The Linux security model is a robust framework designed to protect system resources, enforce access control, and maintain the integrity, confidentiality, and availability of data.
1. User and Group Model.
• Users: Every process and file is associated with a user, identified by a User ID (UID). The root user (UID 0) has unrestricted access to the system.
• Groups: Users can belong to one or more groups (identified by Group ID, GID), enabling shared access to resources.
• Authentication: Managed through mechanisms like /etc/passwd for user info and /etc/shadow for hashed passwords, with support for PAM (Pluggable Authentication Modules) for flexible authentication policies.
2. File System Permissions.
• Standard Permissions: Files and directories have permissions (read, write, execute) for three categories: owner, group, and others. Represented as rwxr-xr-x or octal (e.g., 755).
• Ownership: Each file has an owner (UID) and a group (GID), set via chown or chgrp.
• Special Bits:
• SetUID/SetGID: Allows a process to run with the file owner’s or group’s permissions (e.g., passwd uses SetUID to allow users to change their own passwords).
• Sticky Bit: Restricts deletion of files in a directory to the file owner (e.g., /tmp).
• Access Control Lists (ACLs): Extend permissions to allow fine-grained control beyond the standard model, using setfacl and getfacl.
3. Discretionary Access Control (DAC).
• Users (owners) have discretion over their files’ permissions, deciding who can access what.
• Processes run with the permissions of the user executing them, except when SetUID/SetGID is used.
4. Mandatory Access Control (MAC)
• Systems like SELinux (Security-Enhanced Linux) or AppArmor enforce stricter, system-wide policies:
• SELinux: Uses labels (contexts) for subjects (processes) and objects (files, sockets). Policies define rules for access based on these labels, enforcing confinement even for root.
• AppArmor: Focuses on path-based confinement, restricting processes to specific resources (e.g., files, network).
• MAC overrides DAC, ensuring even privileged users adhere to system policies.
5. Privilege Management
• sudo: Allows users to execute commands with elevated privileges (configured in /etc/sudoers).
• Capabilities: Splits root privileges into granular capabilities (e.g., CAP_NET_ADMIN for network configuration), reducing the need for full root access.
• Privilege Escalation Protections: Mechanisms like restricted shells and noexec mounts limit unauthorized privilege escalation.
6. Process Isolation
• Namespaces: Isolate resources like process IDs, network stacks, and file systems (used in containers like Docker).
• cgroups: Limit and control resource usage (CPU, memory) for processes.
• Seccomp: Restricts system calls a process can make, reducing the attack surface (e.g., used in sandboxing).
7. Kernel-Level Security
• System Call Filtering: Tools like seccomp-bpf limit the system calls available to a process.
• Address Space Layout Randomization (ASLR): Randomizes memory locations to mitigate exploits.
• Kernel Hardening: Features like KASLR (Kernel ASLR), SMAP (Supervisor Mode Access Prevention), and SMEP (Supervisor Mode Execution Prevention) protect against kernel exploits.
8. Network Security
• iptables/nftables: Firewall rules to control incoming and outgoing traffic.
• Network Namespaces: Isolate network interfaces and routing tables.
• TLS/SSL: Secure communication for services like SSH and HTTPS.
• IPsec: Provides encrypted network communication.
9. Audit and Logging
• Auditd: Tracks security-relevant events (e.g., file access, system calls) for monitoring and forensics.
• Syslog: Logs system and security events for analysis.
• File Integrity Monitoring: Tools like AIDE detect unauthorized changes to critical files.
10. Filesystem and Storage Security
• Mount Options: Options like noexec, nosuid, and nodev restrict what can be done on mounted filesystems.
• Encrypted Filesystems: Tools like LUKS (via dm-crypt) encrypt data at rest.
• Immutable Files: Using chattr +i to prevent modification of critical files.
11. Additional Security Mechanisms
• Firejail: A lightweight sandboxing tool to restrict process environments.
• Grsecurity/PaX: Kernel patches for enhanced security (e.g., memory protection, role-based access).
• Fail2ban: Monitors logs and bans IPs after repeated failed login attempts.
Key Principles
• Least Privilege: Processes and users have only the permissions necessary for their tasks.
• Defense in Depth: Multiple layers (permissions, MAC, kernel hardening) ensure no single failure compromises the system.
• Separation of Duties: Divides administrative tasks to reduce the risk of abuse.
Practical Tools and Commands
• Check permissions: ls -l, getfacl
• Modify permissions: chmod, chown, setfacl
• Monitor processes: ps, top, htop
• Audit system: auditctl, ausearch
• Configure SELinux: semanage, setsebool
• Firewall management: iptables, nft
For deeper insights, you can explore specific components like SELinux policies or kernel capabilities, or I can analyze relevant X posts or web resources for real-time discussions on Linux security trends. Let me know if you want to focus on a specific aspect or need a practical example!
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Linux-Security