Re: [PATCH v4,bpf-next] bpf: Don't redirect packets with invalid pkt_len

[Stanislav Fomichev]

On Fri, Jul 15, 2022 at 4:51 AM Zhengchao Shao <shaozhengchao@xxxxxxxxxx> 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 or others like bpf_prog_test 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Zhengchao Shao <shaozhengchao@xxxxxxxxxx>

Reviewed-by: Stanislav Fomichev <sdf@xxxxxxxxxx>

Looks like it addresses everything?

> ---
> v3: modify debug print
> v2: need move checking to convert___skb_to_skb and add debug info
> v1: should not check len in fast path
>
>  include/linux/skbuff.h | 8 ++++++++
>  net/bpf/test_run.c     | 3 +++
>  net/core/dev.c         | 1 +
>  3 files changed, 12 insertions(+)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index f6a27ab19202..82e8368ba6e6 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -2459,6 +2459,14 @@ 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 (WARN_ONCE(!skb->len, "%s\n", __func__))
> +               DO_ONCE_LITE(skb_dump, KERN_ERR, skb, false);
> +#endif /* CONFIG_DEBUG_NET */
> +}
> +
>  /*
>   *     Add data to an sk_buff
>   */
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index 2ca96acbc50a..dc9dc0bedca0 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -955,6 +955,9 @@ 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;
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index d588fd0a54ce..716df64fcfa5 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4168,6 +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);
> --
> 2.17.1
>