net/sched: sch_codel: clamp default mtu to avoid disabling CoDel

net/sched: sch_codel: clamp default mtu to avoid disabling CoDel

codel_init() sets q->params.mtu = 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. In
codel_should_drop() the test "*backlog <= params->mtu" then compares
the backlog against ~2 GiB; with the default sch->limit of
DEFAULT_CODEL_LIMIT (1000) packets the backlog can never reach it, so
the test is always true and CoDel is silently and completely disabled
i.e no drops, no ECN marking, codel degrades to a tail-drop FIFO.
codel_change() never updates params.mtu, so the init path is the only
place to clamp it. Constrain to [256, 1 << 20], matching the fq_codel
bound; 256 is a sane floor that only makes CoDel slightly more willing
to act on very small queues, which is the safe direction.

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: 76e3cc126bb2 ("codel: Controlled Delay 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/20260822195509.112717-4-jhs@mojatatu.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>