netdev - Re: [Patch net] fib: relax source validation check for loopback packets

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Date:   Sat, 13 Jul 2019 23:30:56 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     David Ahern <dsahern@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Julian Anastasov <ja@....bg>
Subject: Re: [Patch net] fib: relax source validation check for loopback packets

On Sat, Jul 13, 2019 at 3:42 PM David Ahern <dsahern@...il.com> wrote:
>
> On 7/12/19 2:17 PM, Cong Wang wrote:
> > diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
> > index 317339cd7f03..8662a44a28f9 100644
> > --- a/net/ipv4/fib_frontend.c
> > +++ b/net/ipv4/fib_frontend.c
> > @@ -388,6 +388,12 @@ static int __fib_validate_source(struct sk_buff *skb, __be32 src, __be32 dst,
> >       fib_combine_itag(itag, &res);
> >
> >       dev_match = fib_info_nh_uses_dev(res.fi, dev);
> > +     /* This is rare, loopback packets retain skb_dst so normally they
> > +      * would not even hit this slow path.
> > +      */
> > +     dev_match = dev_match || (res.type == RTN_LOCAL &&
> > +                               dev == net->loopback_dev &&
>
> The dev should not be needed. res.type == RTN_LOCAL should be enough, no?
>
> > +                               IN_DEV_ACCEPT_LOCAL(idev));
>
> Why is this check needed? Can you give an example use that is fixed -

I am not sure if I should have this check either, my initial version didn't
have it either, later I add it because I find out it is checked for rp_filter=0
case too.

On the other hand, loopback always accepts local traffic, so it may be
redundant to check it. So, I am not sure.

What do you think?

> and add one to selftests/net/fib_tests.sh?

It's complicated, Mesos network isolation uses this case:
https://cgit.twitter.biz/mesos/tree/src/slave/containerizer/mesos/isolators/network/port_mapping.cpp

Even if I use a simplified case, it still has to use TC filters and mirred
action to redirect the packet, which I am not sure they fit in fib_tests.sh.

Thanks.