CVE-2026-90075net/sched: fq_codel: clamp default quantum and mtu

Official 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.

Affected packages & fixed versions

Affected products (as reported)

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