2Linux Security Modules: General Security Hooks for Linux
   5:Author: Stephen Smalley
   6:Author: Timothy Fraser
   7:Author: Chris Vance
   9.. note::
  11   The APIs described in this book are outdated.
  16In March 2001, the National Security Agency (NSA) gave a presentation
  17about Security-Enhanced Linux (SELinux) at the 2.5 Linux Kernel Summit.
  18SELinux is an implementation of flexible and fine-grained
  19nondiscretionary access controls in the Linux kernel, originally
  20implemented as its own particular kernel patch. Several other security
  21projects (e.g. RSBAC, Medusa) have also developed flexible access
  22control architectures for the Linux kernel, and various projects have
  23developed particular access control models for Linux (e.g. LIDS, DTE,
  24SubDomain). Each project has developed and maintained its own kernel
  25patch to support its security needs.
  27In response to the NSA presentation, Linus Torvalds made a set of
  28remarks that described a security framework he would be willing to
  29consider for inclusion in the mainstream Linux kernel. He described a
  30general framework that would provide a set of security hooks to control
  31operations on kernel objects and a set of opaque security fields in
  32kernel data structures for maintaining security attributes. This
  33framework could then be used by loadable kernel modules to implement any
  34desired model of security. Linus also suggested the possibility of
  35migrating the Linux capabilities code into such a module.
  37The Linux Security Modules (LSM) project was started by WireX to develop
  38such a framework. LSM was a joint development effort by several security
  39projects, including Immunix, SELinux, SGI and Janus, and several
  40individuals, including Greg Kroah-Hartman and James Morris, to develop a
  41Linux kernel patch that implements this framework. The work was
  42incorporated in the mainstream in December of 2003. This technical
  43report provides an overview of the framework and the capabilities
  44security module.
  46LSM Framework
  49The LSM framework provides a general kernel framework to support
  50security modules. In particular, the LSM framework is primarily focused
  51on supporting access control modules, although future development is
  52likely to address other security needs such as sandboxing. By itself, the
  53framework does not provide any additional security; it merely provides
  54the infrastructure to support security modules. The LSM framework is
  55optional, requiring `CONFIG_SECURITY` to be enabled. The capabilities
  56logic is implemented as a security module.
  57This capabilities module is discussed further in
  58`LSM Capabilities Module`_.
  60The LSM framework includes security fields in kernel data structures and
  61calls to hook functions at critical points in the kernel code to
  62manage the security fields and to perform access control.
  63It also adds functions for registering security modules.
  64An interface `/sys/kernel/security/lsm` reports a comma separated list
  65of security modules that are active on the system.
  67The LSM security fields are simply ``void*`` pointers.
  68The data is referred to as a blob, which may be managed by
  69the framework or by the individual security modules that use it.
  70Security blobs that are used by more than one security module are
  71typically managed by the framework.
  72For process and
  73program execution security information, security fields are included in
  74:c:type:`struct task_struct <task_struct>` and
  75:c:type:`struct cred <cred>`.
  76For filesystem
  77security information, a security field is included in :c:type:`struct
  78super_block <super_block>`. For pipe, file, and socket security
  79information, security fields are included in :c:type:`struct inode
  80<inode>` and :c:type:`struct file <file>`.
  81For System V IPC security information,
  82security fields were added to :c:type:`struct kern_ipc_perm
  83<kern_ipc_perm>` and :c:type:`struct msg_msg
  84<msg_msg>`; additionally, the definitions for :c:type:`struct
  85msg_msg <msg_msg>`, struct msg_queue, and struct shmid_kernel
  86were moved to header files (``include/linux/msg.h`` and
  87``include/linux/shm.h`` as appropriate) to allow the security modules to
  88use these definitions.
  90For packet and
  91network device security information, security fields were added to
  92:c:type:`struct sk_buff <sk_buff>` and
  93:c:type:`struct scm_cookie <scm_cookie>`.
  94Unlike the other security module data, the data used here is a
  9532-bit integer. The security modules are required to map or otherwise
  96associate these values with real security attributes.
  98LSM hooks are maintained in lists. A list is maintained for each
  99hook, and the hooks are called in the order specified by CONFIG_LSM.
 100Detailed documentation for each hook is
 101included in the `include/linux/lsm_hooks.h` header file.
 103The LSM framework provides for a close approximation of
 104general security module stacking. It defines
 105security_add_hooks() to which each security module passes a
 106:c:type:`struct security_hooks_list <security_hooks_list>`,
 107which are added to the lists.
 108The LSM framework does not provide a mechanism for removing hooks that
 109have been registered. The SELinux security module has implemented
 110a way to remove itself, however the feature has been deprecated.
 112The hooks can be viewed as falling into two major
 113categories: hooks that are used to manage the security fields and hooks
 114that are used to perform access control. Examples of the first category
 115of hooks include the security_inode_alloc() and security_inode_free()
 116These hooks are used to allocate
 117and free security structures for inode objects.
 118An example of the second category of hooks
 119is the security_inode_permission() hook.
 120This hook checks permission when accessing an inode.
 122LSM Capabilities Module
 125The POSIX.1e capabilities logic is maintained as a security module
 126stored in the file ``security/commoncap.c``. The capabilities
 127module uses the order field of the :c:type:`lsm_info` description
 128to identify it as the first security module to be registered.
 129The capabilities security module does not use the general security
 130blobs, unlike other modules. The reasons are historical and are
 131based on overhead, complexity and performance concerns.