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net/sched: fq_codel: clamp default quantum and mtu

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fq_codel_init() sets q->quantum = psched_mtu(qdisc_dev(sch)) without
clamping. A device with a huge MTU (e.g. dummy with max_mtu == 0
accepting MTU 2147483634) makes psched_mtu() return 0x80000000, which
overflows the signed flow->deficit to INT_MIN in fq_codel_dequeue(),
causing an infinite loop and soft lockup. Emulate fq_codel_change()
and constrain to [256, FQ_CODEL_QUANTUM_MAX].

The same unclamped psched_mtu() is assigned to q->cparams.mtu a bit
below, and fq_codel_change() never updates it. codel_should_drop()
tests "*backlog <= params->mtu"; with mtu == 0x80000000 (~2 GiB) and
the default 32 MiB memory_limit, the test is always true, so CoDel is
silently and completely disabled (no drops, no ECN). Declare a single
clamped mtu and assign both q->quantum and q->cparams.mtu from it,
which also removes the double psched_mtu() call.

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: 4b549a2ef4 ("fq_codel: Fair Queue Codel AQM")
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/20260822195509.112717-3-jhs@mojatatu.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Jamal Hadi Salim 2026-08-22 15:55:05 -04:00 committed by Paolo Abeni
parent 709f34f7c2
commit d9ebd8f9aa
1 changed files with 4 additions and 2 deletions
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6
net/sched/sch_fq_codel.c
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@ -509,6 +509,7 @@ static int fq_codel_init(struct Qdisc *sch, struct nlattr *opt,
struct netlink_ext_ack *extack)
{
struct fq_codel_sched_data *q = qdisc_priv(sch);
u32 mtu;
int i;
int err;
@ -516,13 +517,14 @@ static int fq_codel_init(struct Qdisc *sch, struct nlattr *opt,
q->flows_cnt = 1024;
q->memory_limit = 32 << 20; /* 32 MBytes */
q->drop_batch_size = 64;
q->quantum = psched_mtu(qdisc_dev(sch));
mtu = clamp_t(u32, psched_mtu(qdisc_dev(sch)), 256, FQ_CODEL_QUANTUM_MAX);
q->quantum = mtu;
INIT_LIST_HEAD(&q->new_flows);
INIT_LIST_HEAD(&q->old_flows);
codel_params_init(&q->cparams);
codel_stats_init(&q->cstats);
q->cparams.ecn = true;
q->cparams.mtu = psched_mtu(qdisc_dev(sch));
q->cparams.mtu = mtu;
if (opt) {
err = fq_codel_change(sch, opt, extack);