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net/sched: fq: add overflow bounds to quantum and initial quantum

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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: afe4fd0624 ("pkt_sched: fq: Fair Queue packet scheduler")
Reported-by: vega@nebusec.ai
Tested-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20260822195509.112717-2-jhs@mojatatu.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Jamal Hadi Salim 2026-08-22 15:55:04 -04:00 committed by Paolo Abeni
parent 728836ebca
commit 709f34f7c2
1 changed files with 4 additions and 2 deletions
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6
net/sched/sch_fq.c
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@ -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;