Lsm

SELinux (Security-Enhanced Linux) is a Mandatory Access Control (MAC) implementation developed by the NSA and released as open source in 2000, merged into the mainline Linux kernel in 2.6 via the LSM framework in 2003. Its defining characteristic is default deny: unlike the standard Linux Discretionary Access Control model (file permission bits), where anything not explicitly forbidden is permitted, SELinux refuses all access that is not explicitly allowed by policy. Every process and every object — every file, socket, pipe, device node, and IPC object — carries a security context (also called a label) of the form user:role:type:level. The policy is a compiled set of rules, loaded at boot, that defines precisely which combinations of process context and object context may interact and how. An Apache web server process running in the httpd_t domain can read files labelled httpd_sys_content_t but is denied access to files labelled user_home_t or shadow_t, regardless of what Unix file permission bits say. If the web server is compromised, the attacker is confined to what httpd_t permits — typically a narrow, well-defined set of files and network operations — rather than having the full access of the user account running Apache.

AppArmor (Application Armor) is a Mandatory Access Control (MAC) system implemented as a major LSM (Linux Security Module), developed originally by Immunix and now maintained by Canonical. It is the default MAC system on Ubuntu, Debian, and their derivatives, and the default container confinement mechanism for containerd and Docker on those distributions. Where SELinux assigns security labels to every object on the system and enforces policy based on label interactions, AppArmor takes a fundamentally different approach: it confines programs by filesystem path. A profile for nginx lists the specific file paths that nginx is allowed to read, write, and execute, the network operations it may perform, and the Linux capabilities it may use — anything not listed is denied. No relabelling of the filesystem is required and no extended attributes are set: AppArmor’s confinement decisions are made purely from the path of the file being accessed and the identity of the confined process. This path-based model makes AppArmor profiles far simpler to read, write, and audit than SELinux policy, and eliminates the mislabelled-file failure mode that is the most common SELinux operational problem.