Re: [PATCH] veth: don't modify ip-summed; doing so treats packets with bad checksums as good.

[Cong Wang]

On Sat, Dec 19, 2015 at 1:01 PM, Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
> On Fri, Dec 18, 2015 at 11:42 AM, Vijay Pandurangan <vijayp@xxxxxxxxx> wrote:
>> Evan and I have demonstrated this bug on Kubernetes as well, so it's
>> not just a problem in Mesos. (See
>> https://github.com/kubernetes/kubernetes/issues/18898)
>>
>
> Interesting... then this problem is much more serious than I thought.
>
> Looks like in RX path the bridge sets the checksum to CHECKSUM_NONE
> too:
>
> static inline void skb_forward_csum(struct sk_buff *skb)
> {
>         /* Unfortunately we don't support this one.  Any brave souls? */
>         if (skb->ip_summed == CHECKSUM_COMPLETE)
>                 skb->ip_summed = CHECKSUM_NONE;
> }
>
> I guess this is probably why Docker/Kubernetes could be affected too.

Hmm, no, actually this is due to netem does the software checksum
and sets it to CHECKSUM_NONE:

        if (q->corrupt && q->corrupt >= get_crandom(&q->corrupt_cor)) {
                if (!(skb = skb_unshare(skb, GFP_ATOMIC)) ||
                    (skb->ip_summed == CHECKSUM_PARTIAL &&
                     skb_checksum_help(skb)))
                        return qdisc_drop(skb, sch);

                skb->data[prandom_u32() % skb_headlen(skb)] ^=
                        1<<(prandom_u32() % 8);
        }


But anyway, your patch still looks correct to me.
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