kcm: switch order of device registration to fix a crash

When kcm is loaded while many processes try to create a KCM socket, a
crash occurs:
 BUG: unable to handle kernel NULL pointer dereference at 000000000000000e
 IP: mutex_lock+0x27/0x40 kernel/locking/mutex.c:240
 PGD 8000000016ef2067 P4D 8000000016ef2067 PUD 3d6e9067 PMD 0
 Oops: 0002 [#1] SMP KASAN PTI
 CPU: 0 PID: 7005 Comm: syz-executor.5 Not tainted 4.12.14-396-default #1 SLE15-SP1 (unreleased)
 RIP: 0010:mutex_lock+0x27/0x40 kernel/locking/mutex.c:240
 RSP: 0018:ffff88000d487a00 EFLAGS: 00010246
 RAX: 0000000000000000 RBX: 000000000000000e RCX: 1ffff100082b0719
 ...
 CR2: 000000000000000e CR3: 000000004b1bc003 CR4: 0000000000060ef0
 Call Trace:
  kcm_create+0x600/0xbf0 [kcm]
  __sock_create+0x324/0x750 net/socket.c:1272
 ...

This is due to race between sock_create and unfinished
register_pernet_device. kcm_create tries to do "net_generic(net,
kcm_net_id)". but kcm_net_id is not initialized yet.

So switch the order of the two to close the race.

This can be reproduced with mutiple processes doing socket(PF_KCM, ...)
and one process doing module removal.

Fixes: ab7ac4eb ("kcm: Kernel Connection Multiplexor module")
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/kcm/kcmsock.c

@@ -2054,14 +2054,14 @@ static int __init kcm_init(void)
 	if (err)
 		goto fail;
 
-	err = sock_register(&kcm_family_ops);
-	if (err)
-		goto sock_register_fail;
-
 	err = register_pernet_device(&kcm_net_ops);
 	if (err)
 		goto net_ops_fail;
 
+	err = sock_register(&kcm_family_ops);
+	if (err)
+		goto sock_register_fail;
+
 	err = kcm_proc_init();
 	if (err)
 		goto proc_init_fail;
@@ -2069,12 +2069,12 @@ static int __init kcm_init(void)
 	return 0;
 
 proc_init_fail:
-	unregister_pernet_device(&kcm_net_ops);
-
-net_ops_fail:
 	sock_unregister(PF_KCM);
 
 sock_register_fail:
+	unregister_pernet_device(&kcm_net_ops);
+
+net_ops_fail:
 	proto_unregister(&kcm_proto);
 
 fail:
@@ -2090,8 +2090,8 @@ static int __init kcm_init(void)
 static void __exit kcm_exit(void)
 {
 	kcm_proc_exit();
-	unregister_pernet_device(&kcm_net_ops);
 	sock_unregister(PF_KCM);
+	unregister_pernet_device(&kcm_net_ops);
 	proto_unregister(&kcm_proto);
 	destroy_workqueue(kcm_wq);