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

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Date:	Mon, 28 Mar 2016 20:17:03 -0700
From:	Tom Herbert <tom@...bertland.com>
To:	Jesse Gross <jesse@...nel.org>
Cc:	Alexander Duyck <aduyck@...antis.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [net PATCH] gro: Allow tunnel stacking in the case of FOU/GUE

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