[v2] netfilter: fix bad checksum on IPv6 when NAT is performed

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@freebox.fr>
---
 net/ipv6/netfilter/nf_nat_l3proto_ipv6.c | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

On Thu, Oct 8, 2015 at 1:26 PM, Maxime Bizon <mbizon@freebox.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@freebox.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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On Thu, 2015-10-08 at 14:09 -0700, Tom Herbert wrote:

> I think inet_proto_csum_replace16 should be called here.

inet_proto_csum_replace16() wants a non NULL checksum pointer to update,
and there is no such thing here.

I could pass a dummy value, but inet_proto_csum_replace16() will do
twice more work for nothing

or I could modify inet_proto_csum_replace16() to allow a NULL sum
argument.

On Thu, Oct 8, 2015 at 2:26 PM, Maxime Bizon <mbizon@freebox.fr> wrote:
>
> On Thu, 2015-10-08 at 14:09 -0700, Tom Herbert wrote:
>
>> I think inet_proto_csum_replace16 should be called here.
>
> inet_proto_csum_replace16() wants a non NULL checksum pointer to update,
> and there is no such thing here.
>
> I could pass a dummy value, but inet_proto_csum_replace16() will do
> twice more work for nothing
>
> or I could modify inet_proto_csum_replace16() to allow a NULL sum
> argument.
>
If I am reading the code correctly, it looks like
inet_proto_csum_replace16 is called from
{tcp,dccp,tcp.udp,udplite}_manip_pkt via l3proto->csum_update which is
a call to nf_nat_ipv6_csum_update for IPv6. So for these protocols
when the transport checksum is updated skb->csum is also correctly
updated. For other protocols or when UDP checksum is zero or for
fragments-- I don't see where skb->csum is being properly updated.
This potentially seems to be a major bug.

I would suggest:
1) Modify the inet_proto_csum_replace* routines to check for NULL
check pointer and and only write it when non-NULL
2) Call l3proto->csum_update from unknown_manip_pkt with check == NULL
3) In udp_manip_pkt call l3proto->csum_update with check = NULL when
UDP checksum is zero
4) For IPv6 fragments call l3proto->csum_update with check = NULL

Tom



> --
> Maxime
>
>
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