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1 <?xml version="1.0" encoding="UTF-8"?> 2 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN" 3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" > 4 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>firstname.lastname@example.org</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>email@example.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>firstname.lastname@example.org</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>