========================================================
Linux Security Modules: General Security Hooks for Linux
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:Author: Stephen Smalley
:Author: Timothy Fraser
:Author: Chris Vance

.. note::

   The APIs described in this book are outdated.

Introduction
============

In March 2001, the National Security Agency (NSA) gave a presentation
about Security-Enhanced Linux (SELinux) at the 2.5 Linux Kernel Summit.
SELinux is an implementation of flexible and fine-grained
nondiscretionary access controls in the Linux kernel, originally
implemented as its own particular kernel patch. Several other security
projects (e.g. RSBAC, Medusa) have also developed flexible access
control architectures for the Linux kernel, and various projects have
developed particular access control models for Linux (e.g. LIDS, DTE,
SubDomain). Each project has developed and maintained its own kernel
patch to support its security needs.

In response to the NSA presentation, Linus Torvalds made a set of
remarks that described a security framework he would be willing to
consider for inclusion in the mainstream Linux kernel. He described a
general framework that would provide a set of security hooks to control
operations on kernel objects and a set of opaque security fields in
kernel data structures for maintaining security attributes. This
framework could then be used by loadable kernel modules to implement any
desired model of security. Linus also suggested the possibility of
migrating the Linux capabilities code into such a module.

The Linux Security Modules (LSM) project was started by WireX to develop
such a framework. LSM is a joint development effort by several security
projects, including Immunix, SELinux, SGI and Janus, and several
individuals, including Greg Kroah-Hartman and James Morris, to develop a
Linux kernel patch that implements this framework. The patch is
currently tracking the 2.4 series and is targeted for integration into
the 2.5 development series. This technical report provides an overview
of the framework and the example capabilities security module provided
by the LSM kernel patch.

LSM Framework
=============

The LSM kernel patch provides a general kernel framework to support
security modules. In particular, the LSM framework is primarily focused
on supporting access control modules, although future development is
likely to address other security needs such as auditing. By itself, the
framework does not provide any additional security; it merely provides
the infrastructure to support security modules. The LSM kernel patch
also moves most of the capabilities logic into an optional security
module, with the system defaulting to the traditional superuser logic.
This capabilities module is discussed further in
`LSM Capabilities Module`_.

The LSM kernel patch adds security fields to kernel data structures and
inserts calls to hook functions at critical points in the kernel code to
manage the security fields and to perform access control. It also adds
functions for registering and unregistering security modules, and adds a
general :c:func:`security()` system call to support new system calls
for security-aware applications.

The LSM security fields are simply ``void*`` pointers. For process and
program execution security information, security fields were added to
:c:type:`struct task_struct <task_struct>` and
:c:type:`struct linux_binprm <linux_binprm>`. For filesystem
security information, a security field was added to :c:type:`struct
super_block <super_block>`. For pipe, file, and socket security
information, security fields were added to :c:type:`struct inode
<inode>` and :c:type:`struct file <file>`. For packet and
network device security information, security fields were added to
:c:type:`struct sk_buff <sk_buff>` and :c:type:`struct
net_device <net_device>`. For System V IPC security information,
security fields were added to :c:type:`struct kern_ipc_perm
<kern_ipc_perm>` and :c:type:`struct msg_msg
<msg_msg>`; additionally, the definitions for :c:type:`struct
msg_msg <msg_msg>`, struct msg_queue, and struct shmid_kernel
were moved to header files (``include/linux/msg.h`` and
``include/linux/shm.h`` as appropriate) to allow the security modules to
use these definitions.

Each LSM hook is a function pointer in a global table, security_ops.
This table is a :c:type:`struct security_operations
<security_operations>` structure as defined by
``include/linux/security.h``. Detailed documentation for each hook is
included in this header file. At present, this structure consists of a
collection of substructures that group related hooks based on the kernel
object (e.g. task, inode, file, sk_buff, etc) as well as some top-level
hook function pointers for system operations. This structure is likely
to be flattened in the future for performance. The placement of the hook
calls in the kernel code is described by the "called:" lines in the
per-hook documentation in the header file. The hook calls can also be
easily found in the kernel code by looking for the string
"security_ops->".

