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-<?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
- "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
-
-<article class="whitepaper" id="LinuxSecurityModule" lang="en">
- <articleinfo>
- <title>Linux Security Modules: General Security Hooks for Linux</title>
- <authorgroup>
- <author>
- <firstname>Stephen</firstname>
- <surname>Smalley</surname>
- <affiliation>
- <orgname>NAI Labs</orgname>
- <address><email>ssmalley@nai.com</email></address>
- </affiliation>
- </author>
- <author>
- <firstname>Timothy</firstname>
- <surname>Fraser</surname>
- <affiliation>
- <orgname>NAI Labs</orgname>
- <address><email>tfraser@nai.com</email></address>
- </affiliation>
- </author>
- <author>
- <firstname>Chris</firstname>
- <surname>Vance</surname>
- <affiliation>
- <orgname>NAI Labs</orgname>
- <address><email>cvance@nai.com</email></address>
- </affiliation>
- </author>
- </authorgroup>
- </articleinfo>
-
-<sect1 id="Introduction"><title>Introduction</title>
-
-<para>
-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.
-</para>
-
-<para>
-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.
-</para>
-
-<para>
-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.
-</para>
-
-</sect1>
-
-<sect1 id="framework"><title>LSM Framework</title>
-
-<para>
-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 <xref linkend="cap"/>.
-</para>
-
-<para>
-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 <function>security</function> system call
-to support new system calls for security-aware applications.
-</para>
-
-<para>
-The LSM security fields are simply <type>void*</type> pointers. For
-process and program execution security information, security fields
-were added to <structname>struct task_struct</structname> and
-<structname>struct linux_binprm</structname>. For filesystem security
-information, a security field was added to
-<structname>struct super_block</structname>. For pipe, file, and socket
-security information, security fields were added to
-<structname>struct inode</structname> and
-<structname>struct file</structname>. For packet and network device security
-information, security fields were added to
-<structname>struct sk_buff</structname> and
-<structname>struct net_device</structname>. For System V IPC security
-information, security fields were added to
-<structname>struct kern_ipc_perm</structname> and
-<structname>struct msg_msg</structname>; additionally, the definitions
-for <structname>struct msg_msg</structname>, <structname>struct
-msg_queue</structname>, and <structname>struct
-shmid_kernel</structname> were moved to header files
-(<filename>include/linux/msg.h</filename> and
-<filename>include/linux/shm.h</filename> as appropriate) to allow
-the security modules to use these definitions.
-</para>
-
-<para>
-Each LSM hook is a function pointer in a global table,
-security_ops. This table is a
-<structname>security_operations</structname> structure as defined by
-<filename>include/linux/security.h</filename>. 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->".
-
-</para>
-
-<para>
-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.
-</para>
-
-<para>
-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 <function>register_security</function>
-function (in <filename>security/security.c</filename>) is provided to
-allow a security module to set security_ops to refer to its own hook
-functions, and an <function>unregister_security</function> 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.
-</para>
-
-<para>
-LSM also provides a simple mechanism for stacking additional security
-modules with the primary security module. It defines
-<function>register_security</function> and
-<function>unregister_security</function> hooks in the
-<structname>security_operations</structname> structure and provides
-<function>mod_reg_security</function> and
-<function>mod_unreg_security</function> 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.
-</para>
-
-<para>
-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
-<function>alloc_security</function> and
-<function>free_security</function> 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 <function>post_lookup</function>
-hook in <structname>struct inode_security_ops</structname>. This hook
-is used to set security information for inodes after successful lookup
-operations. An example of the second category of hooks is the
-<function>permission</function> hook in
-<structname>struct inode_security_ops</structname>. This hook checks
-permission when accessing an inode.
-</para>
-
-</sect1>
-
-<sect1 id="cap"><title>LSM Capabilities Module</title>
-
-<para>
-The LSM kernel patch moves most of the existing POSIX.1e capabilities
-logic into an optional security module stored in the file
-<filename>security/capability.c</filename>. 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.
-</para>
-
-<para>
-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.
-</para>
-
-<para>
-At present, the capabilities logic for computing process capabilities
-on <function>execve</function> and <function>set*uid</function>,
-checking capabilities for a particular process, saving and checking
-capabilities for netlink messages, and handling the
-<function>capget</function> and <function>capset</function> 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:
-<itemizedlist>
-<listitem><para><filename>fs/open.c</filename>:<function>sys_access</function></para></listitem>
-<listitem><para><filename>fs/lockd/host.c</filename>:<function>nlm_bind_host</function></para></listitem>
-<listitem><para><filename>fs/nfsd/auth.c</filename>:<function>nfsd_setuser</function></para></listitem>
-<listitem><para><filename>fs/proc/array.c</filename>:<function>task_cap</function></para></listitem>
-</itemizedlist>
-</para>
-
-</sect1>
-
-</article>