bridge: netlink: fix slave_changelink/br_setport race conditions

commit 963ad948 upstream.

Since slave_changelink support was added there have been a few race
conditions when using br_setport() since some of the port functions it
uses require the bridge lock. It is very easy to trigger a lockup due to
some internal spin_lock() usage without bh disabled, also it's possible to
get the bridge into an inconsistent state.
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Fixes: 3ac636b8 ("bridge: implement rtnl_link_ops->slave_changelink")
Reviewed-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

net/bridge/br_netlink.c

@@ -466,9 +466,17 @@ static int br_port_slave_changelink(struct net_device *brdev,
 				    struct nlattr *tb[],
 				    struct nlattr *data[])
 {
+	struct net_bridge *br = netdev_priv(brdev);
+	int ret;
+
 	if (!data)
 		return 0;
-	return br_setport(br_port_get_rtnl(dev), data);
+
+	spin_lock_bh(&br->lock);
+	ret = br_setport(br_port_get_rtnl(dev), data);
+	spin_unlock_bh(&br->lock);
+
+	return ret;
 }
 
 static int br_port_fill_slave_info(struct sk_buff *skb,