netdev - Re: [PATCH v2] xfrm: interface: Don't hide plain packets from netfilter

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Date:   Thu, 10 Dec 2020 12:10:18 +0100
From:   Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:     Eyal Birger <eyal.birger@...il.com>, Phil Sutter <phil@....cc>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        linux-crypto@...r.kernel.org, netfilter-devel@...r.kernel.org,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH v2] xfrm: interface: Don't hide plain packets from
 netfilter

Le 09/12/2020 à 15:40, Eyal Birger a écrit :
> Hi Phil,
> 
> On Tue, Dec 8, 2020 at 8:51 PM Phil Sutter <phil@....cc> wrote:
>>
>> Hi Eyal,
>>
>> On Tue, Dec 08, 2020 at 04:47:02PM +0200, Eyal Birger wrote:
>>> On Mon, Dec 7, 2020 at 4:07 PM Phil Sutter <phil@....cc> wrote:
[snip]
>>
>> The packet appears twice being sent to eth1, the second time as ESP
>> packet. I understand xfrm interface as a collector of to-be-xfrmed
>> packets, dropping those which do not match a policy.
>>
>>>> Fix this by looping packets transmitted from xfrm_interface through
>>>> NF_INET_LOCAL_OUT before passing them on to dst_output(), which makes
>>>> behaviour consistent again from netfilter's point of view.
>>>
>>> When an XFRM interface is used when forwarding, why would it be correct
>>> for NF_INET_LOCAL_OUT to observe the inner packet?
I think it is valid because:
 - it would be consistent with ip tunnels (see iptunnel_xmit())
 - it would be consistent with the standard xfrm path see [1]
 - from the POV of the forwarder, the packet is locally emitted, the src @ is
   owned by the forwarder.

[1] https://upload.wikimedia.org/wikipedia/commons/3/37/Netfilter-packet-flow.svg

>>
>> A valid question, indeed. One could interpret packets being forwarded by
>> those tunneling devices emit the packets one feeds them from the local
>> host. I just checked and ip_vti behaves identical to xfrm_interface
>> prior to my patch, so maybe my patch is crap and the inability to match
>> on ipsec context data when using any of those devices is just by design.
There was no real design for vti[6] interfaces, it's why xfrmi interfaces have
been added. But they should be consistent I think, so this patch should handle
xfrmi and vti[6] together.


Regards,
Nicolas

>>
> 
> I would find such interpretation and behavior to be surprising for an IPsec
> forwarder...
> I guess some functionality of policy matching is lost with these
> devices; although they do offer the ability to match ipsec traffic based on
> the destination interface it is possible to have multiple ipsec flows share
> the same device so netfilter doesn't provide the ability to distinguish
> between different flows on the outbound direction in such cases.
> 
> Thanks,
> Eyal.
>