X Access Control Extension Specification

Eamon F. Walsh

X Version 11, Release 7.7

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2009

Revision History
Revision 1.0      19 Oct 2006    efw
Initial Version
Revision 2.0      10 Mar 2008    efw
Version 2.0
Revision 2.1      19 Jun 2009    efw
Version 2.1 (XI2)
Revision 2.2      29 Jun 2009    efw
Version 2.2 (Property post-data hook)

The X Access Control Extension (XACE) is a set of generic "hooks" that can be
used by other X extensions to perform access checks. The goal of XACE is to
prevent clutter in the core dix/os code by providing a common mechanism for
doing these sorts of checks. The concept is identical to the Linux Security
Module (LSM) in the Linux Kernel.

XACE version 1.0 was a generalization of the SECURITY extension, which provides
a simple on/off trust model where "untrusted" clients are restricted in certain
areas. Its hooks were for the most part straight replacements of the old
SECURITY logic with generic hook calls. XACE version 2.0 has substantially
modified many of the hooks, adding additional parameters and many new access
types. Coverage has also been extended to many additional extensions, such as
Render and Composite. Finally, there is new support for polyinstantiation, or
duplicate, window properties and selections.

This paper describes the implementation of XACE version 2.0, changes to the
core server DIX and OS layers that have been made or are being considered, and
each of the security hooks that XACE offers at the current time and their
function. It is expected that changes to XACE be documented here. Please notify
the authors of this document of any changes to XACE so that they may be
properly documented.

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Table of Contents

Introduction
    Prerequisites
    Purpose
    Prior Work
    Version 2.0 Changes
    Future Work
Usage
    Storing Security State
    Using Hooks
Protocol
    Requests
    Events
    Errors

Introduction

Prerequisites

This document is targeted to programmers who are writing security extensions
for X. It is assumed that the reader is familiar with the C programming
language. It is assumed that the reader understands the general workings of the
X protocol and X server.

Purpose

XACE makes it easier to implement new security models for X by providing a set
of pluggable hooks that extension writers can use. The idea is to provide an
abstraction layer between security extensions and the core DIX/OS code of the X
server. This prevents security extensions writers from having to understand the
inner workings of the X server and it prevents X server maintainers from having
to deal with multiple security subsystems, each with its own intrusive code.

For example, consider the X.Org X server's resource subsystem, which is used to
track different types of server objects using ID numbers. The act of looking up
an object by its ID number is a security-relevant operation which security
extension writers would likely wish to control. For one or two security
extensions it may be acceptable to simply insert the extension's code directly
into the resource manager code, bracketed by ifdef's. However for more
extensions this approach leads to a tangle of code, particularly when results
need to be logically combined, as in if statement conditions. Additionally,
different extension writers might place their resource checking code in
different places in the server, leading to difficulty in tracking down where
exactly a particular lookup operation is being blocked. Finally, this approach
may lead to unexpected interactions between the code of different extensions,
since there is no collaboration between extension writers.

The solution employed by the X Access Control Extension is to place hooks
(calls into XACE) at security-relevant places, such as the resource subsystem
mentioned above. Other extensions, typically in their initialization routines,
can register callback functions on these hooks. When the hook is called from
the server code, each callback function registered on it is called in turn. The
callback function is provided with necessary arguments needed to make a
security decision, including a return value argument which can be set to
indicate the result. XACE itself does not make security decisions, or even know
or care how such decisions are made. XACE merely enforces the result of the
decision, such as by returning a BadAccess error to the requesting client.

This separation between the decision-making logic and the enforcement logic is
advantageous because it allows a great variety of security models to be
developed without resorting to intrusive modifications to the core systems
being secured. The challenge is to ensure that the hook framework itself
provides hooks everywhere they need to be provided. Once created, however, a
hook can be used by everyone, leading to less duplication of effort.

Prior Work

Security Extension

XACE was initially based on the SECURITY extension. This extension introduced
the concept of "trusted" and "untrusted" client connections, with the trust
level established by the authorization token used in the initial client
connection. Untrusted clients are restricted in several areas, notably in the
use of background "None" windows, access to server resources owned by trusted
clients, and certain keyboard input operations. Server extensions are also
declared "trusted" or "untrusted," with only untrusted extensions being visible
to untrusted client connections.

