netdev - Re: [PATCH net] l2tp: remove skb_dst_set() from l2tp_xmit

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Date:   Wed, 8 Jul 2020 04:08:09 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     James Chapman <jchapman@...alix.com>
Cc:     network dev <netdev@...r.kernel.org>, davem <davem@...emloft.net>,
        Guillaume Nault <gnault@...hat.com>
Subject: Re: [PATCH net] l2tp: remove skb_dst_set() from l2tp_xmit_skb()

On Wed, Jul 8, 2020 at 1:24 AM James Chapman <jchapman@...alix.com> wrote:
>
> On  Tue, Jul 07, 2020 at 02:02:32 +0800, Xin Long wrote:
> > In the tx path of l2tp, l2tp_xmit_skb() calls skb_dst_set() to set
> > skb's dst. However, it will eventually call inet6_csk_xmit() or
> > ip_queue_xmit() where skb's dst will be overwritten by:
> >
> >    skb_dst_set_noref(skb, dst);
> >
> > without releasing the old dst in skb. Then it causes dst/dev refcnt leak:
> >
> >   unregister_netdevice: waiting for eth0 to become free. Usage count = 1
> >
> > This can be reproduced by simply running:
> >
> >   # modprobe l2tp_eth && modprobe l2tp_ip
> >   # sh ./tools/testing/selftests/net/l2tp.sh
> >
> > So before going to inet6_csk_xmit() or ip_queue_xmit(), skb's dst
> > should be dropped. This patch is to fix it by removing skb_dst_set()
> > from l2tp_xmit_skb() and moving skb_dst_drop() into l2tp_xmit_core().
> >
> > Fixes: 3557baabf280 ("[L2TP]: PPP over L2TP driver core")
> > Reported-by: Hangbin Liu <liuhangbin@...il.com>
> > Signed-off-by: Xin Long <lucien.xin@...il.com>
> > ---
> >  net/l2tp/l2tp_core.c | 5 +----
> >  1 file changed, 1 insertion(+), 4 deletions(-)
> >
> > diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> > index fcb53ed..df133c24 100644
> > --- a/net/l2tp/l2tp_core.c
> > +++ b/net/l2tp/l2tp_core.c
> > @@ -1028,6 +1028,7 @@ static void l2tp_xmit_core(struct l2tp_session *session, struct sk_buff *skb,
> >
> >       /* Queue the packet to IP for output */
> >       skb->ignore_df = 1;
> > +     skb_dst_drop(skb);
> >  #if IS_ENABLED(CONFIG_IPV6)
> >       if (l2tp_sk_is_v6(tunnel->sock))
> >               error = inet6_csk_xmit(tunnel->sock, skb, NULL);
> > @@ -1099,10 +1100,6 @@ int l2tp_xmit_skb(struct l2tp_session *session, struct sk_buff *skb, int hdr_len
> >               goto out_unlock;
> >       }
> >
> > -     /* Get routing info from the tunnel socket */
> > -     skb_dst_drop(skb);
> > -     skb_dst_set(skb, sk_dst_check(sk, 0));
> > -
> >       inet = inet_sk(sk);
> >       fl = &inet->cork.fl;
> >       switch (tunnel->encap) {
> > --
> > 2.1.0
> >
>
> This patch doesn't seem right.

Hi James,
>
> For ipv4, the skb dst is used by skb_rtable. In ip_queue_xmit, if
> skb_rtable returns a route, it follows the packet_routed label and
> skb_dst_set_noref isn't done. Your patch is forcing every ipv4 l2tp
> packet to be routed, which isn't what we want.
Without this patch,
it does skb_dst_drop() in l2tp_xmit_skb(),
then do:
skb_dst_set(skb, sk_dst_check(sk, 0));

With this patch:
it does skb_dst_drop() in l2tp_xmit_core()

then in ip_queue_xmit(), it will do:
rt = (struct rtable *)__sk_dst_check(sk, 0);
skb_dst_set_noref(skb, &rt->dst);

so I don't think this patch drops any useful dst for ipv4.

>
> I ran l2tp.sh and found that the issue happens only for l2tp tests
> that use IPv6 IPSec in a routed topology.
>
> Perhaps the real problem is that l2tp shouldn't be using
> inet6_csk_xmit and should instead use ip6_xmit?
>
> Please hold off on applying this patch while I look into it.