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

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Message-ID: <CAHA+R7Oium7-Zj_TKOL4qOTiEUmzsykO2UPf4M9Gt0DS8ABt0g@mail.gmail.com>
Date:	Sat, 19 Dec 2015 13:34:31 -0800
From:	Cong Wang <cwang@...pensource.com>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	Vijay Pandurangan <vijayp@...ayp.ca>,
	Nicolas Dichtel <nicolas.dichtel@...nd.com>,
	Phil Sutter <phil@....cc>,
	Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Evan Jones <ej@...njones.ca>, Tom Herbert <tom@...bertland.com>
Subject: Re: [PATCH] veth: don't modify ip-summed; doing so treats packets
 with bad checksums as good.

On Sat, Dec 19, 2015 at 1:01 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
> On Fri, Dec 18, 2015 at 11:42 AM, Vijay Pandurangan <vijayp@...ayp.ca> 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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