tuntap: forbid calling TUNSETIFF when detached

Michael points out that even after Stefan's fix the TUNSETIFF is still allowed
to create a new tap device. This because we only check tfile->tun but the
tfile->detached were introduced. Fix this by failing early in tun_set_iff() if
the file is detached. After this fix, there's no need to do the check again in
tun_set_iff(), so this patch removes it.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -488,8 +488,6 @@ static int tun_attach(struct tun_struct *tun, struct file *file)
 	err = -EINVAL;
 	if (rtnl_dereference(tfile->tun))
 		goto out;
-	if (tfile->detached && tun != tfile->detached)
-		goto out;
 
 	err = -EBUSY;
 	if (!(tun->flags & TUN_TAP_MQ) && tun->numqueues == 1)
@@ -1543,6 +1541,9 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
 	struct net_device *dev;
 	int err;
 
+	if (tfile->detached)
+		return -EINVAL;
+
 	dev = __dev_get_by_name(net, ifr->ifr_name);
 	if (dev) {
 		if (ifr->ifr_flags & IFF_TUN_EXCL)