MAC(9) MidnightBSD Kernel Developer’s Manual MAC(9)
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.
MidnightBSD 0.3 July 10, 2006 MidnightBSD 0.3