Re: [PATCH net] l2tp: remove skb_dst_set() from l2tp_xmit_skb()

[James Chapman <jchapman@katalix.com>]

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@gmail.com>
> Signed-off-by: Xin Long <lucien.xin@gmail.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.

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.

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.