Re: [PATCH net] netfilter: set skb transport_header before calling sctp_compute_cksum

[Xin Long <lucien.xin@gmail.com>]

On Mon, Mar 11, 2019 at 7:08 PM Neil Horman <nhorman@tuxdriver.com> wrote:
>
> On Sat, Mar 09, 2019 at 10:24:34AM +0100, Florian Westphal wrote:
> > Xin Long <lucien.xin@gmail.com> wrote:
> > >   https://marc.info/?l=linux-netdev&m=155109395226858&w=2
> > > But from sctp side, Neil preferred sctp_hdr().
> > >
> > > We need to either add skb_set_transport_header() in sctp_s/dnat_handler()
> > > and sctp_manip_pkt(), or bring that patch back?
> > >
> > > Now it seems not good to set skb->transport_header in netfilter code.
> >
> > I think its fine, but I wonder why we need to do it.
> >
> > Since 21d1196a35f5686c4323e42a62fdb4b23b0ab4a3 ipv4 input path sets
> > transport header before netfilter.  The only problem is that linear
> > access is illegal without may_pull checks, but in this case the
> > make_writable call takes care of this already.
> >
> Yes, this.  It seems to me we should be setting the transport header prior to
> ever getting into the netfilter code, which does imply that we need the may_pull
> check to linearize enough of the packet to do so, just like tcp and udp do.
>
> > So, why was this patch needed?

The issue was reported when going to nf_conntrack by br_netfilter's
bridge-nf-call-iptables, which could be:

br_prerouting->inet_prerouting->
br_forward->inet_forward->
br_postrouting->inet_postrouting

> > If we need it, do we also need to add it in other locations that
> > deal with sctp csum (e.g. in ipvs?).
So ipvs hooks are in inet_local_in/out, sctp_s/dnat_handler() won't
be affected.
But the one in sctp_manip_pkt() that may be in inet_pre/postrouting
will need to set it.

I will post v2 with skb_set_transport_header(skb, dataoff) added in
sctp_manip_pkt().

agreed?

> >
> This is a fair question, and I'm not yet sure of the answer.
>
> > Thanks,
> > Florian
> >