In the Linux kernel, the following vulnerability has been resolved:
net/sched: fq_codel: clamp default quantum and mtu
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 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.
