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NAME

     mac - TrustedBSD Mandatory Access Control framework

SYNOPSIS

     #include <sys/types.h>
     #include <sys/mac.h>

     In the kernel configuration file:
     options MAC
     options MAC_DEBUG

DESCRIPTION

   Introduction
     The TrustedBSD mandatory access control framework permits dynamically
     introduced system security modules to modify system security
     functionality.  This can be used to support a variety of new security
     services, including traditional labeled mandatory access control models.
     The framework provides a series of entry points which must be called by
     code supporting various kernel services, especially with respects to
     access control points and object creation.  The framework then calls out
     to security modules to offer them the opportunity to modify security
     behavior at those MAC API entry points.  Both consumers of the API
     (normal kernel services) and security modules must be aware of the
     semantics of the API calls, particularly with respect to synchronization
     primitives (such as locking).

   Note on Appropriateness for Production Use
     The TrustedBSD MAC Framework included in FreeBSD 5.0 is considered
     experimental, and should not be deployed in production environments
     without careful consideration of the risks associated with the use of
     experimental operating system features.

   Kernel Objects Supported by the Framework
     The MAC framework manages labels on a variety of types of in-kernel
     objects, including process credentials, vnodes, devfs_dirents, mount
     points, sockets, mbufs, bpf descriptors, network interfaces, IP fragment
     queues, and pipes.  Label data on kernel objects, represented by struct
     label, is policy-unaware, and may be used in the manner seen fit by
     policy modules.

   API for Consumers
     The MAC API provides a large set of entry points, too broad to
     specifically document here.  In general, these entry points represent an
     access control check or other MAC-relevant operations, accept one or more
     subjects (credentials) authorizing the activity, a set of objects on
     which the operation is to be performed, and a set of operation arguments
     providing information about the type of operation being requested.

   Locking for Consumers
     Consumers of the MAC API must be aware of the locking requirements for
     each API entry point: generally, appropriate locks must be held over each
     subject or object being passed into the call, so that MAC modules may
     make use of various aspects of the object for access control purposes.
     For example, vnode locks are frequently required in order that the MAC
     framework and modules may retrieve security labels and attributes from
     the vnodes for the purposes of access control.  Similarly, the caller
     must be aware of the reference counting semantics of any subject or
     object passed into the MAC API: all calls require that a valid reference
     to the object be held for the duration of the (potentially lengthy) MAC
     API call.  Under some circumstances, objects must be held in either a
     shared or exclusive manner.

   API for Module Writers
     Each module exports a structure describing the MAC API operations that
     the module chooses to implement, including initialization and destruction
     API entry points, a variety of object creation and destruction calls, and
     a large set of access control check points.  In the future, additional
     audit entry points will also be present.  Module authors may choose to
     only implement a subset of the entry points, setting API function
     pointers in the description structure to NULL, permitting the framework
     to avoid calling into the module.

   Locking for Module Writers
     Module writers must be aware of the locking semantics of entry points
     that they implement: MAC API entry points will have specific locking or
     reference counting semantics for each argument, and modules must follow
     the locking and reference counting protocol or risk a variety of failure
     modes (including race conditions, inappropriate pointer dereferences,
     etc).

     MAC module writers must also be aware that MAC API entry points will
     frequently be invoked from deep in a kernel stack, and as such must be
     careful to avoid violating more global locking requirements, such as
     global lock order requirements.  For example, it may be inappropriate to
     lock additional objects not specifically maintained and ordered by the
     policy module, or the policy module might violate a global ordering
     requirement relating to those additional objects.

     Finally, MAC API module implementors must be careful to avoid
     inappropriately calling back into the MAC framework: the framework makes
     use of locking to prevent inconsistencies during policy module attachment
     and detachment.  MAC API modules should avoid producing scenarios in
     which deadlocks or inconsistencies might occur.

   Adding New MAC Entry Points
     The MAC API is intended to be easily expandable as new services are added
     to the kernel.  In order that policies may be guaranteed the opportunity
     to ubiquitously protect system subjects and objects, it is important that
     kernel developers maintain awareness of when security checks or relevant
     subject or object operations occur in newly written or modified kernel
     code.  New entry points must be carefully documented so as to prevent any
     confusion regarding lock orders and semantics.  Introducing new entry
     points requires four distinct pieces of work: introducing new MAC API
     entries reflecting the operation arguments, scattering these MAC API
     entry points throughout the new or modified kernel service, extending the
     front-end implementation of the MAC API framework, and modifying
     appropriate modules to take advantage of the new entry points so that
     they may consistently enforce their policies.

ENTRY POINTS

     System service and module authors should reference the FreeBSD
     Architecture Handbook for information on the MAC Framework APIs.

SEE ALSO

     acl(3), mac(3), posix1e(3), mac_biba(4), mac_bsdextended(4),
     mac_ifoff(4), mac_lomac(4), mac_mls(4), mac_none(4), mac_partition(4),
     mac_seeotheruids(4), mac_test(4), ucred(9), vaccess(9),
     vaccess_acl_posix1e(9), VFS(9)

     The FreeBSD Architecture Handbook,
     http://www.FreeBSD.org/doc/en_US.ISO8859-1/books/arch-handbook/.

HISTORY

     The TrustedBSD MAC Framework first appeared in FreeBSD 5.0.

AUTHORS

     This manual page was written by Robert Watson.  This software was
     contributed to the FreeBSD Project by Network Associates Laboratories,
     the Security Research Division of Network Associates Inc. under
     DARPA/SPAWAR contract N66001-01-C-8035 (“CBOSS”), as part of the DARPA
     CHATS research program.

     The TrustedBSD MAC Framework was designed by Robert Watson, and
     implemented by the Network Associates Laboratories Network Security
     (NETSEC), Secure Execution Environment (SEE), and Adaptive Network
     Defense research groups.  Network Associates Laboratory staff
     contributing to the CBOSS Project include (in alphabetical order): Lee
     Badger, Brian Feldman, Hrishikesh Dandekar, Tim Fraser, Doug Kilpatrick,
     Suresh Krishnaswamy, Adam Migus, Wayne Morrison, Andrew Reisse, Chris
     Vance, and Robert Watson.

     Sub-contracted staff include: Chris Costello, Poul-Henning Kamp, Jonathan
     Lemon, Kirk McKusick, Dag-Erling Smørgrav.

     Additional contributors include: Pawel Dawidek, Chris Faulhaber, Ilmar
     Habibulin, Mike Halderman, Bosko Milekic, Thomas Moestl, Andrew Reiter,
     and Tim Robbins.

BUGS

     See the earlier section in this document concerning appropriateness for
     production use.  The TrustedBSD MAC Framework is considered experimental
     in FreeBSD.

     While the MAC Framework design is intended to support the containment of
     the root user, not all attack channels are currently protected by entry
     point checks.  As such, MAC Framework policies should not be relied on,
     in isolation, to protect against a malicious privileged user.