[PATCH] Documentation: clarify the purpose of LSMs

[Kees Cook]

Clarify the purpose of the LSM interface with some brief examples and
pointers to additional documentation.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 Documentation/security/00-INDEX        |    2 ++
 Documentation/security/LSM.txt         |   26 ++++++++++++++++++++++++++
 Documentation/security/credentials.txt |    6 +++---
 3 files changed, 31 insertions(+), 3 deletions(-)
 create mode 100644 Documentation/security/LSM.txt

diff --git a/Documentation/security/00-INDEX b/Documentation/security/00-INDEX
index 1f33b73..eeed1de 100644
--- a/Documentation/security/00-INDEX
+++ b/Documentation/security/00-INDEX
@@ -1,5 +1,7 @@
 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
diff --git a/Documentation/security/LSM.txt b/Documentation/security/LSM.txt
new file mode 100644
index 0000000..ec93803
--- /dev/null
+++ b/Documentation/security/LSM.txt
@@ -0,0 +1,26 @@
+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.
+
+For extensive documentation on the available LSM hook interfaces, please
+see include/linux/security.h.
diff --git a/Documentation/security/credentials.txt b/Documentation/security/credentials.txt
index fc0366c..8625705 100644
--- a/Documentation/security/credentials.txt
+++ b/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.
 
-- 
1.7.5.4


-- 
Kees Cook                                            @outflux.net
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