Linus mentioned per-process security hooks in his original remarks as a
possible alternative to global security hooks. However, if LSM were to
start from the perspective of per-process hooks, then the base framework
would have to deal with how to handle operations that involve multiple
processes (e.g. kill), since each process might have its own hook for
controlling the operation. This would require a general mechanism for
composing hooks in the base framework. Additionally, LSM would still
need global hooks for operations that have no process context (e.g.
network input operations). Consequently, LSM provides global security
hooks, but a security module is free to implement per-process hooks
(where that makes sense) by storing a security_ops table in each
process' security field and then invoking these per-process hooks from
the global hooks. The problem of composition is thus deferred to the
module.

The global security_ops table is initialized to a set of hook functions
provided by a dummy security module that provides traditional superuser
logic. A :c:func:`register_security()` function (in
``security/security.c``) is provided to allow a security module to set
security_ops to refer to its own hook functions, and an
:c:func:`unregister_security()` function is provided to revert
security_ops to the dummy module hooks. This mechanism is used to set
the primary security module, which is responsible for making the final
decision for each hook.

LSM also provides a simple mechanism for stacking additional security
modules with the primary security module. It defines
:c:func:`register_security()` and
:c:func:`unregister_security()` hooks in the :c:type:`struct
security_operations <security_operations>` structure and
provides :c:func:`mod_reg_security()` and
:c:func:`mod_unreg_security()` functions that invoke these hooks
after performing some sanity checking. A security module can call these
functions in order to stack with other modules. However, the actual
details of how this stacking is handled are deferred to the module,
which can implement these hooks in any way it wishes (including always
returning an error if it does not wish to support stacking). In this
manner, LSM again defers the problem of composition to the module.

Although the LSM hooks are organized into substructures based on kernel
object, all of the hooks can be viewed as falling into two major
categories: hooks that are used to manage the security fields and hooks
that are used to perform access control. Examples of the first category
of hooks include the :c:func:`alloc_security()` and
:c:func:`free_security()` hooks defined for each kernel data
structure that has a security field. These hooks are used to allocate
and free security structures for kernel objects. The first category of
hooks also includes hooks that set information in the security field
after allocation, such as the :c:func:`post_lookup()` hook in
:c:type:`struct inode_security_ops <inode_security_ops>`.
This hook is used to set security information for inodes after
successful lookup operations. An example of the second category of hooks
is the :c:func:`permission()` hook in :c:type:`struct
inode_security_ops <inode_security_ops>`. This hook checks
permission when accessing an inode.

LSM Capabilities Module
=======================

The LSM kernel patch moves most of the existing POSIX.1e capabilities
logic into an optional security module stored in the file
``security/capability.c``. This change allows users who do not want to
use capabilities to omit this code entirely from their kernel, instead
using the dummy module for traditional superuser logic or any other
module that they desire. This change also allows the developers of the
capabilities logic to maintain and enhance their code more freely,
without needing to integrate patches back into the base kernel.

In addition to moving the capabilities logic, the LSM kernel patch could
move the capability-related fields from the kernel data structures into
the new security fields managed by the security modules. However, at
present, the LSM kernel patch leaves the capability fields in the kernel
data structures. In his original remarks, Linus suggested that this
might be preferable so that other security modules can be easily stacked
with the capabilities module without needing to chain multiple security
structures on the security field. It also avoids imposing extra overhead
on the capabilities module to manage the security fields. However, the
LSM framework could certainly support such a move if it is determined to
be desirable, with only a few additional changes described below.

At present, the capabilities logic for computing process capabilities on
:c:func:`execve()` and :c:func:`set\*uid()`, checking
capabilities for a particular process, saving and checking capabilities
for netlink messages, and handling the :c:func:`capget()` and
:c:func:`capset()` system calls have been moved into the
capabilities module. There are still a few locations in the base kernel
where capability-related fields are directly examined or modified, but
the current version of the LSM patch does allow a security module to
completely replace the assignment and testing of capabilities. These few
locations would need to be changed if the capability-related fields were
moved into the security field. The following is a list of known
locations that still perform such direct examination or modification of
capability-related fields:

-  ``fs/open.c``::c:func:`sys_access()`

-  ``fs/lockd/host.c``::c:func:`nlm_bind_host()`

-  ``fs/nfsd/auth.c``::c:func:`nfsd_setuser()`

-  ``fs/proc/array.c``::c:func:`task_cap()`