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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>
+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
+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.
+<sect1 id="framework"><title>LSM Framework</title>
+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"/>.
+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.
+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.
+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->".
+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 <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.
+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.
+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.
+<sect1 id="cap"><title>LSM Capabilities Module</title>
+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.
+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 <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: