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).

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.