Date:   Sat, 22 Oct 2022 19:36:27 +0800
From:   shaozhengchao <shaozhengchao@...wei.com>
To:     Stanislav Fomichev <sdf@...gle.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 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?
> }
> 
>>>>>
>>>>>
>>>>>
>>>>>>            ctx = bpf_ctx_init(kattr, sizeof(struct __sk_buff));
>>>>>>            if (IS_ERR(ctx)) {
>>>>>>                    kfree(data);
>>>>>> --
>>>>>> 2.17.1
>>>>>>

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