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diff --git a/Documentation/DocBook/lsm.tmpl b/Documentation/DocBook/lsm.tmpl new file mode 100644 index 000000000..fe7664ce9 --- /dev/null +++ b/Documentation/DocBook/lsm.tmpl @@ -0,0 +1,265 @@ +<?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> + +<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> |