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

From: Lorenzo Bianconi <[email-masked]>
Date: Tue, 9 Jun 2026 12:28:08 +0200

On Jun 09, Pablo Neira Ayuso wrote:
> 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 <[email-masked]>
> > ---
> >  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.

I think you are right, here we need to run:

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

in order to be sure we can pull ctx->tun.hdr_size in nf_flow_ip_tunnel_pop().
Doing so, we can roll-back to the original skb_header_pointer() to access the
outer ip6 header here. What do you think?

Regards,
Lorenzo

> 
> >  	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 <[email-masked]>
> >