netdev - Re: [PATCH v2] netfilter: fix bad checksum on IPv6 when NAT is performed

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Message-ID: <CALx6S37AdH=icFEqXjH5hDJ2O0dkwLEx6_KqpFkoPYF2sr1N_Q@mail.gmail.com>
Date:	Thu, 8 Oct 2015 14:09:00 -0700
From:	Tom Herbert <tom@...bertland.com>
To:	mbizon@...ebox.fr
Cc:	netdev <netdev@...r.kernel.org>, davem <davem@...emloft.net>,
	kaber <kaber@...sh.net>, pablo@...filter.org,
	netfilter-devel <netfilter-devel@...r.kernel.org>
Subject: Re: [PATCH v2] netfilter: fix bad checksum on IPv6 when NAT is performed

On Thu, Oct 8, 2015 at 1:26 PM, Maxime Bizon <mbizon@...ebox.fr> wrote:
>
> With this setup:
>
> * non IPv6 checksumming capable network hardware
> * GRO off
> * IPv6 SNAT
>
> I get this when I receive an UDPv6 reply: "<unknown>: hw csum failure"
>
> Call trace:
>
> * nf_ip6_checksum() calls __skb_checksum_complete()
> * nf_nat_ipv6_csum_update() & nf_nat_ipv6_manip_pkt()
> * __udp6_lib_rcv() => udp6_csum_init()
> * __skb_checksum_validate_complete() "fastpath" fails because
>   skb->csum is incorrect.
> * udpv6_recvmsg() => skb_copy_and_csum_datagram_msg()
>
> The last call computes a valid checksum despite CHECKSUM_COMPLETE and
> triggers the warning.
>
> When we perform NAT on IPv4, we also update the IPv4 checksum, so
> there is no side effect on skb->csum (since the csum over a valid IPv4
> header area was already zero).
>
> But IPv6 doesn't have such checksum, so when performing NAT we need to
> update skb->csum.
>
> Signed-off-by: Maxime Bizon <mbizon@...ebox.fr>
> ---
>  net/ipv6/netfilter/nf_nat_l3proto_ipv6.c | 23 +++++++++++++++++++----
>  1 file changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/net/ipv6/netfilter/nf_nat_l3proto_ipv6.c b/net/ipv6/netfilter/nf_nat_l3proto_ipv6.c
> index 70fbaed..e44af9c 100644
> --- a/net/ipv6/netfilter/nf_nat_l3proto_ipv6.c
> +++ b/net/ipv6/netfilter/nf_nat_l3proto_ipv6.c
> @@ -81,6 +81,8 @@ static bool nf_nat_ipv6_manip_pkt(struct sk_buff *skb,
>                                   enum nf_nat_manip_type maniptype)
>  {
>         struct ipv6hdr *ipv6h;
> +       const __be32 *to;
> +       __be32 *from;
>         __be16 frag_off;
>         int hdroff;
>         u8 nexthdr;
> @@ -100,11 +102,24 @@ static bool nf_nat_ipv6_manip_pkt(struct sk_buff *skb,
>                                 target, maniptype))
>                 return false;
>  manip_addr:
> -       if (maniptype == NF_NAT_MANIP_SRC)
> -               ipv6h->saddr = target->src.u3.in6;
> -       else
> -               ipv6h->daddr = target->dst.u3.in6;
> +       if (maniptype == NF_NAT_MANIP_SRC) {
> +               from = ipv6h->saddr.s6_addr32;
> +               to = target->src.u3.in6.s6_addr32;
> +       } else {
> +               from = ipv6h->daddr.s6_addr32;
> +               to = target->dst.u3.in6.s6_addr32;
> +       }
> +
> +       if (skb->ip_summed == CHECKSUM_COMPLETE) {
> +               __be32 diff[] = {
> +                       ~from[0], ~from[1], ~from[2], ~from[3],
> +                       to[0], to[1], to[2], to[3],
> +               };
> +
> +               skb->csum = ~csum_partial(diff, sizeof(diff), ~skb->csum);
> +       }
>
I think inet_proto_csum_replace16 should be called here.

> +       memcpy(from, to, sizeof (struct in6_addr));
>         return true;
>  }
>
> --
> 1.9.1
>
>
> --
> Maxime
>
>
> --
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