On Fri, Oct 21, 2022 at 5:13 PM Kuniyuki Iwashima <kuniyu@xxxxxxxxxx> wrote: > > +Stanislav, bpf > > From: Zhengchao Shao <shaozhengchao@xxxxxxxxxx> > 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@xxxxxxxxxxxxxx/ > > """ > 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? > > Fixes: 4b549a2ef4be ("fq_codel: Fair Queue Codel AQM") > > Signed-off-by: Zhengchao Shao <shaozhengchao@xxxxxxxxxx> > > --- > > 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 >
