RDS: TCP: avoid bad page reference in rds_tcp_listen_data_ready

As the existing comments in rds_tcp_listen_data_ready() indicate,
it is possible under some race-windows to get to this function with the
accept() socket. If that happens, we could run into a sequence whereby

   thread 1				thread 2

rds_tcp_accept_one() thread
sets up new_sock via ->accept().
The sk_user_data is now
sock_def_readable
					data comes in for new_sock,
					->sk_data_ready is called, and
					we land in rds_tcp_listen_data_ready
rds_tcp_set_callbacks()
takes the sk_callback_lock and
sets up sk_user_data to be the cp
					read_lock sk_callback_lock
					ready = cp
					unlock sk_callback_lock
					page fault on ready

In the above sequence, we end up with a panic on a bad page reference
when trying to execute (*ready)(). Instead we need to call
sock_def_readable() safely, which is what this patch achieves.
Acked-by: NSantosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: NSowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/rds/tcp.c

@@ -551,6 +551,13 @@ static void rds_tcp_kill_sock(struct net *net)
 	}
 }
 
+void *rds_tcp_listen_sock_def_readable(struct net *net)
+{
+	struct rds_tcp_net *rtn = net_generic(net, rds_tcp_netid);
+
+	return rtn->rds_tcp_listen_sock->sk->sk_user_data;
+}
+
 static int rds_tcp_dev_event(struct notifier_block *this,
 			     unsigned long event, void *ptr)
 {

net/rds/tcp.h

@@ -70,6 +70,7 @@ void rds_tcp_listen_stop(struct socket *);
 void rds_tcp_listen_data_ready(struct sock *sk);
 int rds_tcp_accept_one(struct socket *sock);
 int rds_tcp_keepalive(struct socket *sock);
+void *rds_tcp_listen_sock_def_readable(struct net *net);
 
 /* tcp_recv.c */
 int rds_tcp_recv_init(void);

net/rds/tcp_listen.c

@@ -183,6 +183,8 @@ void rds_tcp_listen_data_ready(struct sock *sk)
 	 */
 	if (sk->sk_state == TCP_LISTEN)
 		rds_tcp_accept_work(sk);
+	else
+		ready = rds_tcp_listen_sock_def_readable(sock_net(sk));
 
 out:
 	read_unlock_bh(&sk->sk_callback_lock);