266 lines
12 KiB
Cheetah
266 lines
12 KiB
Cheetah
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
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<article class="whitepaper" id="LinuxSecurityModule" lang="en">
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<articleinfo>
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<title>Linux Security Modules: General Security Hooks for Linux</title>
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<authorgroup>
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<author>
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<firstname>Stephen</firstname>
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<surname>Smalley</surname>
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<affiliation>
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<orgname>NAI Labs</orgname>
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<address><email>ssmalley@nai.com</email></address>
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</affiliation>
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</author>
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<author>
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<firstname>Timothy</firstname>
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<surname>Fraser</surname>
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<affiliation>
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<orgname>NAI Labs</orgname>
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<address><email>tfraser@nai.com</email></address>
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</affiliation>
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</author>
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<author>
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<firstname>Chris</firstname>
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<surname>Vance</surname>
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<affiliation>
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<orgname>NAI Labs</orgname>
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<address><email>cvance@nai.com</email></address>
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</affiliation>
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</author>
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</authorgroup>
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</articleinfo>
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<sect1 id="Introduction"><title>Introduction</title>
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<para>
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In March 2001, the National Security Agency (NSA) gave a presentation
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about Security-Enhanced Linux (SELinux) at the 2.5 Linux Kernel
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Summit. SELinux is an implementation of flexible and fine-grained
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nondiscretionary access controls in the Linux kernel, originally
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implemented as its own particular kernel patch. Several other
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security projects (e.g. RSBAC, Medusa) have also developed flexible
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access control architectures for the Linux kernel, and various
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projects have developed particular access control models for Linux
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(e.g. LIDS, DTE, SubDomain). Each project has developed and
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maintained its own kernel patch to support its security needs.
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</para>
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<para>
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In response to the NSA presentation, Linus Torvalds made a set of
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remarks that described a security framework he would be willing to
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consider for inclusion in the mainstream Linux kernel. He described a
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general framework that would provide a set of security hooks to
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control operations on kernel objects and a set of opaque security
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fields in kernel data structures for maintaining security attributes.
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This framework could then be used by loadable kernel modules to
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implement any desired model of security. Linus also suggested the
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possibility of migrating the Linux capabilities code into such a
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module.
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</para>
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<para>
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The Linux Security Modules (LSM) project was started by WireX to
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develop such a framework. LSM is a joint development effort by
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several security projects, including Immunix, SELinux, SGI and Janus,
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and several individuals, including Greg Kroah-Hartman and James
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Morris, to develop a Linux kernel patch that implements this
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framework. The patch is currently tracking the 2.4 series and is
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targeted for integration into the 2.5 development series. This
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technical report provides an overview of the framework and the example
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capabilities security module provided by the LSM kernel patch.
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</para>
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</sect1>
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<sect1 id="framework"><title>LSM Framework</title>
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<para>
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The LSM kernel patch provides a general kernel framework to support
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security modules. In particular, the LSM framework is primarily
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focused on supporting access control modules, although future
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development is likely to address other security needs such as
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auditing. By itself, the framework does not provide any additional
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security; it merely provides the infrastructure to support security
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modules. The LSM kernel patch also moves most of the capabilities
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logic into an optional security module, with the system defaulting
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to the traditional superuser logic. This capabilities module
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is discussed further in <xref linkend="cap"/>.
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</para>
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<para>
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The LSM kernel patch adds security fields to kernel data structures
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and inserts calls to hook functions at critical points in the kernel
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code to manage the security fields and to perform access control. It
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also adds functions for registering and unregistering security
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modules, and adds a general <function>security</function> system call
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to support new system calls for security-aware applications.
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</para>
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<para>
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The LSM security fields are simply <type>void*</type> pointers. For
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process and program execution security information, security fields
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were added to <structname>struct task_struct</structname> and
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<structname>struct linux_binprm</structname>. For filesystem security
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information, a security field was added to
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<structname>struct super_block</structname>. For pipe, file, and socket
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security information, security fields were added to
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<structname>struct inode</structname> and
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<structname>struct file</structname>. For packet and network device security
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information, security fields were added to
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<structname>struct sk_buff</structname> and
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<structname>struct net_device</structname>. For System V IPC security
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information, security fields were added to
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<structname>struct kern_ipc_perm</structname> and
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<structname>struct msg_msg</structname>; additionally, the definitions
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for <structname>struct msg_msg</structname>, <structname>struct
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msg_queue</structname>, and <structname>struct
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shmid_kernel</structname> were moved to header files
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(<filename>include/linux/msg.h</filename> and
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<filename>include/linux/shm.h</filename> as appropriate) to allow
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the security modules to use these definitions.
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</para>
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<para>
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Each LSM hook is a function pointer in a global table,
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security_ops. This table is a
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<structname>security_operations</structname> structure as defined by
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<filename>include/linux/security.h</filename>. Detailed documentation
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for each hook is included in this header file. At present, this
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structure consists of a collection of substructures that group related
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hooks based on the kernel object (e.g. task, inode, file, sk_buff,
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etc) as well as some top-level hook function pointers for system
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operations. This structure is likely to be flattened in the future
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for performance. The placement of the hook calls in the kernel code
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is described by the "called:" lines in the per-hook documentation in
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the header file. The hook calls can also be easily found in the
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kernel code by looking for the string "security_ops->".
