netdev - Re: [PATCH net-next 4/4] net/sched: act_mirred: Implement ingress actions

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Date:   Mon, 26 Sep 2016 17:12:18 +0200
From:   Hannes Frederic Sowa <hannes@...essinduktion.org>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Florian Westphal <fw@...len.de>,
        Jamal Hadi Salim <jhs@...atatu.com>
Cc:     Shmulik Ladkani <shmulik.ladkani@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        WANG Cong <xiyou.wangcong@...il.com>,
        Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 4/4] net/sched: act_mirred: Implement ingress
 actions

On 26.09.2016 16:53, Daniel Borkmann wrote:
> On 09/26/2016 04:43 PM, Hannes Frederic Sowa wrote:
>> On 26.09.2016 03:35, Florian Westphal wrote:
>>> Jamal Hadi Salim <jhs@...atatu.com> wrote:
>>>> On 16-09-25 02:31 PM, Florian Westphal wrote:
>>>>> Shmulik Ladkani <shmulik.ladkani@...il.com> wrote:
>>>>>> We can later address any loop-detection improvements in mirred.
>>>>>> WDYT?
>>>>>
>>>>> You can address this after fixing infamous spinlock recursion hard
>>>>> lockup (which has existed forever):
>>>>>
>>>>> tc qdisc add dev eth0 root handle 1: prio
>>>>> tc filter add dev eth0 parent 1: protocol ip u32 match u32 0 0 flowid
>>>>> 1:2 action mirred egress redirect dev eth0
>>>>>
>>>>> (only do this on toy vm)
>>>>
>>>> Realize didnt respond to this. Seems very simple to fix:
>>>> if skb->dev->ifindex and m->tcfm_dev->ifindex are the
>>>> same, then you can drop the packet.
>>>
>>> Yes, but I think we get same issue when we deal with stacked
>>> interfaces, and redirect is to e.g. vlan on top of physical device.
>>
>> We do have the adjacent upper lists in all netdevices, calculating if a
>> mirred actions would insert the skb on a stacked device above us should
>> be as easy as querying netdev_has_upper_dev and should be possible to
>> check that during config time.
> 
> But that would still not be enough, no? In the sense that with above
> scenario, you could redirect to some arbitrary device that redirects
> this back to the original device if on purpose configured as such,
> thus they don't necessarily need to have a stacked relationship.

Yes, it would only help with the scenario Florian described above.

Personally, I would only try to fix and warn against the easy to detect
cases. It is easy enough to just create a loop with your local attached
L2 which brings your box into a endless loop processing the same packet
again and again. Because it is out of control of the kernel you cannot
do anything at all.

I would just care that we sometimes reschedule and don't do everything
in one stack so we don't corrupt the machine and an admin has still a
chance to solve the problem.

Bye,
Hannes