Re: [PATCH] veth: donʼt modify ip_summed; doing so treats packets with bad checksums as good.

[David Miller]

From: Vijay Pandurangan <vijayp@xxxxxxxxx>
Date: Fri, 18 Dec 2015 14:34:59 -0500

> 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.
> 
> 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 didnʼt 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).
> 
> 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>

Applied and queued up for -stable, thanks.
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