l2tp: avoid use-after-free caused by l2tp_ip_backlog_recv

commit 51fb60eb162ab84c5edf2ae9c63cf0b878e5547e upstream.

l2tp_ip_backlog_recv may not return -1 if the packet gets dropped.
The return value is passed up to ip_local_deliver_finish, which treats
negative values as an IP protocol number for resubmission.

Signed-off-by: Paul Hüber <phueber@kernsp.in>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/net/l2tp/l2tp_ip.c b/net/l2tp/l2tp_ip.c
index a753c9b..6f12fd6 100644 (file)
--- a/net/l2tp/l2tp_ip.c
+++ b/net/l2tp/l2tp_ip.c
@@ -432,7 +432,7 @@ static int l2tp_ip_backlog_recv(struct sock *sk, struct sk_buff *skb)
 drop:
        IP_INC_STATS(&init_net, IPSTATS_MIB_INDISCARDS);
        kfree_skb(skb);
-       return -1;
+       return 0;
 }
 
 /* Userspace will call sendmsg() on the tunnel socket to send L2TP