static int tun_attach(struct tun_struct *tun, struct file *file, bool skip_filter)
{
	struct tun_file *tfile = file->private_data;
	struct net_device *dev = tun->dev;
	int err;

	err = security_tun_dev_attach(tfile->socket.sk, tun->security);
	if (err < 0)
		goto out;

	err = -EINVAL;
	if (rtnl_dereference(tfile->tun) && !tfile->detached)
		goto out;

	err = -EBUSY;
	if (!(tun->flags & IFF_MULTI_QUEUE) && tun->numqueues == 1)
		goto out;

	err = -E2BIG;
	if (!tfile->detached &&
	    tun->numqueues + tun->numdisabled == MAX_TAP_QUEUES)
		goto out;

	err = 0;

	/* Re-attach the filter to persist device */
	if (!skip_filter && (tun->filter_attached == true)) {
		lock_sock(tfile->socket.sk);
		err = sk_attach_filter(&tun->fprog, tfile->socket.sk);
		release_sock(tfile->socket.sk);
		if (!err)
			goto out;
	}

	if (!tfile->detached &&
	    skb_array_init(&tfile->tx_array, dev->tx_queue_len, GFP_KERNEL)) {
		err = -ENOMEM;
		goto out;
	}

	tfile->queue_index = tun->numqueues;
	tfile->socket.sk->sk_shutdown &= ~RCV_SHUTDOWN;
	rcu_assign_pointer(tfile->tun, tun);
	rcu_assign_pointer(tun->tfiles[tun->numqueues], tfile);
	tun->numqueues++;

	if (tfile->detached)
		tun_enable_queue(tfile);
	else
		sock_hold(&tfile->sk);

	tun_set_real_num_queues(tun);

	/* device is allowed to go away first, so no need to hold extra
	 * refcnt.
	 */

out:
	return err;
}

The product dereferences a pointer that it expects to be valid but is NULL.