netdev - Re: [PATCH net 1/3] taprio: Fix kernel panic in taprio

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Date:   Wed, 28 Aug 2019 19:51:16 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Vinicius Costa Gomes <vinicius.gomes@...el.com>
Cc:     jhs@...atatu.com, xiyou.wangcong@...il.com,
        Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>, vedang.patel@...el.com,
        leandro.maciel.dorileo@...el.com, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net 1/3] taprio: Fix kernel panic in taprio_destroy

Hi Vinicius,

On Wed, 28 Aug 2019 at 19:31, Vinicius Costa Gomes
<vinicius.gomes@...el.com> wrote:
>
> Hi,
>
> Vladimir Oltean <olteanv@...il.com> writes:
>
> > taprio_init may fail earlier than this line:
> >
> >       list_add(&q->taprio_list, &taprio_list);
> >
> > i.e. due to the net device not being multi queue.
>
> Good catch.
>
> >
> > Attempting to remove q from the global taprio_list when it is not part
> > of it will result in a kernel panic.
> >
> > Fix it by iterating through the list and removing it only if found.
> >
> > Signed-off-by: Vladimir Oltean <olteanv@...il.com>
> > ---
> >  net/sched/sch_taprio.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
> > index 540bde009ea5..f1eea8c68011 100644
> > --- a/net/sched/sch_taprio.c
> > +++ b/net/sched/sch_taprio.c
> > @@ -1199,12 +1199,17 @@ static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
> >
> >  static void taprio_destroy(struct Qdisc *sch)
> >  {
> > -     struct taprio_sched *q = qdisc_priv(sch);
> > +     struct taprio_sched *p, *q = qdisc_priv(sch);
> >       struct net_device *dev = qdisc_dev(sch);
> > +     struct list_head *pos, *tmp;
> >       unsigned int i;
> >
> >       spin_lock(&taprio_list_lock);
> > -     list_del(&q->taprio_list);
> > +     list_for_each_safe(pos, tmp, &taprio_list) {
> > +             p = list_entry(pos, struct taprio_sched, taprio_list);
> > +             if (p == q)
> > +                     list_del(&q->taprio_list);
> > +     }
>
> Personally, I would do things differently, I am thinking: adding the
> taprio instance earlier to the list in taprio_init(), and keeping
> taprio_destroy() the way it is now. But take this more as a suggestion
> :-)
>

While I don't strongly oppose your proposal (keep the list removal
unconditional, but match it better in placement to the list addition),
I think it's rather fragile and I do see this bug recurring in the
future. Anyway if you want to keep it "simpler" I can respin it like
that.

>
> Cheers,
> --
> Vinicius
>

Regards,
-Vladimir