netdev - Re: [PATCH net] netfilter: bridge: set skb transport_header before entering NF_INET_PRE

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Date:   Thu, 14 Mar 2019 15:19:05 +0100
From:   Pablo Neira Ayuso <pablo@...filter.org>
To:     Florian Westphal <fw@...len.de>
Cc:     Neil Horman <nhorman@...driver.com>,
        Xin Long <lucien.xin@...il.com>,
        network dev <netdev@...r.kernel.org>,
        netfilter-devel@...r.kernel.org,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Subject: Re: [PATCH net] netfilter: bridge: set skb transport_header before
 entering NF_INET_PRE_ROUTING

On Wed, Mar 13, 2019 at 12:59:48PM +0100, Florian Westphal wrote:
> Neil Horman <nhorman@...driver.com> wrote:
> > On Wed, Mar 13, 2019 at 04:33:29PM +0800, Xin Long wrote:
> > > Since Commit 21d1196a35f5 ("ipv4: set transport header earlier"),
> > > skb->transport_header has been always set before entering INET
> > > netfilter. This patch is to set skb->transport_header for bridge
> > > before entering INET netfilter by bridge-nf-call-iptables.
> > > 
> > > It also fixes an issue that sctp_error() couldn't compute a right
> > > csum due to unset skb->transport_header.
> > > 
> > > Fixes: e6d8b64b34aa ("net: sctp: fix and consolidate SCTP checksumming code")
> > > Reported-by: Li Shuang <shuali@...hat.com>
> > > Suggested-by: Pablo Neira Ayuso <pablo@...filter.org>
> > > Signed-off-by: Xin Long <lucien.xin@...il.com>
> > > ---
> > >  net/bridge/br_netfilter_hooks.c | 1 +
> > >  net/bridge/br_netfilter_ipv6.c  | 2 ++
> > >  2 files changed, 3 insertions(+)
> > > 
> > > diff --git a/net/bridge/br_netfilter_hooks.c b/net/bridge/br_netfilter_hooks.c
> > > index c93c35b..4d09a33 100644
> > > --- a/net/bridge/br_netfilter_hooks.c
> > > +++ b/net/bridge/br_netfilter_hooks.c
> > > @@ -502,6 +502,7 @@ static unsigned int br_nf_pre_routing(void *priv,
> > >  	nf_bridge->ipv4_daddr = ip_hdr(skb)->daddr;
> > >  
> > >  	skb->protocol = htons(ETH_P_IP);
> > > +	skb->transport_header = skb->network_header + ip_hdr(skb)->ihl * 4;
> > >  
> > >  	NF_HOOK(NFPROTO_IPV4, NF_INET_PRE_ROUTING, state->net, state->sk, skb,
> > >  		skb->dev, NULL,
> > > diff --git a/net/bridge/br_netfilter_ipv6.c b/net/bridge/br_netfilter_ipv6.c
> > > index 564710f..e88d664 100644
> > > --- a/net/bridge/br_netfilter_ipv6.c
> > > +++ b/net/bridge/br_netfilter_ipv6.c
> > > @@ -235,6 +235,8 @@ unsigned int br_nf_pre_routing_ipv6(void *priv,
> > >  	nf_bridge->ipv6_daddr = ipv6_hdr(skb)->daddr;
> > >  
> > >  	skb->protocol = htons(ETH_P_IPV6);
> > > +	skb->transport_header = skb->network_header + sizeof(struct ipv6hdr);
> > > +
> > >  	NF_HOOK(NFPROTO_IPV6, NF_INET_PRE_ROUTING, state->net, state->sk, skb,
> > >  		skb->dev, NULL,
> > >  		br_nf_pre_routing_finish_ipv6);
> > > -- 
> > > 2.1.0
> > > 
> > > 
> > Acked-by: Neil Horman <nhorman@...driver.com>
> 
> Acked-by: Florian Westphal <fw@...len.de>
> 
> ... because this fixes discrepancy of normal stack vs. bridge path.
> 
> However, I think we still need a fix for the ipv6 case as
> there is no guarantee ipv6 nexthdr will be the sctp one.
> 
> We already pass the sctp offset as argument to sctp_compute_cksum(),
> why can't we just replace sctp_hdr(skb) with skb->data + dataoff, as
> Xin Long originally suggested?
> 
> IOW, whats the problem with
> https://marc.info/?l=linux-netdev&m=155109395226858&w=2 ?

That patch looks fine too.

> (In theory conntrack can also inspect transport header inside icmp error
>  messages like pkttoobig and so on,
>  although this won't be the case here as we can't validate csum anyway
>  if the packet isn't complete).
> 
> Just pointing out that we can't rely on skb transport header being
> "correct" in all cases from netfilter point of view.

Indeed.