Linux Security Modules

Linux Security Module framework The Linux Security Module (LSM) framework provides a mechanism for various security checks to be hooked by new kernel extensions. The name "module" is a bit of a misnomer since these extensions are not actually loadable kernel modules. Instead, they are selectable at build-time via CONFIG DEFAULT SECURITY and can be overridden at boot-time via the "security=..." kernel command line argument, in the case where multiple LSMs were built into a given kernel. The primary users of the LSM interface are Mandatory Access Control (MAC) extensions which provide a comprehensive security policy. Examples include SELinux, Smack, Tomoyo, and AppArmor. In addition to the larger MAC extensions, other extensions can be built using the LSM to provide specific changes to system operation when these tweaks are not available in the core functionality of Linux itself. Without a specific LSM built into the kernel, the default LSM will be the Linux capabilities system. Most LSMs choose to extend the capabilities system, building their checks on top of the defined capability hooks. For more details on capabilities, see capabilities(7) in the Linux man-pages project. A list of the active security modules can be found by reading /sys/kernel/security/lsm. This is a comma separated list, and will always include the capability module. The list reflects the order in which checks are made. The capability module will always be first, followed by any "minor" modules (e.g. Yama) and then the one "major" module (e.g. SELinux) if there is one configured. Based on a new LSM is accepted into the kernel when its intent (a description of what it tries to protect against and in what cases one would expect to use it) has been appropriately documented in Documentation/security/. This allows an LSM's code to be easily compared to its goals, and so that end users and distros can make a more informed decision about which LSMs suit their requirements. For extensive documentation on the available LSM hook interfaces, please see include/linux/security.h.