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

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Date:   Sat, 29 Jun 2019 17:33:18 -0700
From:   Pravin Shelar <pshelar@....org>
To:     John Hurley <john.hurley@...ronome.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Simon Horman <simon.horman@...ronome.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        oss-drivers@...ronome.com
Subject: Re: [PATCH net 1/1] net: openvswitch: fix csum updates for MPLS actions

On Thu, Jun 27, 2019 at 6:37 AM John Hurley <john.hurley@...ronome.com> wrote:
>
> 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>
> ---

Thanks for fixing it.
Acked-by: Pravin B Shelar <pshelar@....org>

>  net/openvswitch/actions.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
> index 151518d..bd13146 100644
> --- a/net/openvswitch/actions.c
> +++ b/net/openvswitch/actions.c
> @@ -166,8 +166,7 @@ static void update_ethertype(struct sk_buff *skb, struct ethhdr *hdr,
>         if (skb->ip_summed == CHECKSUM_COMPLETE) {
>                 __be16 diff[] = { ~(hdr->h_proto), ethertype };
>
> -               skb->csum = ~csum_partial((char *)diff, sizeof(diff),
> -                                       ~skb->csum);
> +               skb->csum = csum_partial((char *)diff, sizeof(diff), skb->csum);
>         }
>
>         hdr->h_proto = ethertype;
> @@ -259,8 +258,7 @@ static int set_mpls(struct sk_buff *skb, struct sw_flow_key *flow_key,
>         if (skb->ip_summed == CHECKSUM_COMPLETE) {
>                 __be32 diff[] = { ~(stack->label_stack_entry), lse };
>
> -               skb->csum = ~csum_partial((char *)diff, sizeof(diff),
> -                                         ~skb->csum);
> +               skb->csum = csum_partial((char *)diff, sizeof(diff), skb->csum);
>         }
>
>         stack->label_stack_entry = lse;
> --
> 2.7.4
>