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

From: Pablo Neira Ayuso <[email-masked]>
Date: Tue, 9 Jun 2026 13:48:32 +0200

On Tue, Jun 09, 2026 at 12:28:08PM +0200, Lorenzo Bianconi wrote:
> 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?

Yes, initial skb_header_pointer() then pskb_may_pull(skb, hdrlen) to
ensure the entire should be fine.

I think this need one more fix: This needs to resort to classic path
if there are intermediate extension headers sitting in between the
outer and inner headers in IP6IP6, ie. ipv6_ext_hdr() == true. Those
extensions need to be handled by the IPv6 stack.

nf_flow_ip6_tunnel_proto() needs to be fixed to deal with this. 
And I suspect nf_flow_ip4_tunnel_proto() with IP options have the same
problem, the flowtable need to resort to the classic stack path.

Thanks.