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

[Martin KaFai Lau <martin.lau@linux.dev>]

On 7/15/22 4:55 AM, Zhengchao Shao 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@syzkaller.appspotmail.com
> Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
> ---
> 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;

 From another recent report [0], I don't think this change is fixing the report 
from syzbot.  It probably makes sense to revert this patch.

afaict, This '!skb->len' test is done after
	if (is_l2)
		__skb_push(skb, hh_len);

Hence, skb->len is not zero in convert___skb_to_skb().  The proper place to test 
skb->len is before __skb_push() to ensure there is some network header after the 
mac or may as well ensure "data_size_in > ETH_HLEN" at the beginning.

The fix in [0] is applied.  If it turns out there are other cases caused by the 
skb generated by test_run that needs extra fixes in bpf_redirect_*,  it needs to 
revisit an earlier !skb->len check mentioned above and the existing test cases 
outside of test_progs would have to adjust accordingly.

[0]: https://lore.kernel.org/bpf/20221027225537.353077-1-sdf@google.com/

> +
>   	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);