netdev - Re: [net PATCH] gro: Allow tunnel stacking in the case of FOU/GUE

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Date:	Mon, 28 Mar 2016 21:01:47 -0700
From:	Tom Herbert <tom@...bertland.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Jesse Gross <jesse@...nel.org>,
	Alexander Duyck <aduyck@...antis.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [net PATCH] gro: Allow tunnel stacking in the case of FOU/GUE

On Mon, Mar 28, 2016 at 8:27 PM, Alexander Duyck
<alexander.duyck@...il.com> wrote:
> On Mon, Mar 28, 2016 at 8:17 PM, Tom Herbert <tom@...bertland.com> wrote:
>> On Mon, Mar 28, 2016 at 6:54 PM, Jesse Gross <jesse@...nel.org> wrote:
>>> On Mon, Mar 28, 2016 at 6:24 PM, Tom Herbert <tom@...bertland.com> wrote:
>>>> On Mon, Mar 28, 2016 at 4:58 PM, Alexander Duyck <aduyck@...antis.com> wrote:
>>>>> This patch should fix the issues seen with a recent fix to prevent
>>>>> tunnel-in-tunnel frames from being generated with GRO.  The fix itself is
>>>>> correct for now as long as we do not add any devices that support
>>>>> NETIF_F_GSO_GRE_CSUM.  When such a device is added it could have the
>>>>> potential to mess things up due to the fact that the outer transport header
>>>>> points to the outer UDP header and not the GRE header as would be expected.
>>>>>
>>>>> Fixes: fac8e0f579695 ("tunnels: Don't apply GRO to multiple layers of encapsulation.")
>>>>
>>>> This could only fix FOU/GUE. It is very possible someone else could
>>>> happily be doing some other layered encapsulation and never had a
>>>> problem before, so the decision to start enforcing only a single layer
>>>> of encapsulation for GRO would still break them. I still think we
>>>> should revert the patch, and for next version fixes things to that any
>>>> combination/nesting of encapsulation is supported, and if there are
>>>> exceptions to that support they need be clearly documented.
>>>
>>> It was pointed out to me that prior to my patch, it was also possible
>>> to remotely cause a stack overflow by filling up a packet with tunnel
>>> headers and letting GRO descend through them over and over again.
>>>
>> Then the fix would be set set a reasonable limit on the number of
>> encapsulation levels.
>>
>>> Tom, I'm sorry that you don't like how I fixed this issue but there
>>> really, truly is a bug here. I gave you a specific example to be clear
>>> but that doesn't mean that is the only case. I am aware that the bug
>>> is not encountered in all situations and that the fix removes an
>>> optimization in some of those but I think that ensuring correct
>>> behavior must come first.
>>
>> The example you gave results in packet loss, this is not
>> incorrectness. Actually reproduce a real issue that leads to
>> incorrectness and then we can talk about a solution.
>
> Tom,
>
> Just take a look in the __skb_udp_tunnel_segment or gre_gso_segment
> code.  Then tell me how we are supposed to deal with the fact that the
> GSO code expects skb_inner_network_offset() to be valid.  If you have
> more than an inner and an outer network header we cannot.  So we
> cannot put GRE in UDP, or UDP in GRE if there is a network header
> between them.  The FOU/GUE code gets around this because in the IPIP
> and SIT cases you are adding an L4 header between two L3 headers.  The
> GRE case works because you essentially convert the GRE header into a
> tunnel header like VXLAN or GENEVE and we just overwrite the outer
> transport header offset.
>
> What it comes down to is that we can only support 2 network headers
> per frame.  One for the inner and one for the outer.  That is why we
> can have an exception for GUE as it only has 2 network headers.  If we
> had multiple levels of UDP, or GRE, or 2 levels of network headers
> either before or after either UDP or GRE we cannot support
> segmentation because the code will blow up and generate a malformed
> frame.
>
If you apply Edward's jumbo L2 header concept then
Eth|IP|UDP|VXLAN|Eth|IP|UDP|GUE|GRE|IPIP|IPv6|TCP|payload becomes
Eth|IP|UDP|encapsulation-hdrs|IPv6|TCP|Payload. One set of outer
headers, one set of inner headers. The rules that encapsulation_hdrs
don't contain fields that need to be modified for every segment need
to be supported in GRO and the stack when it generates such a
configuration.

> - Alex