Commit 27d53323 authored Jul 07, 2020 by Xin Long Committed by David S. Miller Jul 08, 2020

l2tp: remove skb_dst_set() from l2tp_xmit_skb()



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: 3557baab ("[L2TP]: PPP over L2TP driver core")
Reported-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: James Chapman <jchapman@katalix.com>
Tested-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 1412bb2b

net/l2tp/l2tp_core.c

+1 −4

Original line number	Diff line number	Diff line
		@@ -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) {

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