CVE-2026-90074net/sched: fq_pie: clamp default quantum to avoid signed overflow

Official description

In the Linux kernel, the following vulnerability has been resolved: net/sched: fq_pie: clamp default quantum to avoid signed overflow fq_pie_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_pie_qdisc_dequeue(), causing an infinite loop and soft lockup. Emulate fq_pie_policy which is already bounded to [1, 1 << 20]; clamp the default to [256, 1 << 20]. 256 matches fq_codel's floor and is a sane minimum for a DRR quantum. 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/1f43e4dbc93b7d02761a024d5f895df823c07b72
• https://git.kernel.org/stable/c/f7942b2693bc24c7d609a03d06f97bee3b0bd96a
• https://git.kernel.org/stable/c/6f1be05faecef4c0f9a008200209f66ef2e133f7
• https://git.kernel.org/stable/c/435da3734a8e63d3067016accc977ed1d10aaaa0
• https://git.kernel.org/stable/c/44dad8fe99b58a7fe2961e9eec6a5f07ace5b51f
• https://git.kernel.org/stable/c/044fa2498bbc2900f0c48a6ef2fdd5b2bdfc3795
• https://git.kernel.org/stable/c/bf97fd2163ad1191ef8daa3d4df8372aee19e396
• https://git.kernel.org/stable/c/c86cd7ed0b0e44779a3d1683f03e4353baf4bdc9