From 709f34f7c28dc4dd6c40343d101850f11e172312 Mon Sep 17 00:00:00 2001 From: Jamal Hadi Salim Date: Sat, 22 Aug 2026 15:55:04 -0400 Subject: [PATCH] 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. Fixes: afe4fd062416 ("pkt_sched: fq: Fair Queue packet scheduler") Reported-by: vega@nebusec.ai Tested-by: Victor Nogueira Signed-off-by: Jamal Hadi Salim Link: https://patch.msgid.link/20260822195509.112717-2-jhs@mojatatu.com Signed-off-by: Paolo Abeni --- net/sched/sch_fq.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c index 4b5f6d896c6d..6144b5686f13 100644 --- a/net/sched/sch_fq.c +++ b/net/sched/sch_fq.c @@ -1226,12 +1226,14 @@ static int fq_init(struct Qdisc *sch, struct nlattr *opt, struct netlink_ext_ack *extack) { struct fq_sched_data *q = qdisc_priv(sch); + u32 mtu; int i, err; sch->limit = 10000; q->flow_plimit = 100; - q->quantum = 2 * psched_mtu(qdisc_dev(sch)); - q->initial_quantum = 10 * psched_mtu(qdisc_dev(sch)); + mtu = clamp_t(u32, psched_mtu(qdisc_dev(sch)), 1, 1 << 20); + q->quantum = min_t(u32, 2 * mtu, 1 << 20); + q->initial_quantum = min_t(u32, 10 * mtu, 1 << 20); q->flow_refill_delay = msecs_to_jiffies(40); q->flow_max_rate = ~0UL; q->time_next_delayed_flow = ~0ULL;