net: fix a concurrency bug in l2tp_tunnel_register()

l2tp_tunnel_register() registers a tunnel without fully
initializing its attribute. This can allow another kernel thread
running l2tp_xmit_core() to access the uninitialized data and
then cause a kernel NULL pointer dereference error, as shown below.

Thread 1    Thread 2
//l2tp_tunnel_register()
list_add_rcu(&tunnel->list, &pn->l2tp_tunnel_list);
           //pppol2tp_connect()
           tunnel = l2tp_tunnel_get(sock_net(sk), info.tunnel_id);
           // Fetch the new tunnel
           ...
           //l2tp_xmit_core()
           struct sock *sk = tunnel->sock;
           ...
           bh_lock_sock(sk);
           //Null pointer error happens
tunnel->sock = sk;

Fix this bug by initializing tunnel->sock before adding the
tunnel into l2tp_tunnel_list.

Reviewed-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Sishuai Gong <sishuai@purdue.edu>
Reported-by: Sishuai Gong <sishuai@purdue.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 2ee2074..53486b1 100644 (file)
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1478,11 +1478,15 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
        tunnel->l2tp_net = net;
        pn = l2tp_pernet(net);
 
+       sk = sock->sk;
+       sock_hold(sk);
+       tunnel->sock = sk;
+
        spin_lock_bh(&pn->l2tp_tunnel_list_lock);
        list_for_each_entry(tunnel_walk, &pn->l2tp_tunnel_list, list) {
                if (tunnel_walk->tunnel_id == tunnel->tunnel_id) {
                        spin_unlock_bh(&pn->l2tp_tunnel_list_lock);
-
+                       sock_put(sk);
                        ret = -EEXIST;
                        goto err_sock;
                }
@@ -1490,10 +1494,6 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
        list_add_rcu(&tunnel->list, &pn->l2tp_tunnel_list);
        spin_unlock_bh(&pn->l2tp_tunnel_list_lock);
 
-       sk = sock->sk;
-       sock_hold(sk);
-       tunnel->sock = sk;
-
        if (tunnel->encap == L2TP_ENCAPTYPE_UDP) {
                struct udp_tunnel_sock_cfg udp_cfg = {
                        .sk_user_data = tunnel,