CVE-2026-90075 net/sched: fq_codel: clamp default quantum and mtu
A local attacker with CAP_NET_ADMIN can cause a kernel soft lockup via integer overflow in fq_codel when configuring a device with an extremely large MTU.
- Exploited in the wild
- Not known
- Login needed
- An administrator
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-90075?
CVE-2026-90075 is a vulnerability in Linux, published on 17 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
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.
Which versions are affected by CVE-2026-90075?
How do I fix CVE-2026-90075?
Apply the vendor's patch.
Is CVE-2026-90075 being exploited?
There is no known exploitation of CVE-2026-90075 in the wild and no public exploit code.
Sources
Also known as AZL-101846, BELL-CVE-2026-90075, DEBIAN-CVE-2026-90075, GHSA-cgq5-rq9f-qphv, UBUNTU-CVE-2026-90075.