Documentation/DocBook/lsm.tmpl

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   5 <article class="whitepaper" id="LinuxSecurityModule" lang="en">
   6  <articleinfo>
   7  <title>Linux Security Modules:  General Security Hooks for Linux</title>
   8  <authorgroup>
   9  <author>
  10  <firstname>Stephen</firstname>
  11  <surname>Smalley</surname>
  12  <affiliation>
  13  <orgname>NAI Labs</orgname>
  14  <address><email>ssmalley@nai.com</email></address>
  15  </affiliation>
  16  </author>
  17  <author>
  18  <firstname>Timothy</firstname>
  19  <surname>Fraser</surname>
  20  <affiliation>
  21  <orgname>NAI Labs</orgname>
  22  <address><email>tfraser@nai.com</email></address>
  23  </affiliation>
  24  </author>
  25  <author>
  26  <firstname>Chris</firstname>
  27  <surname>Vance</surname>
  28  <affiliation>
  29  <orgname>NAI Labs</orgname>
  30  <address><email>cvance@nai.com</email></address>
  31  </affiliation>
  32  </author>
  33  </authorgroup>
  34  </articleinfo>
  35
  36 <sect1 id="Introduction"><title>Introduction</title>
  37
  38 <para>
  39 In March 2001, the National Security Agency (NSA) gave a presentation
  40 about Security-Enhanced Linux (SELinux) at the 2.5 Linux Kernel
  41 Summit.  SELinux is an implementation of flexible and fine-grained
  42 nondiscretionary access controls in the Linux kernel, originally
  43 implemented as its own particular kernel patch.  Several other
  44 security projects (e.g. RSBAC, Medusa) have also developed flexible
  45 access control architectures for the Linux kernel, and various
  46 projects have developed particular access control models for Linux
  47 (e.g. LIDS, DTE, SubDomain).  Each project has developed and
  48 maintained its own kernel patch to support its security needs.
  49 </para>
  50
  51 <para>
  52 In response to the NSA presentation, Linus Torvalds made a set of
  53 remarks that described a security framework he would be willing to
  54 consider for inclusion in the mainstream Linux kernel.  He described a
  55 general framework that would provide a set of security hooks to
  56 control operations on kernel objects and a set of opaque security
  57 fields in kernel data structures for maintaining security attributes.
  58 This framework could then be used by loadable kernel modules to
  59 implement any desired model of security.  Linus also suggested the
  60 possibility of migrating the Linux capabilities code into such a
  61 module.
  62 </para>
  63
  64 <para>
  65 The Linux Security Modules (LSM) project was started by WireX to
  66 develop such a framework.  LSM is a joint development effort by
  67 several security projects, including Immunix, SELinux, SGI and Janus,
  68 and several individuals, including Greg Kroah-Hartman and James
  69 Morris, to develop a Linux kernel patch that implements this
  70 framework.  The patch is currently tracking the 2.4 series and is
  71 targeted for integration into the 2.5 development series.  This
  72 technical report provides an overview of the framework and the example
  73 capabilities security module provided by the LSM kernel patch.
  74 </para>
  75
  76 </sect1>
  77
  78 <sect1 id="framework"><title>LSM Framework</title>
  79
  80 <para>
  81 The LSM kernel patch provides a general kernel framework to support
  82 security modules.  In particular, the LSM framework is primarily
  83 focused on supporting access control modules, although future
  84 development is likely to address other security needs such as
  85 auditing.  By itself, the framework does not provide any additional
  86 security; it merely provides the infrastructure to support security
  87 modules.  The LSM kernel patch also moves most of the capabilities
  88 logic into an optional security module, with the system defaulting
  89 to the traditional superuser logic.  This capabilities module
  90 is discussed further in <xref linkend="cap"/>.
