132 строки
6.1 KiB
ReStructuredText
132 строки
6.1 KiB
ReStructuredText
========================================================
|
|
Linux Security Modules: General Security Hooks for Linux
|
|
========================================================
|
|
|
|
: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The APIs described in this book are outdated.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 was 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 work was
|
|
incorporated in the mainstream in December of 2003. This technical
|
|
report provides an overview of the framework and the capabilities
|
|
security module.
|
|
|
|
LSM Framework
|
|
=============
|
|
|
|
The LSM framework 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 sandboxing. By itself, the
|
|
framework does not provide any additional security; it merely provides
|
|
the infrastructure to support security modules. The LSM framework is
|
|
optional, requiring `CONFIG_SECURITY` to be enabled. The capabilities
|
|
logic is implemented as a security module.
|
|
This capabilities module is discussed further in
|
|
`LSM Capabilities Module`_.
|
|
|
|
The LSM framework includes security fields in kernel data structures and
|
|
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 security modules.
|
|
An interface `/sys/kernel/security/lsm` reports a comma separated list
|
|
of security modules that are active on the system.
|
|
|
|
The LSM security fields are simply ``void*`` pointers.
|
|
The data is referred to as a blob, which may be managed by
|
|
the framework or by the individual security modules that use it.
|
|
Security blobs that are used by more than one security module are
|
|
typically managed by the framework.
|
|
For process and
|
|
program execution security information, security fields are included in
|
|
:c:type:`struct task_struct <task_struct>` and
|
|
:c:type:`struct cred <cred>`.
|
|
For filesystem
|
|
security information, a security field is included in :c:type:`struct
|
|
super_block <super_block>`. For pipe, file, and socket security
|
|
information, security fields are included in :c:type:`struct inode
|
|
<inode>` and :c:type:`struct file <file>`.
|
|
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.
|
|
|
|
For packet and
|
|
network device security information, security fields were added to
|
|
:c:type:`struct sk_buff <sk_buff>` and
|
|
:c:type:`struct scm_cookie <scm_cookie>`.
|
|
Unlike the other security module data, the data used here is a
|
|
32-bit integer. The security modules are required to map or otherwise
|
|
associate these values with real security attributes.
|
|
|
|
LSM hooks are maintained in lists. A list is maintained for each
|
|
hook, and the hooks are called in the order specified by CONFIG_LSM.
|
|
Detailed documentation for each hook is
|
|
included in the `include/linux/lsm_hooks.h` header file.
|
|
|
|
The LSM framework provides for a close approximation of
|
|
general security module stacking. It defines
|
|
security_add_hooks() to which each security module passes a
|
|
:c:type:`struct security_hooks_list <security_hooks_list>`,
|
|
which are added to the lists.
|
|
The LSM framework does not provide a mechanism for removing hooks that
|
|
have been registered. The SELinux security module has implemented
|
|
a way to remove itself, however the feature has been deprecated.
|
|
|
|
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 security_inode_alloc() and security_inode_free()
|
|
These hooks are used to allocate
|
|
and free security structures for inode objects.
|
|
An example of the second category of hooks
|
|
is the security_inode_permission() hook.
|
|
This hook checks permission when accessing an inode.
|
|
|
|
LSM Capabilities Module
|
|
=======================
|
|
|
|
The POSIX.1e capabilities logic is maintained as a security module
|
|
stored in the file ``security/commoncap.c``. The capabilities
|
|
module uses the order field of the :c:type:`lsm_info` description
|
|
to identify it as the first security module to be registered.
|
|
The capabilities security module does not use the general security
|
|
blobs, unlike other modules. The reasons are historical and are
|
|
based on overhead, complexity and performance concerns.
|