Re: [net] seg6: fix the iif in the IPv6 socket control block

[David Ahern <dsahern@gmail.com>]

On 12/8/21 12:54 PM, Andrea Mayer wrote:
> When an IPv4 packet is received, the ip_rcv_core(...) sets the receiving
> interface index into the IPv4 socket control block (v5.16-rc4,
> net/ipv4/ip_input.c line 510):
> 
>     IPCB(skb)->iif = skb->skb_iif;
> 
> If that IPv4 packet is meant to be encapsulated in an outer IPv6+SRH
> header, the seg6_do_srh_encap(...) performs the required encapsulation.
> In this case, the seg6_do_srh_encap function clears the IPv6 socket control
> block (v5.16-rc4 net/ipv6/seg6_iptunnel.c line 163):
> 
>     memset(IP6CB(skb), 0, sizeof(*IP6CB(skb)));
> 
> The memset(...) was introduced in commit ef489749aae5 ("ipv6: sr: clear
> IP6CB(skb) on SRH ip4ip6 encapsulation") a long time ago (2019-01-29).
> 
> Since the IPv6 socket control block and the IPv4 socket control block share
> the same memory area (skb->cb), the receiving interface index info is lost
> (IP6CB(skb)->iif is set to zero).
> 
> As a side effect, that condition triggers a NULL pointer dereference if
> commit 0857d6f8c759 ("ipv6: When forwarding count rx stats on the orig
> netdev") is applied.
> 
> To fix that issue, we set the IP6CB(skb)->iif with the index of the
> receiving interface once again.
> 
> Fixes: ef489749aae5 ("ipv6: sr: clear IP6CB(skb) on SRH ip4ip6 encapsulation")
> Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
> ---
>  net/ipv6/seg6_iptunnel.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 

Reviewed-by: David Ahern <dsahern@kernel.org>