CVE-2026-90076net/sched: fq: add overflow bounds to quantum and initial quantum

Official description

In the Linux kernel, the following vulnerability has been resolved: net/sched: fq: add overflow bounds to quantum and initial quantum fq_init() computes quantum = 2 * psched_mtu() and initial_quantum = 10 * psched_mtu() with no overflow check. A device with a huge MTU (e.g. dummy with max_mtu == 0 accepting MTU 2147483634) makes psched_mtu() return 0x80000000; the 2 * and 10 * multiplications wrap to 0 in 32-bit arithmetic, so q->quantum == 0. Then in fq_dequeue() the credit-refill loop adds 0 to f->credit (which stays <= 0) and goto begin loops forever under the qdisc lock, creating a soft lockup. Clamp psched_mtu() to [1, 1 << 20] before multiplying so the product cannot wrap, then cap the result at 1 << 20, matching the bound already enforced on TCA_FQ_QUANTUM in fq_change(). Conditions to recreate the bug: a device whose MTU (plus hard_header_len) is large enough that 2 * psched_mtu() wraps (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/cf398f67d9caa71af3b9fb9319058b22fe6b524e
• https://git.kernel.org/stable/c/fd63409b669f9951f1cc6b02486a5254103800d1
• https://git.kernel.org/stable/c/d16dac3925be95ad46e986d4b139c9898b6e227f
• https://git.kernel.org/stable/c/f6b3e3848a5fca63438984acd6d9eceac80814c1
• https://git.kernel.org/stable/c/e35acd56f244d94355f9ab237c2ecc8fba5e6f04
• https://git.kernel.org/stable/c/709f34f7c28dc4dd6c40343d101850f11e172312