CVE-2026-90072net/sched: sfq: clamp quantum to avoid signed overflow soft lockup

Official description

In the Linux kernel, the following vulnerability has been resolved: net/sched: sfq: clamp quantum to avoid signed overflow soft lockup sfq_init() sets q->quantum = psched_mtu(qdisc_dev(sch)) (unsigned). A device with a huge MTU (e.g. dummy with max_mtu == 0 accepting MTU 2147483634) makes psched_mtu() return 0x80000000, so slot->allot = INT_MIN and INT_MIN + INT_MIN toggles between INT_MIN and 0 forever, spinning sfq_dequeue() under the qdisc lock. Clamp the quantum to [256, 1 << 20] so the refill loop terminates. The lower bound also covers q->quantum == 0 (psched_mtu() returning 0), which spins sfq_dequeue() identically. sfq_change() already rejects a negative quantum, so only the init path was exposed. 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/86799499585af06a9431c9e7782658667bd62650
• https://git.kernel.org/stable/c/2017c355a5a1af77736cd1739fe922f69b5652dc
• https://git.kernel.org/stable/c/6581a82bf08ba5d4e2a6d4e3080df43315b296c4
• https://git.kernel.org/stable/c/9d782c662879c3adaaca30e05c36910ad11b9e54
• https://git.kernel.org/stable/c/d72dbdfaeb5b9b8d884e0e036c3442754379ef74
• https://git.kernel.org/stable/c/aeb8196ecb95146e3410f635628e6cd47352b3e5
• https://git.kernel.org/stable/c/da16c31517cd2c06bb2c78c73e91ae192c01c426
• https://git.kernel.org/stable/c/816e90057ab1879562a5b7cc688e35bb9027ae97