# \[PATCH net v3 5/6\] net/sched: hhf: clamp quantum before hhf\_change() to avoid overflow - From: Jamal Hadi Salim \ - Date: 2026-08-22 19:55:38 - List: netdev - Page: https://ratatoskr.run/netdev/2026/08/17448954 - Thread: https://ratatoskr.run/netdev/2026/08/17448949/t.md ```text hhf_init() sets q->quantum = psched_mtu(qdisc_dev(sch)) with no overflow check. A device with a huge MTU (e.g. dummy with max_mtu == 0 accepting MTU 2147483634) makes weight * quantum overflow the signed deficit in hhf_dequeue(), spinning forever. Clamp q->quantum before hhf_change() so both the opt and !opt paths see a sane quantum. Without this, bare "tc qdisc add ... hhf" succeeds with a clamped quantum but "tc qdisc add ... hhf limit 1000" (any option present) fails with -EINVAL because hhf_change() re-validates the unclamped default (sch_hhf.c:559). 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. Fixes: 10239edf86f1 ("net-qdisc-hhf: Heavy-Hitter Filter (HHF) qdisc") Reported-by: Tested-by: Victor Nogueira Signed-off-by: Jamal Hadi Salim --- net/sched/sch_hhf.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/net/sched/sch_hhf.c b/net/sched/sch_hhf.c index d85cb0263b67..96acab6a8da0 100644 --- a/net/sched/sch_hhf.c +++ b/net/sched/sch_hhf.c @@ -624,6 +624,10 @@ static int hhf_init(struct Qdisc *sch, struct nlattr *opt, q->hhf_evict_timeout = HZ; /* 1 sec */ q->hhf_non_hh_weight = 2; + if ((int)q->quantum <= 0 || + (u64)q->quantum * q->hhf_non_hh_weight > INT_MAX) + q->quantum = 256; + if (opt) { int err = hhf_change(sch, opt, extack); -- 2.43.0 ```