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</para>
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<para>
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Linus mentioned per-process security hooks in his original remarks as a
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possible alternative to global security hooks. However, if LSM were
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to start from the perspective of per-process hooks, then the base
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framework would have to deal with how to handle operations that
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involve multiple processes (e.g. kill), since each process might have
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its own hook for controlling the operation. This would require a
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general mechanism for composing hooks in the base framework.
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Additionally, LSM would still need global hooks for operations that
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have no process context (e.g. network input operations).
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Consequently, LSM provides global security hooks, but a security
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module is free to implement per-process hooks (where that makes sense)
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by storing a security_ops table in each process' security field and
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then invoking these per-process hooks from the global hooks.
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The problem of composition is thus deferred to the module.
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</para>
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<para>
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The global security_ops table is initialized to a set of hook
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functions provided by a dummy security module that provides
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traditional superuser logic. A <function>register_security</function>
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function (in <filename>security/security.c</filename>) is provided to
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allow a security module to set security_ops to refer to its own hook
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functions, and an <function>unregister_security</function> function is
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provided to revert security_ops to the dummy module hooks. This
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mechanism is used to set the primary security module, which is
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responsible for making the final decision for each hook.
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</para>
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<para>
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LSM also provides a simple mechanism for stacking additional security
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modules with the primary security module. It defines
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<function>register_security</function> and
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<function>unregister_security</function> hooks in the
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<structname>security_operations</structname> structure and provides
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<function>mod_reg_security</function> and
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<function>mod_unreg_security</function> functions that invoke these
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hooks after performing some sanity checking. A security module can
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call these functions in order to stack with other modules. However,
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the actual details of how this stacking is handled are deferred to the
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module, which can implement these hooks in any way it wishes
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(including always returning an error if it does not wish to support
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stacking). In this manner, LSM again defers the problem of
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composition to the module.
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</para>
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<para>
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Although the LSM hooks are organized into substructures based on
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kernel object, all of the hooks can be viewed as falling into two
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major categories: hooks that are used to manage the security fields
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and hooks that are used to perform access control. Examples of the
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first category of hooks include the
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<function>alloc_security</function> and
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<function>free_security</function> hooks defined for each kernel data
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structure that has a security field. These hooks are used to allocate
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and free security structures for kernel objects. The first category
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of hooks also includes hooks that set information in the security
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field after allocation, such as the <function>post_lookup</function>
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hook in <structname>struct inode_security_ops</structname>. This hook
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is used to set security information for inodes after successful lookup
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operations. An example of the second category of hooks is the
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<function>permission</function> hook in
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<structname>struct inode_security_ops</structname>. This hook checks
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permission when accessing an inode.
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</para>
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</sect1>
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<sect1 id="cap"><title>LSM Capabilities Module</title>
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<para>
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The LSM kernel patch moves most of the existing POSIX.1e capabilities
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logic into an optional security module stored in the file
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<filename>security/capability.c</filename>. This change allows
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users who do not want to use capabilities to omit this code entirely
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from their kernel, instead using the dummy module for traditional
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superuser logic or any other module that they desire. This change
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also allows the developers of the capabilities logic to maintain and
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enhance their code more freely, without needing to integrate patches
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back into the base kernel.
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</para>
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<para>
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In addition to moving the capabilities logic, the LSM kernel patch
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could move the capability-related fields from the kernel data
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structures into the new security fields managed by the security
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modules. However, at present, the LSM kernel patch leaves the
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capability fields in the kernel data structures. In his original
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remarks, Linus suggested that this might be preferable so that other
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security modules can be easily stacked with the capabilities module
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without needing to chain multiple security structures on the security field.
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It also avoids imposing extra overhead on the capabilities module
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to manage the security fields. However, the LSM framework could
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certainly support such a move if it is determined to be desirable,
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with only a few additional changes described below.
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</para>
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<para>
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At present, the capabilities logic for computing process capabilities
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on <function>execve</function> and <function>set*uid</function>,
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checking capabilities for a particular process, saving and checking
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capabilities for netlink messages, and handling the
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<function>capget</function> and <function>capset</function> system
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calls have been moved into the capabilities module. There are still a
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few locations in the base kernel where capability-related fields are
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directly examined or modified, but the current version of the LSM
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patch does allow a security module to completely replace the
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assignment and testing of capabilities. These few locations would
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need to be changed if the capability-related fields were moved into
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the security field. The following is a list of known locations that
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still perform such direct examination or modification of
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capability-related fields:
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<itemizedlist>
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<listitem><para><filename>fs/open.c</filename>:<function>sys_access</function></para></listitem>
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<listitem><para><filename>fs/lockd/host.c</filename>:<function>nlm_bind_host</function></para></listitem>
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<listitem><para><filename>fs/nfsd/auth.c</filename>:<function>nfsd_setuser</function></para></listitem>
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<listitem><para><filename>fs/proc/array.c</filename>:<function>task_cap</function></para></listitem>
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</itemizedlist>
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</para>
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</sect1>
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</article>
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