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Subject[PATCH v2] 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 <>
- based on feedback from James Morris, added reference to

Documentation/security/00-INDEX | 2 +
Documentation/security/LSM.txt | 34 ++++++++++++++++++++++++++++++++
Documentation/security/credentials.txt | 6 ++--
3 files changed, 39 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 @@
- this file.
+ - description of the Linux Security Module framework.
- how to get started with the SELinux security enhancement.
diff --git a/Documentation/security/LSM.txt b/Documentation/security/LSM.txt
new file mode 100644
index 0000000..7245316
--- /dev/null
+++ b/Documentation/security/LSM.txt
@@ -0,0 +1,34 @@
+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,
+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.
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.


Kees Cook

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