netdev - Re: [PATCH net] netfilter: set skb transport_header before calling sctp_compute

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Message-ID: <20190311110752.GA30241@hmswarspite.think-freely.org>
Date:   Mon, 11 Mar 2019 07:07:52 -0400
From:   Neil Horman <nhorman@...driver.com>
To:     Florian Westphal <fw@...len.de>
Cc:     Xin Long <lucien.xin@...il.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        network dev <netdev@...r.kernel.org>,
        netfilter-devel@...r.kernel.org,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Subject: Re: [PATCH net] netfilter: set skb transport_header before calling
 sctp_compute_cksum

On Sat, Mar 09, 2019 at 10:24:34AM +0100, Florian Westphal wrote:
> Xin Long <lucien.xin@...il.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?
> If we need it, do we also need to add it in other locations that
> deal with sctp csum (e.g. in ipvs?).
> 
This is a fair question, and I'm not yet sure of the answer.

> Thanks,
> Florian
>