  91 </para>
  92
  93 <para>
  94 The LSM kernel patch adds security fields to kernel data structures
  95 and inserts calls to hook functions at critical points in the kernel
  96 code to manage the security fields and to perform access control.  It
  97 also adds functions for registering and unregistering security
  98 modules, and adds a general <function>security</function> system call
  99 to support new system calls for security-aware applications.
 100 </para>
 101
 102 <para>
 103 The LSM security fields are simply <type>void*</type> pointers.  For
 104 process and program execution security information, security fields
 105 were added to <structname>struct task_struct</structname> and
 106 <structname>struct linux_binprm</structname>.  For filesystem security
 107 information, a security field was added to
 108 <structname>struct super_block</structname>.  For pipe, file, and socket
 109 security information, security fields were added to
 110 <structname>struct inode</structname> and
 111 <structname>struct file</structname>.  For packet and network device security
 112 information, security fields were added to
 113 <structname>struct sk_buff</structname> and
 114 <structname>struct net_device</structname>.  For System V IPC security
 115 information, security fields were added to
 116 <structname>struct kern_ipc_perm</structname> and
 117 <structname>struct msg_msg</structname>; additionally, the definitions
 118 for <structname>struct msg_msg</structname>, <structname>struct
 119 msg_queue</structname>, and <structname>struct
 120 shmid_kernel</structname> were moved to header files
 121 (<filename>include/linux/msg.h</filename> and
 122 <filename>include/linux/shm.h</filename> as appropriate) to allow
 123 the security modules to use these definitions.
 124 </para>
 125
 126 <para>
 127 Each LSM hook is a function pointer in a global table,
 128 security_ops. This table is a
 129 <structname>security_operations</structname> structure as defined by
 130 <filename>include/linux/security.h</filename>.  Detailed documentation
 131 for each hook is included in this header file.  At present, this
 132 structure consists of a collection of substructures that group related
 133 hooks based on the kernel object (e.g. task, inode, file, sk_buff,
 134 etc) as well as some top-level hook function pointers for system
 135 operations.  This structure is likely to be flattened in the future
 136 for performance.  The placement of the hook calls in the kernel code
 137 is described by the "called:" lines in the per-hook documentation in
 138 the header file.  The hook calls can also be easily found in the
 139 kernel code by looking for the string "security_ops->".
 140
 141 </para>
 142
 143 <para>
 144 Linus mentioned per-process security hooks in his original remarks as a
 145 possible alternative to global security hooks.  However, if LSM were
 146 to start from the perspective of per-process hooks, then the base
 147 framework would have to deal with how to handle operations that
 148 involve multiple processes (e.g. kill), since each process might have
 149 its own hook for controlling the operation.  This would require a
 150 general mechanism for composing hooks in the base framework.
 151 Additionally, LSM would still need global hooks for operations that
 152 have no process context (e.g. network input operations).
 153 Consequently, LSM provides global security hooks, but a security
 154 module is free to implement per-process hooks (where that makes sense)
 155 by storing a security_ops table in each process' security field and
 156 then invoking these per-process hooks from the global hooks.
 157 The problem of composition is thus deferred to the module.
 158 </para>
 159
 160 <para>
 161 The global security_ops table is initialized to a set of hook
 162 functions provided by a dummy security module that provides
 163 traditional superuser logic.  A <function>register_security</function>
 164 function (in <filename>security/security.c</filename>) is provided to
 165 allow a security module to set security_ops to refer to its own hook
 166 functions, and an <function>unregister_security</function> function is
 167 provided to revert security_ops to the dummy module hooks.  This
 168 mechanism is used to set the primary security module, which is
 169 responsible for making the final decision for each hook.
 170 </para>
 171
 172 <para>
 173 LSM also provides a simple mechanism for stacking additional security
 174 modules with the primary security module.  It defines
 175 <function>register_security</function> and
 176 <function>unregister_security</function> hooks in the
 177 <structname>security_operations</structname> structure and provides
 178 <function>mod_reg_security</function> and
 179 <function>mod_unreg_security</function> functions that invoke these
 180 hooks after performing some sanity checking.  A security module can
 181 call these functions in order to stack with other modules.  However,
 182 the actual details of how this stacking is handled are deferred to the
 183 module, which can implement these hooks in any way it wishes
 184 (including always returning an error if it does not wish to support
 185 stacking).  In this manner, LSM again defers the problem of
 186 composition to the module.
