[NETLINK]: Don't attach callback to a going-away netlink socket

There is a race between netlink_dump_start() and netlink_release()
that can lead to the situation when a netlink socket with non-zero
callback is freed.

Here it is:

CPU1:                           CPU2
netlink_release():              netlink_dump_start():

                                sk = netlink_lookup(); /* OK */

netlink_remove();

spin_lock(&nlk->cb_lock);
if (nlk->cb) { /* false */
  ...
}
spin_unlock(&nlk->cb_lock);

                                spin_lock(&nlk->cb_lock);
                                if (nlk->cb) { /* false */
                                         ...
                                }
                                nlk->cb = cb;
                                spin_unlock(&nlk->cb_lock);
                                ...
sock_orphan(sk);
/*
 * proceed with releasing
 * the socket
 */

The proposal it to make sock_orphan before detaching the callback
in netlink_release() and to check for the sock to be SOCK_DEAD in
netlink_dump_start() before setting a new callback.
Signed-off-by: Denis Lunev <den@openvz.org>
Signed-off-by: Kirill Korotaev <dev@openvz.org>
Signed-off-by: Pavel Emelianov <xemul@openvz.org>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -443,6 +443,7 @@ static int netlink_release(struct socket *sock)
 		return 0;
 
 	netlink_remove(sk);
+	sock_orphan(sk);
 	nlk = nlk_sk(sk);
 
 	spin_lock(&nlk->cb_lock);
@@ -457,7 +458,6 @@ static int netlink_release(struct socket *sock)
 	/* OK. Socket is unlinked, and, therefore,
 	   no new packets will arrive */
 
-	sock_orphan(sk);
 	sock->sk = NULL;
 	wake_up_interruptible_all(&nlk->wait);
 
@@ -1412,9 +1412,9 @@ int netlink_dump_start(struct sock *ssk, struct sk_buff *skb,
 		return -ECONNREFUSED;
 	}
 	nlk = nlk_sk(sk);
-	/* A dump is in progress... */
+	/* A dump or destruction is in progress... */
 	spin_lock(&nlk->cb_lock);
-	if (nlk->cb) {
+	if (nlk->cb || sock_flag(sk, SOCK_DEAD)) {
 		spin_unlock(&nlk->cb_lock);
 		netlink_destroy_callback(cb);
 		sock_put(sk);