Documentation: clarify the purpose of LSMs

Clarify the purpose of the LSM interface with some brief examples and
pointers to additional documentation.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <jmorris@namei.org>

Documentation/security/00-INDEX

 00-INDEX
 	- this file.
+LSM.txt
+	- description of the Linux Security Module framework.
 SELinux.txt
 	- how to get started with the SELinux security enhancement.
 Smack.txt

Documentation/security/LSM.txt

+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.
+
+Based on http://kerneltrap.org/Linux/Documenting_Security_Module_Intent,
+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.

Documentation/security/credentials.txt

@@ -221,10 +221,10 @@ The Linux kernel supports the following types of credentials:
  (5) LSM
 
      The Linux Security Module allows extra controls to be placed over the
-     operations that a task may do.  Currently Linux supports two main
-     alternate LSM options: SELinux and Smack.
+     operations that a task may do.  Currently Linux supports several LSM
+     options.
 
-     Both work by labelling the objects in a system and then applying sets of
+     Some work by labelling the objects in a system and then applying sets of
      rules (policies) that say what operations a task with one label may do to
      an object with another label.