===========================
Linux Security Module Usage
===========================

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.

The Linux capabilities modules will always be included. This may be
followed by any number of "minor" modules and at most one "major" module.
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.

Process attributes associated with "major" security modules should
be accessed and maintained using the special files in ``/proc/.../attr``.
A security module may maintain a module specific subdirectory there,
named after the module. ``/proc/.../attr/smack`` is provided by the Smack
security module and contains all its special files. The files directly
in ``/proc/.../attr`` remain as legacy interfaces for modules that provide
subdirectories.

.. toctree::
   :maxdepth: 1

   apparmor
   LoadPin
   SELinux
   Smack
   tomoyo
   Yama
