rds: Incorrect reference counting in TCP socket creation

Commit 0933a578cd55 ("rds: tcp: use sock_create_lite() to create the
accept socket") has a reference counting issue in TCP socket creation
when accepting a new connection.  The code uses sock_create_lite() to
create a kernel socket.  But it does not do __module_get() on the
socket owner.  When the connection is shutdown and sock_release() is
called to free the socket, the owner's reference count is decremented
and becomes incorrect.  Note that this bug only shows up when the socket
owner is configured as a kernel module.

v2: Update comments

Fixes: 0933a578cd55 ("rds: tcp: use sock_create_lite() to create the accept socket")
Signed-off-by: Ka-Cheong Poon <ka-cheong.poon@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Acked-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/rds/tcp_listen.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2006 Oracle.  All rights reserved.
+ * Copyright (c) 2006, 2018 Oracle.  All rights reserved.
  *
  * This software is available to you under a choice of one of two
  * licenses.  You may choose to be licensed under the terms of the GNU
@@ -142,12 +142,20 @@ int rds_tcp_accept_one(struct socket *sock)
 	if (ret)
 		goto out;
 
-	new_sock->type = sock->type;
-	new_sock->ops = sock->ops;
 	ret = sock->ops->accept(sock, new_sock, O_NONBLOCK, true);
 	if (ret < 0)
 		goto out;
 
+	/* sock_create_lite() does not get a hold on the owner module so we
+	 * need to do it here.  Note that sock_release() uses sock->ops to
+	 * determine if it needs to decrement the reference count.  So set
+	 * sock->ops after calling accept() in case that fails.  And there's
+	 * no need to do try_module_get() as the listener should have a hold
+	 * already.
+	 */
+	new_sock->ops = sock->ops;
+	__module_get(new_sock->ops->owner);
+
 	ret = rds_tcp_keepalive(new_sock);
 	if (ret < 0)
 		goto out;