Re: [PATCH net v3 4/6] net/sched: fq_pie: clamp default quantum to avoid signed overflow

[Jamal Hadi Salim <jhs@mojatatu.com>]

On Tue, Aug 25, 2026 at 4:33 AM Paolo Abeni <pabeni@redhat.com> wrote:
>
> Hi,
>
> On 8/22/26 9:55 PM, Jamal Hadi Salim wrote:
> > fq_pie_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_pie_qdisc_dequeue(),
> > causing an infinite loop and soft lockup. Emulate fq_pie_policy which
> > is already bounded to [1, 1 << 20]; clamp the default to [256, 1 << 20].
> > 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: ec97ecf1ebe4 ("net: sched: add Flow Queue PIE packet scheduler")
> > Reported-by: vega@nebusec.ai
> > Tested-by: Victor Nogueira <victor@mojatatu.com>
> > Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
> > ---
> >  net/sched/sch_fq_pie.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/net/sched/sch_fq_pie.c b/net/sched/sch_fq_pie.c
> > index 069e1facd413..b27d95418707 100644
> > --- a/net/sched/sch_fq_pie.c
> > +++ b/net/sched/sch_fq_pie.c
> > @@ -427,7 +427,8 @@ static int fq_pie_init(struct Qdisc *sch, struct nlattr *opt,
> >       pie_params_init(&q->p_params);
> >       sch->limit = 10 * 1024;
> >       q->p_params.limit = sch->limit;
> > -     q->quantum = psched_mtu(qdisc_dev(sch));
> > +     q->quantum = clamp_t(u32, psched_mtu(qdisc_dev(sch)),
> > +                          256, 1 << 20);
>
> Sashiko thinks that the soft lookup is still reachable via pie_change:
>
> https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260822195509.112717-1-jhs%40mojatatu.com
>
> and has similar concerns for patch 6/6, too. It marks the issues as
> pre-existing, but AFAICS they overlap with the things addressed here.
>
> WDYT?

You are right, they overlap. I had them as followups (with a few
others derived from the sashiko feedback with justification that the
v3 init-path clamps are independently correct and the stab cap already
mitigates the change-path worst case to a stall; but those two a
(adding max(256U, ...) to both fq_pie_change() and sfq_change(),
matching the fq_codel_change()) are more serious.
So if you'd prefer a v4 respin of the whole series, I can do that.

Sashiko is a double edge sword - i think code quality is improving but
it feels like the work load has doubled ;->
Here's what i had as followups (some still to be vetted, just noting
what sashiko is stating to be reviewed later when cycles available and
potential followup patches sent):
- sch_dualpi2 unclamped psched_mtu
- sch_pie unclamped psched_mtu → AQM disable / div-by-zero
- hhf TCA_HHF_HH_FLOWS_LIMIT unbounded
- fq_pie_change() / sfq_change() 256 floor missing (one that you bring up here)
- DRR/ETS quantum=0 spin (have a patch, was reported already as a bug by vega@)
- Consider two separate clamps for fq_codel/sch_codel (nipa
gpt-5-6-sol-3-15): quantum in [256, FQ_CODEL_QUANTUM_MAX], mtubounded
separately (no 256 floor on mtu)


cheers,
jamal
>