 187 </para>
 188
 189 <para>
 190 Although the LSM hooks are organized into substructures based on
 191 kernel object, all of the hooks can be viewed as falling into two
 192 major categories: hooks that are used to manage the security fields
 193 and hooks that are used to perform access control.  Examples of the
 194 first category of hooks include the
 195 <function>alloc_security</function> and
 196 <function>free_security</function> hooks defined for each kernel data
 197 structure that has a security field.  These hooks are used to allocate
 198 and free security structures for kernel objects.  The first category
 199 of hooks also includes hooks that set information in the security
 200 field after allocation, such as the <function>post_lookup</function>
 201 hook in <structname>struct inode_security_ops</structname>.  This hook
 202 is used to set security information for inodes after successful lookup
 203 operations.  An example of the second category of hooks is the
 204 <function>permission</function> hook in
 205 <structname>struct inode_security_ops</structname>.  This hook checks
 206 permission when accessing an inode.
 207 </para>
 208
 209 </sect1>
 210
 211 <sect1 id="cap"><title>LSM Capabilities Module</title>
 212
 213 <para>
 214 The LSM kernel patch moves most of the existing POSIX.1e capabilities
 215 logic into an optional security module stored in the file
 216 <filename>security/capability.c</filename>.  This change allows
 217 users who do not want to use capabilities to omit this code entirely
 218 from their kernel, instead using the dummy module for traditional
 219 superuser logic or any other module that they desire.  This change
 220 also allows the developers of the capabilities logic to maintain and
 221 enhance their code more freely, without needing to integrate patches
 222 back into the base kernel.
 223 </para>
 224
 225 <para>
 226 In addition to moving the capabilities logic, the LSM kernel patch
 227 could move the capability-related fields from the kernel data
 228 structures into the new security fields managed by the security
 229 modules.  However, at present, the LSM kernel patch leaves the
 230 capability fields in the kernel data structures.  In his original
 231 remarks, Linus suggested that this might be preferable so that other
 232 security modules can be easily stacked with the capabilities module
 233 without needing to chain multiple security structures on the security field.
 234 It also avoids imposing extra overhead on the capabilities module
 235 to manage the security fields.  However, the LSM framework could
 236 certainly support such a move if it is determined to be desirable,
 237 with only a few additional changes described below.
 238 </para>
 239
 240 <para>
 241 At present, the capabilities logic for computing process capabilities
 242 on <function>execve</function> and <function>set*uid</function>,
 243 checking capabilities for a particular process, saving and checking
 244 capabilities for netlink messages, and handling the
 245 <function>capget</function> and <function>capset</function> system
 246 calls have been moved into the capabilities module.  There are still a
 247 few locations in the base kernel where capability-related fields are
 248 directly examined or modified, but the current version of the LSM
 249 patch does allow a security module to completely replace the
 250 assignment and testing of capabilities.  These few locations would
 251 need to be changed if the capability-related fields were moved into
 252 the security field.  The following is a list of known locations that
 253 still perform such direct examination or modification of
 254 capability-related fields:
 255 <itemizedlist>
 256 <listitem><para><filename>fs/open.c</filename>:<function>sys_access</function></para></listitem>
 257 <listitem><para><filename>fs/lockd/host.c</filename>:<function>nlm_bind_host</function></para></listitem>
 258 <listitem><para><filename>fs/nfsd/auth.c</filename>:<function>nfsd_setuser</function></para></listitem>
 259 <listitem><para><filename>fs/proc/array.c</filename>:<function>task_cap</function></para></listitem>
 260 </itemizedlist>
 261 </para>
 262
 263 </sect1>
 264
 265 </article>