[PATCH net] l2tp: hold tunnel socket when handling control frames in l2tp_ip and l2tp_ip6

[Guillaume Nault]

The code following l2tp_tunnel_find() expects that a new reference is
held on sk. Either sk_receive_skb() or the discard_put error path will
drop a reference from the tunnel's socket.

This issue exists in both l2tp_ip and l2tp_ip6.

Fixes: a3c18422a4b4 ("l2tp: hold socket before dropping lock in l2tp_ip{, 
6}_recv()")
Signed-off-by: Guillaume Nault <g.na...@alphalink.fr>
---
 net/l2tp/l2tp_ip.c  | 5 +++--
 net/l2tp/l2tp_ip6.c | 5 +++--
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/net/l2tp/l2tp_ip.c b/net/l2tp/l2tp_ip.c
index d25038cfd64e..7208fbe5856b 100644
--- a/net/l2tp/l2tp_ip.c
+++ b/net/l2tp/l2tp_ip.c
@@ -178,9 +178,10 @@ static int l2tp_ip_recv(struct sk_buff *skb)
 
        tunnel_id = ntohl(*(__be32 *) &skb->data[4]);
        tunnel = l2tp_tunnel_find(net, tunnel_id);
-       if (tunnel != NULL)
+       if (tunnel) {
                sk = tunnel->sock;
-       else {
+               sock_hold(sk);
+       } else {
                struct iphdr *iph = (struct iphdr *) skb_network_header(skb);
 
                read_lock_bh(&l2tp_ip_lock);
diff --git a/net/l2tp/l2tp_ip6.c b/net/l2tp/l2tp_ip6.c
index a4abcbc4c09a..516d7ce24ba7 100644
--- a/net/l2tp/l2tp_ip6.c
+++ b/net/l2tp/l2tp_ip6.c
@@ -191,9 +191,10 @@ static int l2tp_ip6_recv(struct sk_buff *skb)
 
        tunnel_id = ntohl(*(__be32 *) &skb->data[4]);
        tunnel = l2tp_tunnel_find(net, tunnel_id);
-       if (tunnel != NULL)
+       if (tunnel) {
                sk = tunnel->sock;
-       else {
+               sock_hold(sk);
+       } else {
                struct ipv6hdr *iph = ipv6_hdr(skb);
 
                read_lock_bh(&l2tp_ip6_lock);
-- 
2.11.0