Date:   Thu, 27 Oct 2022 09:37:29 -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 Thu, Oct 27, 2022 at 4:58 AM shaozhengchao <shaozhengchao@...wei.com> wrote:
>
>
>
> On 2022/10/25 1:13, Stanislav Fomichev wrote:
> > On Sat, Oct 22, 2022 at 4:36 AM shaozhengchao <shaozhengchao@...wei.com> wrote:
> >>
> >>
> >>
> >> On 2022/10/22 2:16, Stanislav Fomichev wrote:
> >>> 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 ?
> >>> }
> >>>
> >>>
> >> I pad one byte, and test OK.
> >> if (!is_l2 && !skb->len)
> >>       __skb_push(skb, 1);
> >>
> >> Does it look OK to you?
> >
> > Nope, this will eat a byte out of the l2 header. We need to skb_put
> > and make sure we allocate enough to make that skb_put succeed.
> >
> > But stepping back a bit: it feels like it's all unnecessary? The only
> > valid use-case of this is probing for the BPF_PROG_TEST_RUN as cilium
> > does. This is mostly about testing, so fixing it in the users seems
> > fair? No real production code is expected to generate these zero-len
> > packets. Or are we concerned that this will leak into stable kernels?
> >
> > I feel like we are trying to add more complexity here for no apparent reason.
> >
> I agree with you. users should make sure the correct skb len and
> configurations are passed into kernel. Incorrect configurations should
> be discarded to ensure kernel stability.
>
> Lorenz, Can you modify the user-mode test code?

Lorenz already fixed it for Cilium. I think the discussion here is
around other potential users out there.
Let's wait for them to appear if it is indeed a problem?

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