netdev - Re: [PATCH net] net: sched: fq_codel: fix null-ptr-deref issue in fq_codel

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Date:   Mon, 24 Oct 2022 10:04:39 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Stanislav Fomichev <sdf@...gle.com>
Cc:     Kuniyuki Iwashima <kuniyu@...zon.com>, shaozhengchao@...wei.com,
        davem@...emloft.net, jhs@...atatu.com, jiri@...nulli.us,
        kuba@...nel.org, netdev@...r.kernel.org, pabeni@...hat.com,
        weiyongjun1@...wei.com, xiyou.wangcong@...il.com,
        yuehaibing@...wei.com, bpf@...r.kernel.org
Subject: Re: [PATCH net] net: sched: fq_codel: fix null-ptr-deref issue in fq_codel_enqueue()

On Mon, Oct 24, 2022 at 9:54 AM Stanislav Fomichev <sdf@...gle.com> wrote:
>
> On Fri, Oct 21, 2022 at 5:13 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> >
> > +Stanislav, bpf
> >
> > From:   Zhengchao Shao <shaozhengchao@...wei.com>
> > Date:   Fri, 21 Oct 2022 16:40:58 +0800
> > > As [0] see, it will cause null-ptr-deref issue.
> > > The following is the process of triggering the problem:
> > > fq_codel_enqueue()
> > >       ...
> > >       idx = fq_codel_classify()        --->if idx != 0
> > >       flow = &q->flows[idx];
> > >       flow_queue_add(flow, skb);       --->add skb to flow[idex]
> > >       q->backlogs[idx] += qdisc_pkt_len(skb); --->backlogs = 0
> > >       ...
> > >       fq_codel_drop()          --->set sch->limit = 0, always
> > >                                    drop packets
> > >               ...
> > >               idx = i          --->because backlogs in every
> > >                                    flows is 0, so idx = 0
> > >               ...
> > >               flow = &q->flows[idx];   --->get idx=0 flow
> > >               ...
> > >               dequeue_head()
> > >                       skb = flow->head; --->flow->head = NULL
> > >                       flow->head = skb->next; --->cause null-ptr-deref
> > >
> > > So, only need to discard the packets whose len is 0 on dropping path of
> > > enqueue. Then, the correct flow id can be obtained by fq_codel_drop() on
> > > next enqueuing.
> > >
> > > [0]: https://syzkaller.appspot.com/bug?id=0b84da80c2917757915afa89f7738a9d16ec96c5
> >
> > This can be caused by BPF, but there seems to be no consensus yet.
> > https://lore.kernel.org/netdev/CAKH8qBsOMxVaemF0Oy=vE1V0vKO8ORUcVGB5YANS3HdKOhVjjw@mail.gmail.com/
> >
> > """
> > I think the consensus here is that the stack, in general, doesn't
> > expect the packets like this. So there are probably more broken things
> > besides fq_codel. Thus, it's better if we remove the ability to
> > generate them from the bpf side instead of fixing the individual users
> > like fq_codel.
> > """
>
> That shouldn't happen after commit fd1894224407 ("bpf: Don't redirect
> packets with invalid pkt_len"), so not sure why this patch is needed
> at all?

This patch keeps coming, I have already explained it is silly to add such tests.

Most drivers will crash if skb->len == 0 anyway.

>
> > > Fixes: 4b549a2ef4be ("fq_codel: Fair Queue Codel AQM")
> > > Signed-off-by: Zhengchao Shao <shaozhengchao@...wei.com>
> > > ---
> > >  net/sched/sch_fq_codel.c | 8 ++++++++
> > >  1 file changed, 8 insertions(+)
> > >
> > > diff --git a/net/sched/sch_fq_codel.c b/net/sched/sch_fq_codel.c
> > > index 99d318b60568..3bbe7f69dfb5 100644
> > > --- a/net/sched/sch_fq_codel.c
> > > +++ b/net/sched/sch_fq_codel.c
> > > @@ -187,6 +187,7 @@ static int fq_codel_enqueue(struct sk_buff *skb, struct Qdisc *sch,
> > >       struct fq_codel_sched_data *q = qdisc_priv(sch);
> > >       unsigned int idx, prev_backlog, prev_qlen;
> > >       struct fq_codel_flow *flow;
> > > +     struct sk_buff *drop_skb;
> >
> > We can move this into the if-block below or remove.
> >
> >
> > >       int ret;
> > >       unsigned int pkt_len;
> > >       bool memory_limited;
> > > @@ -222,6 +223,13 @@ static int fq_codel_enqueue(struct sk_buff *skb, struct Qdisc *sch,
> > >
> > >       /* save this packet length as it might be dropped by fq_codel_drop() */
> > >       pkt_len = qdisc_pkt_len(skb);
> > > +
> > > +     /* drop skb if len = 0, so fq_codel_drop could get the right flow idx*/
> > > +     if (unlikely(!pkt_len)) {
> > > +             drop_skb = dequeue_head(flow);
> > > +             __qdisc_drop(drop_skb, to_free);
> >
> > just            __qdisc_drop(dequeue_head(flow), to_free);
> >
> >
> > > +             return NET_XMIT_SUCCESS;
> > > +     }
> > >       /* fq_codel_drop() is quite expensive, as it performs a linear search
> > >        * in q->backlogs[] to find a fat flow.
> > >        * So instead of dropping a single packet, drop half of its backlog
> > > --
> > > 2.17.1
> >