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| -rw-r--r-- | Documentation/security/lsm.rst | 202 |
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diff --git a/Documentation/security/lsm.rst b/Documentation/security/lsm.rst index aadf47c808c0..6a2a2e973080 100644 --- a/Documentation/security/lsm.rst +++ b/Documentation/security/lsm.rst @@ -35,47 +35,50 @@ 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 is a joint development effort by several security +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 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. +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. LSM Framework ============= -The LSM kernel patch provides a general kernel framework to support +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 auditing. By itself, the +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 kernel patch -also moves most of the capabilities logic into an optional security -module, with the system defaulting to the traditional superuser logic. +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 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 :c:func:`security()` system call to support new system calls -for security-aware applications. - -The LSM security fields are simply ``void*`` pointers. For process and -program execution security information, security fields were added to +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 ``void*`` pointers. +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 :c:type:`struct task_struct <task_struct>` and -:c:type:`struct linux_binprm <linux_binprm>`. For filesystem -security information, a security field was added to :c:type:`struct +:c:type:`struct cred <cred>`. +For filesystem +security information, a security field is included in :c:type:`struct super_block <super_block>`. For pipe, file, and socket security -information, security fields were added to :c:type:`struct inode -<inode>` and :c:type:`struct file <file>`. For packet and -network device security information, security fields were added to -:c:type:`struct sk_buff <sk_buff>` and :c:type:`struct -net_device <net_device>`. For System V IPC security information, +information, security fields are included in :c:type:`struct inode +<inode>` and :c:type:`struct file <file>`. +For System V IPC security information, security fields were added to :c:type:`struct kern_ipc_perm <kern_ipc_perm>` and :c:type:`struct msg_msg <msg_msg>`; additionally, the definitions for :c:type:`struct @@ -84,118 +87,45 @@ were moved to header files (``include/linux/msg.h`` and ``include/linux/shm.h`` as appropriate) to allow the security modules to use these definitions. -Each LSM hook is a function pointer in a global table, security_ops. -This table is a :c:type:`struct security_operations -<security_operations>` structure as defined by -``include/linux/security.h``. 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->". - -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. - -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 :c:func:`register_security()` function (in -``security/security.c``) is provided to allow a security module to set -security_ops to refer to its own hook functions, and an -:c:func:`unregister_security()` 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. - -LSM also provides a simple mechanism for stacking additional security -modules with the primary security module. It defines -:c:func:`register_security()` and -:c:func:`unregister_security()` hooks in the :c:type:`struct -security_operations <security_operations>` structure and -provides :c:func:`mod_reg_security()` and -:c:func:`mod_unreg_security()` 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. - -Although the LSM hooks are organized into substructures based on kernel -object, all of the hooks can be viewed as falling into two major +For packet and +network device security information, security fields were added to +:c:type:`struct sk_buff <sk_buff>` and +:c:type:`struct scm_cookie <scm_cookie>`. +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 +:c:type:`struct security_hooks_list <security_hooks_list>`, +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 :c:func:`alloc_security()` and -:c:func:`free_security()` 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 :c:func:`post_lookup()` hook in -:c:type:`struct inode_security_ops <inode_security_ops>`. -This hook is used to set security information for inodes after -successful lookup operations. An example of the second category of hooks -is the :c:func:`permission()` hook in :c:type:`struct -inode_security_ops <inode_security_ops>`. This hook checks -permission when accessing an inode. +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 LSM kernel patch moves most of the existing POSIX.1e capabilities -logic into an optional security module stored in the file -``security/capability.c``. 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. - -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. - -At present, the capabilities logic for computing process capabilities on -:c:func:`execve()` and :c:func:`set\*uid()`, checking -capabilities for a particular process, saving and checking capabilities -for netlink messages, and handling the :c:func:`capget()` and -:c:func:`capset()` 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: - -- ``fs/open.c``::c:func:`sys_access()` - -- ``fs/lockd/host.c``::c:func:`nlm_bind_host()` - -- ``fs/nfsd/auth.c``::c:func:`nfsd_setuser()` - -- ``fs/proc/array.c``::c:func:`task_cap()` +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 :c:type:`lsm_info` description +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. |