Re: [PATCH nf] netfilter: flowtable: use pskb_may_pull() in nf_flow_ip6_tunnel_proto()

[Pablo Neira Ayuso <pablo@netfilter.org>]

Hi Lorenzo,

On Mon, Jun 08, 2026 at 07:06:52PM +0200, Lorenzo Bianconi wrote:
> Switch nf_flow_ip6_tunnel_proto() from skb_header_pointer() to
> pskb_may_pull() for header validation, aligning it with the approach
> used in nf_flow_ip4_tunnel_proto().
> Move ctx->offset update inside the IPPROTO_IPV6 conditional block since
> it should only be adjusted when a tunnel is actually detected.
> While at it, use nexthdr instead of the hardcoded IPPROTO_IPV6 constant
> when setting ctx->tun.proto.
> 
> Fixes: d98103575dcdd ("netfilter: flowtable: Add IP6IP6 rx sw acceleration")
> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> ---
>  net/netfilter/nf_flow_table_ip.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/net/netfilter/nf_flow_table_ip.c b/net/netfilter/nf_flow_table_ip.c
> index 9c05a50d6013..2946399ab715 100644
> --- a/net/netfilter/nf_flow_table_ip.c
> +++ b/net/netfilter/nf_flow_table_ip.c
> @@ -347,15 +347,15 @@ static bool nf_flow_ip6_tunnel_proto(struct nf_flowtable_ctx *ctx,
>  				     struct sk_buff *skb)
>  {
>  #if IS_ENABLED(CONFIG_IPV6)
> -	struct ipv6hdr *ip6h, _ip6h;
> +	struct ipv6hdr *ip6h;
>  	__be16 frag_off;
>  	u8 nexthdr;
>  	int hdrlen;
>  
> -	ip6h = skb_header_pointer(skb, ctx->offset, sizeof(*ip6h), &_ip6h);
> -	if (!ip6h)
> +	if (!pskb_may_pull(skb, sizeof(*ip6h) + ctx->offset))
>  		return false;
>  
> +	ip6h = (struct ipv6hdr *)(skb_network_header(skb) + ctx->offset);
>  	if (ip6h->hop_limit <= 1)
>  		return false;

Not shown in the patch, but is there still a corner case here that
needs to be covered?

ipv6_skip_exthdr() uses skb_header_pointer() internal, then another
pskb_may_pull() is needed to make sure no other IPv6 extension header
sits between the outer and the inner IPPROTO_IPV6 header, allowing to
be in a non-linear area of the skb?        

> @@ -367,9 +367,9 @@ static bool nf_flow_ip6_tunnel_proto(struct nf_flowtable_ctx *ctx,
>  

I mean:

        if (!pskb_may_pull(skb, hdrlen))
                return false;

where hdrlen is what ipv6_skip_exthdr() returns.

Then, I think it should be safe to call skb_pull() on
ctx->tun.hdr_size.

Let me know, thanks.

>  	if (nexthdr == IPPROTO_IPV6) {
>  		ctx->tun.hdr_size = hdrlen;
> -		ctx->tun.proto = IPPROTO_IPV6;
> +		ctx->tun.proto = nexthdr;
> +		ctx->offset += ctx->tun.hdr_size;
>  	}
> -	ctx->offset += ctx->tun.hdr_size;
>  
>  	return true;
>  #else
> 
> ---
> base-commit: 9772589b57e44aedc240211c5c3f7a684a034d3a
> change-id: 20260608-b4-nf_flow_ip6_tunnel_proto-update-8b64903825b4
> 
> Best regards,
> -- 
> Lorenzo Bianconi <lorenzo@kernel.org>
>