On Sat, Oct 22, 2022 at 4:36 AM shaozhengchao <shaozhengchao@xxxxxxxxxx> wrote: > > > > On 2022/10/22 2:16, Stanislav Fomichev wrote: > > 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 ? > > } > > > > > 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.
