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

[Cong Wang]

(Cc'ing Eric B and Tom)

On Fri, Dec 18, 2015 at 9:54 AM, Vijay Pandurangan <vijayp@xxxxxxxxx> 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 didnât 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@xxxxxxxxxxxx>
> Signed-off-by: Evan Jones <ej@xxxxxxxxxxxx>
> Cc: Nicolas Dichtel <nicolas.dichtel@xxxxxxxxx>
> Cc: Phil Sutter <phil@xxxxxx>
> Cc: Toshiaki Makita <makita.toshiaki@xxxxxxxxxxxxx>
> Cc: netdev@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Vijay Pandurangan <vijayp@xxxxxxxxx>

Your patch looks good to me but your email client corrupts your patch,
so please resend.
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