netdev - Re: [PATCH net 1/1] net: openvswitch: fix csum updates for MPLS actions

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Date:   Sun, 30 Jun 2019 18:45:31 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     john.hurley@...ronome.com
Cc:     netdev@...r.kernel.org, pshelar@....org,
        simon.horman@...ronome.com, jakub.kicinski@...ronome.com,
        oss-drivers@...ronome.com
Subject: Re: [PATCH net 1/1] net: openvswitch: fix csum updates for MPLS
 actions

From: John Hurley <john.hurley@...ronome.com>
Date: Thu, 27 Jun 2019 14:37:30 +0100

> Skbs may have their checksum value populated by HW. If this is a checksum
> calculated over the entire packet then the CHECKSUM_COMPLETE field is
> marked. Changes to the data pointer on the skb throughout the network
> stack still try to maintain this complete csum value if it is required
> through functions such as skb_postpush_rcsum.
> 
> The MPLS actions in Open vSwitch modify a CHECKSUM_COMPLETE value when
> changes are made to packet data without a push or a pull. This occurs when
> the ethertype of the MAC header is changed or when MPLS lse fields are
> modified.
> 
> The modification is carried out using the csum_partial function to get the
> csum of a buffer and add it into the larger checksum. The buffer is an
> inversion of the data to be removed followed by the new data. Because the
> csum is calculated over 16 bits and these values align with 16 bits, the
> effect is the removal of the old value from the CHECKSUM_COMPLETE and
> addition of the new value.
> 
> However, the csum fed into the function and the outcome of the
> calculation are also inverted. This would only make sense if it was the
> new value rather than the old that was inverted in the input buffer.
> 
> Fix the issue by removing the bit inverts in the csum_partial calculation.
> 
> The bug was verified and the fix tested by comparing the folded value of
> the updated CHECKSUM_COMPLETE value with the folded value of a full
> software checksum calculation (reset skb->csum to 0 and run
> skb_checksum_complete(skb)). Prior to the fix the outcomes differed but
> after they produce the same result.
> 
> Fixes: 25cd9ba0abc0 ("openvswitch: Add basic MPLS support to kernel")
> Fixes: bc7cc5999fd3 ("openvswitch: update checksum in {push,pop}_mpls")
> Signed-off-by: John Hurley <john.hurley@...ronome.com>
> Reviewed-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> Reviewed-by: Simon Horman <simon.horman@...ronome.com>

Applied and queued up for -stable, thanks.