Linux is a powerful and widely-used operating system, especially favored for servers, cloud environments, and technical workstations. Security is a core focus in Linux, with a well-architected model that governs how resources are accessed and protected. Understanding the Linux security model provides the foundation needed to secure Linux systems effectively.
Core Concepts of Linux Security Model
The Linux security model is designed around several key components and concepts that work together to control access and enforce security policies:
1. User and Group Based Access Control
At its core, Linux controls access to files and resources based on users and groups:
Users: Every person or service process running on Linux is assigned a unique user ID (UID).
Groups: Users can be part of one or more groups, each with a group ID (GID).
Permissions: Every file and directory is associated with an owner (user), a group, and permission bits that determine read, write, and execute access for owner, group members, and others.
2. File System Permissions
Linux uses traditional Unix-style file permissions to regulate access at the file system level: Permissions are divided into three classes (user, group, others). Each class has three types of access: read (r), write (w), and execute (x).
Example: A file permission string like -rwxr-xr-- means the owner can read/write/execute, the group can read/execute, and others can only read.
3. Extended Access Control
Beyond basic permissions, Linux supports more granular controls:
Access Control Lists (ACLs): ACLs provide detailed control over which users or groups can access a file, allowing exceptions beyond the classic owner/group/others model.
Extended Attributes: Metadata that can store additional security information, used by mechanisms like SELinux.
4. Security-Enhanced Linux (SELinux) and Mandatory Access Controls (MAC)
SELinux is a powerful MAC system implemented by the NSA and integrated into many Linux distributions. It provides fine-grained access policies independent of traditional permissions.
SELinux enforces rules that confine programs to the lowest level of privilege necessary, limiting potential damage from vulnerabilities or exploitation. Other MAC systems include AppArmor, which is profile-based and often easier to manage.
5. Process and Kernel-Level Security
In Linux, each process operates within a security context defined by user and group IDs, while the kernel enforces isolation, access control, and fine-grained privileges through capabilities that avoid granting full root access, and namespaces further strengthen security by isolating processes’ views of system resources such as networks and file systems.
6. Authentication and Identity
Linux uses methods including password authentication, key-based authentication (e.g., SSH keys), and PAM (Pluggable Authentication Modules) to manage user identities and control login access. PAM allows the integration of various authentication mechanisms into a unified framework.
7. Auditing and Logging
Linux uses its audit subsystem to record security-relevant events, generating logs for activities such as authentication attempts, file access, and system changes, which supports effective monitoring, incident detection, and forensic investigation.
