netdev - Re: [PATCH v2,bpf-next] bpf: Don't redirect packets with invalid pkt

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Message-ID: <CAKH8qBvd1mc_fM6Ha_=kMppvuy67aB5EzmoHmODmGtVT5pcq7A@mail.gmail.com>
Date:   Thu, 14 Jul 2022 14:26:32 -0700
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Zhengchao Shao <shaozhengchao@...wei.com>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, hawk@...nel.org, ast@...nel.org,
        andrii@...nel.org, martin.lau@...ux.dev, song@...nel.org,
        yhs@...com, john.fastabend@...il.com, kpsingh@...nel.org,
        weiyongjun1@...wei.com, yuehaibing@...wei.com
Subject: Re: [PATCH v2,bpf-next] bpf: Don't redirect packets with invalid pkt_len

On Thu, Jul 14, 2022 at 1:39 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 7/14/22 8:22 PM, Stanislav Fomichev wrote:
> > On Wed, Jul 13, 2022 at 11:05 PM Zhengchao Shao
> > <shaozhengchao@...wei.com> wrote:
> >>
> >> Syzbot found an issue [1]: fq_codel_drop() try to drop a flow whitout any
> >> skbs, that is, the flow->head is null.
> >> The root cause, as the [2] says, is because that bpf_prog_test_run_skb()
> >> run a bpf prog which redirects empty skbs.
> >> So we should determine whether the length of the packet modified by bpf
> >> prog is valid before forwarding it directly.
> >>
> >> LINK: [1] https://syzkaller.appspot.com/bug?id=0b84da80c2917757915afa89f7738a9d16ec96c5
> >> LINK: [2] https://www.spinics.net/lists/netdev/msg777503.html
> >>
> >> Reported-by: syzbot+7a12909485b94426aceb@...kaller.appspotmail.com
> >> Signed-off-by: Zhengchao Shao <shaozhengchao@...wei.com>
> >
> > Reviewed-by: Stanislav Fomichev <sdf@...gle.com>
> >
> > Daniel, do you see any issues with this approach?
>
> I think it's fine, maybe this could be folded into:
>
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index 750d7d173a20..256cd18cfe22 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -957,6 +957,8 @@ static int convert___skb_to_skb(struct sk_buff *skb, struct __sk_buff *__skb)
>
>          if (!__skb)
>                  return 0;
> +       if (!skb->len)
> +               return -EINVAL;

So putting it in convert___skb_to_skb lets us remove a goto :-) Works
for me, whichever you prefer.

>          /* make sure the fields we don't use are zeroed */
>          if (!range_is_zero(__skb, 0, offsetof(struct __sk_buff, mark)))
>
> > I wonder if we might also want to add some WARN_ON to the
> > __bpf_redirect_common routine gated by #ifdef CONFIG_DEBUG_NET ?
> > In case syscaller manages to hit similar issues elsewhere..
>
> Assuming the issue is generic (and CONFIG_DEBUG_NET only enabled by things like
> syzkaller), couldn't we do something like the below?

Based on [1] it's selected by DEBUG_KERNEL, so it feels like it's safe
to assume that it's mostly debugging/fuzzing workloads?
Your suggestion to put it into __dev_queue_xmit looks good to me!

[1] https://lore.kernel.org/netdev/20220509190851.1107955-3-eric.dumazet@gmail.com/

> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index f6a27ab19202..c9988a785294 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -2459,6 +2459,17 @@ static inline void skb_set_tail_pointer(struct sk_buff *skb, const int offset)
>
>   #endif /* NET_SKBUFF_DATA_USES_OFFSET */
>
> +static inline void skb_assert_len(struct sk_buff *skb)
> +{
> +#ifdef CONFIG_DEBUG_NET
> +       if (unlikey(!skb->len)) {
> +               pr_err("%s\n", __func__);
> +               skb_dump(KERN_ERR, skb, false);
> +               WARN_ON_ONCE(1);
> +       }
> +#endif /* CONFIG_DEBUG_NET */
> +}
> +
>   /*
>    *     Add data to an sk_buff
>    */
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 978ed0622d8f..53c4b9fd22c0 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4168,7 +4168,7 @@ int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
>          bool again = false;
>
>          skb_reset_mac_header(skb);
> -
> +       skb_assert_len(skb);
>          if (unlikely(skb_shinfo(skb)->tx_flags & SKBTX_SCHED_TSTAMP))
>                  __skb_tstamp_tx(skb, NULL, NULL, skb->sk, SCM_TSTAMP_SCHED);
>
>
>
> >> ---
> >> v1: should not check len in fast path
> >>
> >>   net/bpf/test_run.c | 6 ++++++
> >>   1 file changed, 6 insertions(+)
> >>
> >> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> >> index 2ca96acbc50a..750d7d173a20 100644
> >> --- a/net/bpf/test_run.c
> >> +++ b/net/bpf/test_run.c
> >> @@ -1152,6 +1152,12 @@ int bpf_prog_test_run_skb(struct bpf_prog *prog, const union bpf_attr *kattr,
> >>          ret = convert___skb_to_skb(skb, ctx);
> >>          if (ret)
> >>                  goto out;
> >> +
> >> +       if (skb->len == 0) {
> >> +               ret = -EINVAL;
> >> +               goto out;
> >> +       }
> >> +
> >>          ret = bpf_test_run(prog, skb, repeat, &retval, &duration, false);
> >>          if (ret)
> >>                  goto out;
> >> --
> >> 2.17.1
> >>
>