Linux Security Modules: General Security Hooks for Linux¶
The APIs described in this book are outdated.
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.
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.
The Linux Security Modules (LSM) project was started by WireX to develop such a framework. LSM was 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 work was incorporated in the mainstream in December of 2003. This technical report provides an overview of the framework and the capabilities security module.
The LSM framework 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 sandboxing. By itself, the framework does not provide any additional security; it merely provides the infrastructure to support security modules. The LSM framework is optional, requiring CONFIG_SECURITY to be enabled. The capabilities logic is implemented as a security module. This capabilities module is discussed further in LSM Capabilities Module.
The LSM framework includes security fields in kernel data structures and 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 security modules. An interface /sys/kernel/security/lsm reports a comma separated list of security modules that are active on the system.
The LSM security fields are simply
The data is referred to as a blob, which may be managed by
the framework or by the individual security modules that use it.
Security blobs that are used by more than one security module are
typically managed by the framework.
For process and
program execution security information, security fields are included in
struct task_struct and
security information, a security field is included in
super_block. For pipe, file, and socket security
information, security fields are included in
struct inode and
For System V IPC security information,
security fields were added to
struct kern_ipc_perm and
struct msg_msg; additionally, the definitions for
msg_msg, struct msg_queue, and struct shmid_kernel
were moved to header files (
include/linux/shm.h as appropriate) to allow the security modules to
use these definitions.
For packet and
network device security information, security fields were added to
struct sk_buff and
Unlike the other security module data, the data used here is a
32-bit integer. The security modules are required to map or otherwise
associate these values with real security attributes.
LSM hooks are maintained in lists. A list is maintained for each hook, and the hooks are called in the order specified by CONFIG_LSM. Detailed documentation for each hook is included in the include/linux/lsm_hooks.h header file.
The LSM framework provides for a close approximation of
general security module stacking. It defines
security_add_hooks() to which each security module passes a
which are added to the lists.
The LSM framework does not provide a mechanism for removing hooks that
have been registered. The SELinux security module has implemented
a way to remove itself, however the feature has been deprecated.
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 security_inode_alloc() and security_inode_free() These hooks are used to allocate and free security structures for inode objects. An example of the second category of hooks is the security_inode_permission() hook. This hook checks permission when accessing an inode.
LSM Capabilities Module¶
The POSIX.1e capabilities logic is maintained as a security module
stored in the file
security/commoncap.c. The capabilities
module uses the order field of the
to identify it as the first security module to be registered.
The capabilities security module does not use the general security
blobs, unlike other modules. The reasons are historical and are
based on overhead, complexity and performance concerns.