Re: [PATCH] ipv6: fix NULL pointer dereference in ip6_output()

[David Ahern]

On 12/7/21 5:21 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 patch
> 0857d6f8c759 ("ipv6: When forwarding count rx stats on the orig netdev") is
> applied.
> 
> To fix that, I can send a patch where we set the IP6CB(skb)->iif to the
> index of the receiving interface, i.e.:
> 
> int seg6_do_srh_encap(struct sk_buff *skb, struct ipv6_sr_hdr *osrh, int proto)
>          [...]
>          ip6_flow_hdr(hdr, 0, flowlabel);
>          hdr->hop_limit = ip6_dst_hoplimit(skb_dst(skb));
> 
>          memset(IP6CB(skb), 0, sizeof(*IP6CB(skb)));
> +        IP6CB(skb)->iif = skb->skb_iif;
>          [...]
> 
> What do you think?
> 

I like that approach over the need for a fall back in core ipv6 code.
Make sure the above analysis is in the commit message. Thanks for the
quick response,