Re: [PATCH] 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);
            //Bug happens
tunnel->sock = sk;

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

Signed-off-by: Sishuai Gong <redacted>
Reported-by: Sishuai Gong <redacted>
---
 net/l2tp/l2tp_core.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 203890e378cb..d61c9879076e 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c

@@ -1478,6 +1478,10 @@ 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;
+