_QUANTUM_MAX].
The same unclamped psched_mtu() is assigned to q->cparams.mtu a bit below, and fq_codel_change() never updates it. codel_should_drop() tests "*backlog <= params->mtu"; with mtu == 0x80000000 (~2 GiB) and the default 32 MiB memory_limit, the test is always true, so CoDel is silently and completely disabled (no drops, no ECN). Declare a single clamped mtu and assign both q->quantum and q->cparams.mtu from it, which also removes the double psched_mtu() call.
Conditions to recreate the bug: a device whose MTU (plus hard_header_len) wraps psched_mtu() into the sign bit (e.g. a dummy device with max_mtu == 0 accepting MTU 2147483634). Requires CAP_NET_ADMIN in a user namespace.
{
"affected": [],
"aliases": [
"CVE-2026-90075"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-09-17T17:16:56Z",
"severity": null
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet/sched: fq_codel: clamp default quantum and mtu\n\nfq_codel_init() sets q-\u003equantum = psched_mtu(qdisc_dev(sch)) without\nclamping. A device with a huge MTU (e.g. dummy with max_mtu == 0\naccepting MTU 2147483634) makes psched_mtu() return 0x80000000, which\noverflows the signed flow-\u003edeficit to INT_MIN in fq_codel_dequeue(),\ncausing an infinite loop and soft lockup. Emulate fq_codel_change()\nand constrain to [256, FQ_CODEL_QUANTUM_MAX].\n\nThe same unclamped psched_mtu() is assigned to q-\u003ecparams.mtu a bit\nbelow, and fq_codel_change() never updates it. codel_should_drop()\ntests \"*backlog \u003c= params-\u003emtu\"; with mtu == 0x80000000 (~2 GiB) and\nthe default 32 MiB memory_limit, the test is always true, so CoDel is\nsilently and completely disabled (no drops, no ECN). Declare a single\nclamped mtu and assign both q-\u003equantum and q-\u003ecparams.mtu from it,\nwhich also removes the double psched_mtu() call.\n\nConditions to recreate the bug: a device whose MTU (plus\nhard_header_len) wraps psched_mtu() into the sign bit (e.g. a dummy\ndevice with max_mtu == 0 accepting MTU 2147483634). Requires\nCAP_NET_ADMIN in a user namespace.",
"id": "GHSA-cgq5-rq9f-qphv",
"modified": "2026-09-17T18:31:50Z",
"published": "2026-09-17T18:31:50Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-90075"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/324f86806673ae4a55f66d28db84577f46f615e1"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/3782067ec0d485628b6f1f9ede3eeeb4f611fcf2"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/397e2b1f71d9f15b8b4e47d24eb620e4dff8878d"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/9f499e5827fdb6d7fdb46a7ce731852f6b1a1bb9"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/a9a5b2943a00df2ab81a2209f31dd9e87016f120"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/d315ee8a07fd1810880227319cf60e6cd925ef22"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/d9ebd8f9aa8b2773235889cb903fafd61f2d8585"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/dfb4b61db886917244284b18b44b23d2254b82c2"
}
],
"schema_version": "1.4.0",
"severity": []
}
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