Re: [PATCH net] net: sched: act_mirred: drop skb's dst_entry in ingress redirection

[Eyal Birger <eyal.birger@gmail.com>]

Hi,

On Fri, 18 Oct 2019 00:33:53 +0800
Zhiyuan Hou <zhiyuan2048@linux.alibaba.com> wrote:

> On 2019/10/16 8:13 下午, Eyal Birger wrote:
> > Hi,
> >
> > On Wed, 16 Oct 2019 01:22:01 +0800
> > Zhiyuan Hou <zhiyuan2048@linux.alibaba.com> wrote:
> >  
> >> On 2019/10/15 1:57 上午, Cong Wang wrote:  
> >>> On Sat, Oct 12, 2019 at 12:16 AM Zhiyuan Hou
> >>> <zhiyuan2048@linux.alibaba.com> wrote:  
> >>>> diff --git a/net/sched/act_mirred.c b/net/sched/act_mirred.c
> >>>> index 9ce073a05414..6108a64c0cd5 100644
> >>>> --- a/net/sched/act_mirred.c
> >>>> +++ b/net/sched/act_mirred.c
> >>>> @@ -18,6 +18,7 @@
> >>>>    #include <linux/gfp.h>
> >>>>    #include <linux/if_arp.h>
> >>>>    #include <net/net_namespace.h>
> >>>> +#include <net/dst.h>
> >>>>    #include <net/netlink.h>
> >>>>    #include <net/pkt_sched.h>
> >>>>    #include <net/pkt_cls.h>
> >>>> @@ -298,8 +299,10 @@ static int tcf_mirred_act(struct sk_buff
> >>>> *skb, const struct tc_action *a,
> >>>>
> >>>>           if (!want_ingress)
> >>>>                   err = dev_queue_xmit(skb2);
> >>>> -       else
> >>>> +       else {
> >>>> +               skb_dst_drop(skb2);
> >>>>                   err = netif_receive_skb(skb2);
> >>>> +       }  
> >>> Good catch!  
> > Indeed! Thanks for fixing this!
> >  
> >>> I don't want to be picky, but it seems this is only needed
> >>> when redirecting from egress to ingress, right? That is,
> >>> ingress to ingress, or ingress to egress is okay? If not,
> >>> please fix all the cases while you are on it?  
> >> Sure. But I think this patch is also needed when redirecting from
> >> ingress to ingress. Because we cannot assure that a skb has null
> >> dst in ingress redirection path. For example, if redirecting a skb
> >> from loopback's ingress to other device's ingress, the skb will
> >> take a dst.
> >>
> >> As commit logs point out, skb with valid dst cannot be made routing
> >> decision in following process. original dst may cause skb loss or
> >> other unexpected behavior.  
> > On the other hand, removing the dst on ingress-to-ingress
> > redirection may remove LWT information on incoming packets, which
> > may be undesired.  
> Sorry, I do not understand why lwt information is needed on
> ingress-to-ingress redirection. lwt is used on output path, isn't it?
> Can you please give more information?

On rx path tunnelled packets parameters received on a collect_md tunnel device
are kept in a metadata dst. See ip_tunnel_rcv() 'tun_dst' parameter.

The rx metadata dst can be matched by a number of mechanisms like routing
rules, eBPF, OVS, and netfilter.

Eyal.