Re: [PATCH net v2] net: ipv6: fix TCP GSO segmentation with NAT

[Eric Dumazet]

On Mon, Mar 10, 2025 at 12:21 PM Felix Fietkau <nbd@xxxxxxxx> wrote:
>
> When updating the source/destination address, the TCP/UDP checksum needs to
> be updated as well.
>
> Fixes: bee88cd5bd83 ("net: add support for segmenting TCP fraglist GSO packets")
> Signed-off-by: Felix Fietkau <nbd@xxxxxxxx>
> ---
> v2: move code to make it similar to __tcpv4_gso_segment_list_csum
>
>  net/ipv6/tcpv6_offload.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
>
> diff --git a/net/ipv6/tcpv6_offload.c b/net/ipv6/tcpv6_offload.c
> index a45bf17cb2a1..34dd0cee3ba6 100644
> --- a/net/ipv6/tcpv6_offload.c
> +++ b/net/ipv6/tcpv6_offload.c
> @@ -94,10 +94,20 @@ INDIRECT_CALLABLE_SCOPE int tcp6_gro_complete(struct sk_buff *skb, int thoff)
>  }
>
>  static void __tcpv6_gso_segment_csum(struct sk_buff *seg,
> +                                    struct in6_addr *oldip,
> +                                    const struct in6_addr *newip,
>                                      __be16 *oldport, __be16 newport)
>  {
>         struct tcphdr *th;
>
> +       if (!ipv6_addr_equal(oldip, newip)) {
> +               inet_proto_csum_replace16(&th->check, seg,

th is not initialized yet.

> +                                         oldip->s6_addr32,
> +                                         newip->s6_addr32,
> +                                         true);
> +               *oldip = *newip;
> +       }
> +
>         if (*oldport == newport)
>                 return;
>
> @@ -129,10 +139,10 @@ static struct sk_buff *__tcpv6_gso_segment_list_csum(struct sk_buff *segs)
>                 th2 = tcp_hdr(seg);
>                 iph2 = ipv6_hdr(seg);
>
> -               iph2->saddr = iph->saddr;
> -               iph2->daddr = iph->daddr;
> -               __tcpv6_gso_segment_csum(seg, &th2->source, th->source);
> -               __tcpv6_gso_segment_csum(seg, &th2->dest, th->dest);
> +               __tcpv6_gso_segment_csum(seg, &iph2->saddr, &iph->saddr,
> +                                        &th2->source, th->source);
> +               __tcpv6_gso_segment_csum(seg, &iph2->daddr, &iph->daddr,
> +                                        &th2->dest, th->dest);
>         }
>
>         return segs;
> --
> 2.47.1
>