Solaris Trusted Extensions

Trusted Extensions for Solaris has an X extension (Xtsol) which adds security
functionality. Some of the XACE hooks in the current set were derived from
security checks made by the Xtsol code. In other places, where the Xtsol and
SECURITY extensions both have checks, a single XACE hook replaces both.

Linux Security Modules

XACE is influenced by the Linux Security Modules project, which provides a
similar framework of security hooks for the Linux kernel.

Version 2.0 Changes

Different Return-Value Semantics

The status value returned by security modules has been changed. Formerly,
security modules were expected to set the "rval" field of the input structure
to "False" if access was to be denied. In version 2.0, this field has been
removed in all hooks. Security modules must now set the "status" field to an X
error code to describe the error. Typically, BadAccess will be returned, but
this change allows security modules to return BadAlloc to report memory
allocation failure and BadMatch to report a polyinstantiated object lookup
failure (the section called “Polyinstantiation”).

DevPrivates Mechanism

The devPrivates mechanism in the X server was substantially revised to better
support security extensions. The interface for using devPrivates has been
unified across the different structures that support private data. Private
space allocation is now independent of whether objects have already been
created, and the private indexes are now global rather than being structure
specific. Callbacks are available to initialize newly allocated space and to
clean up before it is freed. Finally, there is a mechanism for looking up the
offset of the private pointer field in a structure, given the structure's
resource type.

New Access Modes

In the previous version, there were four possible modes for the "access_mode"
field: read, write, create, and destroy. In version 2.0, many new modes have
been introduced to better describe X operations, particularly on window
objects. The access_mode field has also been added to additional hooks as
described in the individual hook changes.

Polyinstantiation

XACE now supports polyinstantiation of selections and window properties. the
section called “Property Access” and the section called “Selection Access”
describe the details, but the basic idea is that the property and selection
access hooks may be used to not only change the return value of a lookup
operation but also to modify the lookup result. This allows more than one
property or selection with the same atom name to be maintained.

Removed Hooks

The "drawable," "map," "window init", and "background" hooks have been removed.
They have been folded into the resource access hook using new access modes. The
"hostlist" hook has been removed and replaced by a new server access hook (see
the section called “Server Access”). The "site policy" and "declare extension
security" hooks have been removed as the SECURITY extension has been revised to
no longer require them.

New Hooks

New "send" and "receive" hooks have been added to allow basic control over
event delivery. "Client" and "server" access hooks have been added to control
access by clients to other clients (for example, through the KillClient call)
and to the server (for example when changing the host access list or changing
the font path). "Screen" and "screen saver" hooks have been added to control
access to screens and screen saver requests. A "selection" hook has been added
to control access to selections.

Changes to Existing Hooks

  • The resource access hook structure now has additional fields to describe a
    "parent" object. They are set only when a resource with a defined parent
    (such as a Window object) is being created, in which case the access mode
    will include DixCreateAccess.

  • The device access hook structure has had the "fromRequest" field removed
    and an access mode field added.

  • The property access hook structure has had the "propertyName" field removed
    and a "ppProp" field added, which contains a pointer to a pointer to the
    property structure itself. The extra level of indirection supports
    polyinstantiation (see the section called “Polyinstantiation”). Note that
    the property structure contains the property name.

  • The extension dispatch/access hook structure now has an access mode field.

Future Work

Security Hooks

It is anticipated that the set of security hooks provided by XACE will change
with time. Some hooks may become deprecated. More hooks will likely be added as
well, as more portions of the X server are subjected to security analysis.
Existing hooks may be added in more places in the code, particularly protocol
extensions. Currently, the only method XACE provides for restricting access to
some protocol extensions is to deny access to them entirely.

It should be noted that XACE includes hooks in the protocol dispatch table,
which allow a security extension to examine any incoming protocol request (core
or extension) and terminate the request before it is handled by the server.
This functionality can be used as a stopgap measure for security checks that
are not supported by the other XACE hooks. The end goal, however, is to have
hooks integrated into the server proper.

