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jeremy 07-12-2013 10:38 AM

Overview of Linux Kernel Security Features

In this article, we'll take a high-level look at the security features of the Linux kernel. We'll start with a brief overview of traditional Unix security, and the rationale for extending that for Linux, then we'll discuss the Linux security extensions.

Unix Security – Discretionary Access Control

Linux was initially developed as a clone of the Unix operating system in the early 1990s. As such, it inherits the core Unix security model—a form of Discretionary Access Control (DAC). The security features of the Linux kernel have evolved significantly to meet modern requirements, although Unix DAC remains as the core model.

Briefly, Unix DAC allows the owner of an object (such as a file) to set the security policy for that object—which is why it's called a discretionary scheme. As a user, you can, for example, create a new file in your home directory and decide who else may read or write the file. This policy is implemented as permission bits attached to the file's inode, which may be set by the owner of the file. Permissions for accessing the file, such as read and write, may be set separately for the owner, a specific group, and other (i.e. everyone else). This is a relatively simple form of access control lists (ACLs).

Programs launched by a user run with all of the rights of that user, whether they need them or not. There is also a superuser—an all-powerful entity which bypasses Unix DAC policy for the purpose of managing the system. Running a program as the superuser provides that program with all rights on the system.

Extending Unix Security

Unix DAC is a relatively simple security scheme, although, designed in 1969, it does not meet all of the needs of security in the Internet age. It does not adequately protect against buggy or misconfigured software, for example, which may be exploited by an attacker seeking unauthorized access to resources. Privileged applications, those running as the superuser (by design or otherwise), are particularly risky in this respect. Once compromised, they can provide full system access to an attacker.

Functional requirements for security have also evolved over time. For example, many users require finer-grained policy than Unix DAC provides, and to control access to resources not covered by Unix DAC such as network packet flows.

It's worth noting that a critical design constraint for integrating new security features into the Linux kernel is that existing applications must not be broken. This is general constraint imposed by Linus for all new features. The option of designing a totally new security system from the ground up is not available—new features have to be retrofitted and compatible with the existing design of the system. In practical terms, this has meant that we end up with a collection of security enhancements rather than a monolithic security architecture.

We'll now take a look at the major Linux security extensions.
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