ax25: fix a use-after-free in ax25_fillin_cb()

There are multiple issues here:

1. After freeing dev->ax25_ptr, we need to set it to NULL otherwise
   we may use a dangling pointer.

2. There is a race between ax25_setsockopt() and device notifier as
   reported by syzbot. Close it by holding RTNL lock.

3. We need to test if dev->ax25_ptr is NULL before using it.

Reported-and-tested-by: syzbot+ae6bb869cbed29b29040@syzkaller.appspotmail.com
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ax25/af_ax25.c

@@ -653,15 +653,22 @@ static int ax25_setsockopt(struct socket *sock, int level, int optname,
 			break;
 		}
 
-		dev = dev_get_by_name(&init_net, devname);
+		rtnl_lock();
+		dev = __dev_get_by_name(&init_net, devname);
 		if (!dev) {
+			rtnl_unlock();
 			res = -ENODEV;
 			break;
 		}
 
 		ax25->ax25_dev = ax25_dev_ax25dev(dev);
+		if (!ax25->ax25_dev) {
+			rtnl_unlock();
+			res = -ENODEV;
+			break;
+		}
 		ax25_fillin_cb(ax25, ax25->ax25_dev);
-		dev_put(dev);
+		rtnl_unlock();
 		break;
 
 	default:

net/ax25/ax25_dev.c

@@ -116,6 +116,7 @@ void ax25_dev_device_down(struct net_device *dev)
 	if ((s = ax25_dev_list) == ax25_dev) {
 		ax25_dev_list = s->next;
 		spin_unlock_bh(&ax25_dev_lock);
+		dev->ax25_ptr = NULL;
 		dev_put(dev);
 		kfree(ax25_dev);
 		return;
@@ -125,6 +126,7 @@ void ax25_dev_device_down(struct net_device *dev)
 		if (s->next == ax25_dev) {
 			s->next = ax25_dev->next;
 			spin_unlock_bh(&ax25_dev_lock);
+			dev->ax25_ptr = NULL;
 			dev_put(dev);
 			kfree(ax25_dev);
 			return;