Re: [PATCH] ipv6: check if dereference of ipv6 header is safe

[Eric Dumazet <eric.dumazet@gmail.com>]

On Fri, 2013-01-18 at 03:06 +0100, Hannes Frederic Sowa wrote:

> [PATCH] ipv6: remove unneeded check to pskb_may_pull
> 
> This is already checked by the caller (tunnel64_rcv) and brings ipip6_rcv
> in line with ipip_rcv.
> 
> Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
> ---
>  net/ipv6/sit.c | 7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
> 
> diff --git a/net/ipv6/sit.c b/net/ipv6/sit.c
> index cfba99b..98fe536 100644
> --- a/net/ipv6/sit.c
> +++ b/net/ipv6/sit.c
> @@ -592,15 +592,10 @@ out:
>  
>  static int ipip6_rcv(struct sk_buff *skb)
>  {
> -	const struct iphdr *iph;
> +	const struct iphdr *iph = ip_hdr(skb);
>  	struct ip_tunnel *tunnel;
>  	int err;
>  
> -	if (!pskb_may_pull(skb, sizeof(struct ipv6hdr)))
> -		goto out;
> -
> -	iph = ip_hdr(skb);
> -
>  	tunnel = ipip6_tunnel_lookup(dev_net(skb->dev), skb->dev,
>  				     iph->saddr, iph->daddr);
>  	if (tunnel != NULL) {

But we use a 'struct iphdr' here, not a ipv6hdr

So we basically implicitely rely on sizeof(struct iphdr) <=
sizeof(struct ipv6hdr)

I would leave the pskb_may_pull() call and fix it, even if not really
needed.