datapath: fix csum updates for MPLS actions

datapath: fix csum updates for MPLS actions

Upstream commit:
    commit 0e3183cd2a64843a95b62f8bd4a83605a4cf0615
    Author: John Hurley <john.hurley@netronome.com>
    Date:   Thu Jun 27 14:37:30 2019 +0100

    net: openvswitch: fix csum updates for MPLS actions

    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@netronome.com>
    Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
    Reviewed-by: Simon Horman <simon.horman@netronome.com>
    Acked-by: Pravin B Shelar <pshelar@ovn.org>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Fixes: ccf4378615e9 ("datapath: Add basic MPLS support to kernel")
Fixes: b51367aad315 ("datapath: update checksum in {push,pop}_mpls")
Cc: John Hurley <john.hurley@netronome.com>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>