On Fri, Oct 21, 2022 at 12:25 AM shaozhengchao <shaozhengchao@xxxxxxxxxx> wrote: > > > > On 2022/10/21 1:45, Stanislav Fomichev wrote: > > On Wed, Oct 19, 2022 at 6:47 PM shaozhengchao <shaozhengchao@xxxxxxxxxx> wrote: > >> > >> > >> > >> On 2022/10/18 0:36, Stanislav Fomichev wrote: > >>> On Sat, Oct 15, 2022 at 2:16 AM Zhengchao Shao <shaozhengchao@xxxxxxxxxx> 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@xxxxxxxxxx> > >>>> --- > >>>> 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 > >>>>
