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  <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>