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

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Date:   Thu, 14 Jul 2022 22:39:32 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Stanislav Fomichev <sdf@...gle.com>,
        Zhengchao Shao <shaozhengchao@...wei.com>
Cc:     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 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;

         /* 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?

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
>>