Date:   Fri, 21 Oct 2022 11:16:51 -0700
From:   Stanislav Fomichev <sdf@...gle.com>
To:     shaozhengchao <shaozhengchao@...wei.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        martin.lau@...ux.dev, song@...nel.org, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org, haoluo@...gle.com,
        jolsa@...nel.org, oss@....io, weiyongjun1@...wei.com,
        yuehaibing@...wei.com
Subject: Re: [PATCH bpf-next] bpf: fix issue that packet only contains l2 is dropped

On Fri, Oct 21, 2022 at 12:25 AM shaozhengchao <shaozhengchao@...wei.com> wrote:
>
>
>
> On 2022/10/21 1:45, Stanislav Fomichev wrote:
> > On Wed, Oct 19, 2022 at 6:47 PM shaozhengchao <shaozhengchao@...wei.com> wrote:
> >>
> >>
> >>
> >> On 2022/10/18 0:36, Stanislav Fomichev wrote:
> >>> On Sat, Oct 15, 2022 at 2:16 AM Zhengchao Shao <shaozhengchao@...wei.com> wrote:
> >>>>
> >>>> As [0] see, bpf_prog_test_run_skb() should allow user space to forward
> >>>> 14-bytes packet via BPF_PROG_RUN instead of dropping packet directly.
> >>>> So fix it.
> >>>>
> >>>> 0: https://github.com/cilium/ebpf/commit/a38fb6b5a46ab3b5639ea4d421232a10013596c0
> >>>>
> >>>> Fixes: fd1894224407 ("bpf: Don't redirect packets with invalid pkt_len")
> >>>> Signed-off-by: Zhengchao Shao <shaozhengchao@...wei.com>
> >>>> ---
> >>>>    net/bpf/test_run.c | 6 +++---
> >>>>    1 file changed, 3 insertions(+), 3 deletions(-)
> >>>>
> >>>> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> >>>> index 13d578ce2a09..aa1b49f19ca3 100644
> >>>> --- a/net/bpf/test_run.c
> >>>> +++ b/net/bpf/test_run.c
> >>>> @@ -979,9 +979,6 @@ static int convert___skb_to_skb(struct sk_buff *skb, struct __sk_buff *__skb)
> >>>>    {
> >>>>           struct qdisc_skb_cb *cb = (struct qdisc_skb_cb *)skb->cb;
> >>>>
> >>>> -       if (!skb->len)
> >>>> -               return -EINVAL;
> >>>> -
> >>>>           if (!__skb)
> >>>>                   return 0;
> >>>>
> >>>> @@ -1102,6 +1099,9 @@ int bpf_prog_test_run_skb(struct bpf_prog *prog, const union bpf_attr *kattr,
> >>>>           if (IS_ERR(data))
> >>>>                   return PTR_ERR(data);
> >>>>
> >>>> +       if (size == ETH_HLEN)
> >>>> +               is_l2 = true;
> >>>> +
> >>>
> >>> Don't think this will work? That is_l2 is there to expose proper l2/l3
> >>> skb for specific hooks; we can't suddenly start exposing l2 headers to
> >>> the hooks that don't expect it.
> >>> Does it make sense to start with a small reproducer that triggers the
> >>> issue first? We can have a couple of cases for
> >>> len=0/ETH_HLEN-1/ETH_HLEN+1 and trigger them from the bpf program that
> >>> redirects to different devices (to trigger dev_is_mac_header_xmit).
> >>>
> >>>
> >> Hi Stanislav:
> >>          Thank you for your review. Is_l2 is the flag of a specific
> >> hook. Therefore, do you mean that if skb->len is equal to 0, just
> >> add the length back?
> >
> > Not sure I understand your question. All I'm saying is - you can't
> > flip that flag arbitrarily. This flag depends on the attach point that
> > you're running the prog against. Some attach points expect packets
> > with l2, some expect packets without l2.
> >
> > What about starting with a small reproducer? Does it make sense to
> > create a small selftest that adds net namespace + fq_codel +
> > bpf_prog_test run and do redirect ingress/egress with len
> > 0/1...tcphdr? Because I'm not sure I 100% understand whether it's only
> > len=0 that's problematic or some other combination as well?
> >
> yes, only skb->len = 0 will cause null-ptr-deref issue.
> The following is the process of triggering the problem:
> enqueue a skb:
> 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                  --->becuase 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, if skb->len !=0，fq_codel_drop() could get the correct idx, and
> then skb!=NULL, it will be OK.
> Maybe, I will fix it in fq_codel.

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.

> But, as I know, skb->len = 0 is just invalid packet. I prefer to add the
> length back, like bellow:
>         if (is_l2 || !skb->len)
>                 __skb_push(skb, hh_len);
> is it OK?

Probably not?

Looking at the original syzkaller report, prog_type is
BPF_PROG_TYPE_LWT_XMIT which does expect a packet without l2 header.
Can we do something like:

if (!is_l2 && !skb->len) {
  // append some dummy byte to the skb ?
}


}

> >>>
> >>>
> >>>
> >>>>           ctx = bpf_ctx_init(kattr, sizeof(struct __sk_buff));
> >>>>           if (IS_ERR(ctx)) {
> >>>>                   kfree(data);
> >>>> --
> >>>> 2.17.1
> >>>>

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