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

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Date:	Fri, 18 Dec 2015 12:42:37 -0700
From:	Vijay Pandurangan <vijayp@...ayp.ca>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	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>,
	Eric Biederman <ebiederm@...stanetworks.com>,
	Tom Herbert <tom@...bertland.com>
Subject: Re: [PATCH] veth: don't modify ip-summed; doing so treats packets
 with bad checksums as good.

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)

Sorry about my email client, I've re-sent the patch in another thread
from git-email as I should have initially.

I'll read through the TX path again to see if we missed something, but
I'd love input from anyone else!

--

Vijay Pandurangan
https://www.twitter.com/vijayp
http://www.vijayp.ca


On Fri, Dec 18, 2015 at 2:00 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
> (Cc'ing Eric B and Tom)
>
> On Fri, Dec 18, 2015 at 9:54 AM, Vijay Pandurangan <vijayp@...ayp.ca> wrote:
>> Packets that arrive from real hardware devices have ip_summed ==
>> CHECKSUM_UNNECESSARY if the hardware verified the checksums, or
>> CHECKSUM_NONE if the packet is bad or it was unable to verify it. The
>> current version of veth will replace CHECKSUM_NONE with
>> CHECKSUM_UNNECESSARY, which causes corrupt packets routed from hardware to
>> a veth device to be delivered to the application. This caused applications
>> at Twitter to receive corrupt data when network hardware was corrupting
>> packets.
>
> Yeah, https://reviews.apache.org/r/41158/.
>
> This is because normally packets to a veth device are _only_ from its pair
> device, Mesos network isolator redirects packets from a hardware interface
> to veth, which violates this expectation. This is also why no one else sees
> this bug. ;)
>
>>
>> We believe this was added as an optimization to skip computing and
>> verifying checksums for communication between containers. However, locally
>> generated packets have ip_summed == CHECKSUM_PARTIAL, so the code as
>> written does nothing for them. As far as we can tell, after removing this
>> code, these packets are transmitted from one stack to another unmodified
>> (tcpdump shows invalid checksums on both sides, as expected), and they are
>> delivered correctly to applications. We didn’t test every possible network
>> configuration, but we tried a few common ones such as bridging containers,
>> using NAT between the host and a container, and routing from hardware
>> devices to containers. We have effectively deployed this in production at
>> Twitter (by disabling RX checksum offloading on veth devices).
>
>
> I am wondering if there is any other CHECKSUM_NONE case in the tx
> path we could miss here. Mesos case is too special not only because
> it redirects packets from hardware to veth, but also because it moves
> packets from RX path to TX path.
>
> Eric? Tom?
>
>>
>> This code dates back to the first version of the driver, commit
>> <e314dbdc1c0dc6a548ecf> ("[NET]: Virtual ethernet device driver"), so I
>> suspect this bug occurred mostly because the driver API has evolved
>> significantly since then. Commit <0b7967503dc97864f283a> ("net/veth: Fix
>> packet checksumming") (in December 2010) fixed this for packets that get
>> created locally and sent to hardware devices, by not changing
>> CHECKSUM_PARTIAL. However, the same issue still occurs for packets coming
>> in from hardware devices.
>>
>> Co-authored-by: Evan Jones <ej@...njones.ca>
>> Signed-off-by: Evan Jones <ej@...njones.ca>
>> Cc: Nicolas Dichtel <nicolas.dichtel@...nd.com>
>> Cc: Phil Sutter <phil@....cc>
>> Cc: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
>> Cc: netdev@...r.kernel.org
>> Cc: linux-kernel@...r.kernel.org
>> Signed-off-by: Vijay Pandurangan <vijayp@...ayp.ca>
>
> Your patch looks good to me but your email client corrupts your patch,
> so please resend.
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