Re: XFRM interface and NF_INET_LOCAL_OUT hook

[Phil Sutter <phil@nwl.cc>]

On Fri, Nov 27, 2020 at 10:55:11AM +0100, Steffen Klassert wrote:
> On Thu, Nov 26, 2020 at 02:12:00PM +0100, Phil Sutter wrote:
> > > > 
> > > > Is this a bug or an expected quirk when using XFRM interface?
> > > 
> > > This is expected behaviour. The xfrm interfaces are plaintext devices,
> > > the plaintext packets are routed to the xfrm interface which guarantees
> > > transformation. So the lookup that assigns skb_dst(skb)->xfrm
> > > happens 'behind' the interface. After transformation,
> > > skb_dst(skb)->xfrm will be cleared. So this assignment exists just
> > > inside xfrm in that case.
> > 
> > OK, thanks for the clarification.
> > 
> > > Does netfilter match against skb_dst(skb)->xfrm? What is the exact case
> > > that does not work?
> > 
> > The reported use-case is a match against tunnel data in output hook:
> > 
> > | table t {
> > |     chain c {
> > |         type filter hook output priority filter
> > |         oifname eth0 ipsec out ip daddr 192.168.1.2
> > |     }
> > | }
> > 
> > The ipsec expression tries to extract that data from skb_dst(skb)->xfrm
> > if present. In xt_policy (for iptables), code is equivalent. The above
> > works when not using xfrm_interface. Initially I assumed one just needs
> > to adjust the oifname match, but even dropping it doesn't help.
> 
> Yes, this does not work with xfrm interfaces. As said, they are plaintext
> devices that guarantee transformation.
> 
> Maybe you can try to match after transformation by using the secpath,
> but not sure if that is what you need.

Secpath is used for input only, no?

I played a bit more with xfrm_interface and noticed that when used,
NF_INET_LOCAL_OUT hook sees the packet (an ICMP reply) only once instead
of twice as without xfrm_interface. I don't think using it should change
behaviour that much apart from packets without matching policy being
dropped. What do you think about the following fix? I checked forwarding
packets as well and it looks like behaviour is identical to plain
policy:

diff --git a/net/xfrm/xfrm_interface.c b/net/xfrm/xfrm_interface.c
index aa4cdcf69d471..24af61c95b4d4 100644
--- a/net/xfrm/xfrm_interface.c
+++ b/net/xfrm/xfrm_interface.c
@@ -317,7 +317,8 @@ xfrmi_xmit2(struct sk_buff *skb, struct net_device *dev, struct flowi *fl)
        skb_dst_set(skb, dst);
        skb->dev = tdev;
 
-       err = dst_output(xi->net, skb->sk, skb);
+       err = NF_HOOK(skb_dst(skb)->ops->family, NF_INET_LOCAL_OUT, xi->net,
+                     skb->sk, skb, NULL, skb_dst(skb)->dev, dst_output);
        if (net_xmit_eval(err) == 0) {
                struct pcpu_sw_netstats *tstats = this_cpu_ptr(dev->tstats);
 
Thanks, Phil