[PATCH 4.14 043/125] l2tp: remove skb_dst_set() from l2tp_xmit_skb()

From: Greg Kroah-Hartman
Date: Mon Jul 20 2020 - 12:41:35 EST


From: Xin Long <lucien.xin@xxxxxxxxx>

[ Upstream commit 27d53323664c549b5bb2dfaaf6f7ad6e0376a64e ]

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@xxxxxxxxx>
Signed-off-by: Xin Long <lucien.xin@xxxxxxxxx>
Acked-by: James Chapman <jchapman@xxxxxxxxxxx>
Tested-by: James Chapman <jchapman@xxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
net/l2tp/l2tp_core.c | 5 +----
1 file changed, 1 insertion(+), 4 deletions(-)

--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1141,6 +1141,7 @@ static int l2tp_xmit_core(struct l2tp_se

/* 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);
@@ -1214,10 +1215,6 @@ int l2tp_xmit_skb(struct l2tp_session *s
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) {