CVE-2026-90075

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.