How to set mandatory access controls to restrict user permissions on files and directories

How to set up mandatory access control to restrict user permissions on files and directories

In the operating system, mandatory access control (Mandatory Access Control, MAC) is a security mechanism used to restrict user permissions on files and directory access rights. Compared with ordinary access control mechanisms, such as Discretionary Access Control (DAC), mandatory access control provides a stricter access control policy to ensure that only users with corresponding permissions can access files and directories.

In this article, we will introduce how to use a common mandatory access control method-Label-based Access Control (LBAC) to implement access control to files and directories. Below is a sample code that demonstrates how to restrict user access to a file by setting labels.

First, we need to create a tag system to assign corresponding tags to files and users. Labels usually include two parts: object labels and subject labels, which represent the security levels of files and users respectively. In this example, we use three different security levels: "LOW", "MEDIUM" and "HIGH".

class LabelSystem:
    def __init__(self):
        self.labels = {}
        
    def assign_label(self, obj, label):
        self.labels[obj] = label
    
    def get_label(self, obj):
        return self.labels.get(obj)
    
    def check_permission(self, user_label, obj_label):
        if user_label <= obj_label:
            return True
        else:
            return False

Next, we create a specific file system to implement mandatory access control on files and directories. In this file system, each file and directory has a unique identifier and corresponding label.

class FileSystem:
    def __init__(self):
        self.files = {}
        
    def create_file(self, name):
        file = File(name)
        self.files[file] = Label("LOW")
        
    def create_directory(self, name):
        directory = Directory(name)
        self.files[directory] = Label("LOW")
        
    def get_file(self, name):
        for file in self.files:
            if file.name == name:
                return file
        return None
    
    def set_label(self, obj, label):
        if obj in self.files:
            self.files[obj] = Label(label)
        
    def get_label(self, obj):
        return self.files.get(obj)
    
    def check_permission(self, user, obj):
        user_label = self.get_label(user)
        obj_label = self.get_label(obj)
        
        if user_label and obj_label:
            return LabelSystem().check_permission(user_label, obj_label)
        else:
            return False
    
class File:
    def __init__(self, name):
        self.name = name
        
class Directory:
    def __init__(self, name):
        self.name = name
        
class Label:
    def __init__(self, level):
        self.level = level

Finally, we can use the above code to create a file system and set the corresponding file and directory labels. Then, you can determine whether the user has permission to access the file based on the user's tag and the file's tag.

if __name__ == "__main__":
    file_system = FileSystem()
    
    # 创建文件和目录
    file_system.create_file("file1.txt")
    file_system.create_directory("dir1")
    
    # 设置文件和目录的标签
    file_system.set_label(file_system.get_file("file1.txt"), "MEDIUM")
    file_system.set_label(file_system.get_file("dir1"), "HIGH")
    
    # 判断用户权限
    user_label = Label("LOW")
    print(file_system.check_permission(user_label, file_system.get_file("file1.txt")))  # True
    print(file_system.check_permission(user_label, file_system.get_file("dir1")))  # False

Through the above sample code, we can see how to use label-based mandatory access control to restrict user access to files and directories. By setting different security level labels, more fine-grained access control can be achieved to protect the security of sensitive data. As an advanced security mechanism, mandatory access control can help us build a more secure system in practical applications.

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