Core X Server

The set of extensions supported by X.org needs to be re-examined. Many of them
are essentially unused and removing them would be easier than attempting to
secure them. The GLX extension and the direct rendering kernel interfaces need
to be secured.

The server's routines for event delivery need to be reworked to allow greater
control by XACE modules. In particular, security extensions need to be able to
associate private data with each event at the time of its generation based on
the context and then have that data available at a decision point just before
the event is delivered to the client. This would allow event delivery to be
better controlled on a per-client basis, and would potentially allow additional
security extension functionality such as piggyback events.

Usage

Storing Security State

The first thing you, the security extension writer, should decide on is the
state information that your extension will be storing and how it will be
stored. XACE itself does not provide any mechanism for storing state.

One method of storing state is global variables in the extension code. Tables
can be kept corresponding to internal server structures, updated to stay
synchronized with the structures themselves. One problem with this method is
that the X server does not have consistent methods for lifecycle management of
its objects, meaning that it might be difficult to keep state up to date with
objects.

Another method of storing state is to attach your extension's security data
directly to the server structures. This method is possible via the devPrivates
mechanism provide by the DIX layer. Structures supporting this mechanism can be
identified by the presence of a "devPrivates" field. Full documentation of the
devPrivates mechanism is described in the core X server documentation.

Using Hooks

Overview

XACE has two header files that security extension code may need to include.
Include Xext/xacestr.h if you need the structure definitions for the XACE hooks
in your source file. Otherwise, include Xext/xace.h, which contains everything
else including constants and function declarations.

XACE hooks use the standard X server callback mechanism. Your security
extension's callback functions should all use the following prototype:

    void MyCallback( CallbackListPtr *pcbl pointer userdata pointer calldata );

When the callback is called, pcbl points to the callback list itself. The X
callback mechanism allows the list to be manipulated in various ways, but XACE
callbacks should not do this. Remember that other security extensions may be
registered on the same hook. userdata is set to the data argument that was
passed to XaceRegisterCallback at registration time; this can be used by your
extension to pass data into the callback. calldata points to a value or
structure which is specific to each XACE hook. These are discussed in the
documentation for each specific hook, below. Your extension must cast this
argument to the appropriate pointer type.

To register a callback on a given hook, use XaceRegisterCallback:

    Bool XaceRegisterCallback( int hook CallbackProcPtr callback pointer
    userdata );

Where hook is the XACE hook you wish to register on, callback is the callback
function you wish to register, and userdata will be passed through to the
callback as its second argument, as described above. See Table 1, “XACE
security hooks.” for the list of XACE hook codes. XaceRegisterCallback is
typically called from the extension initialization code; see the SECURITY
source for examples. The return value is TRUE for success, FALSE otherwise.

To unregister a callback, use XaceDeleteCallback:

    Bool XaceDeleteCallback( int hook CallbackProcPtr callback pointer userdata
    );

where the three arguments are identical to those used in the call to
XaceRegisterCallback. The return value is TRUE for success, FALSE otherwise.

Hooks

The currently defined set of XACE hooks is shown in Table 1, “XACE security
hooks.”. As discussed in the section called “Security Hooks”, the set of hooks
is likely to change in the future as XACE is adopted and further security
analysis of the X server is performed.

Table 1. XACE security hooks.

┌───────────────────────┬──────────────────────┬──────────────────────────────┐
│Hook Identifier        │Callback Argument Type│Refer to                      │
├───────────────────────┼──────────────────────┼──────────────────────────────┤
│XACE_CORE_DISPATCH     │XaceCoreDispatchRec   │the section called “Core      │
│                       │                      │Dispatch”                     │
├───────────────────────┼──────────────────────┼──────────────────────────────┤
│XACE_EXT_DISPATCH      │XaceExtAccessRec      │the section called “Extension │
│                       │                      │Dispatch”                     │
├───────────────────────┼──────────────────────┼──────────────────────────────┤
│XACE_RESOURCE_ACCESS   │XaceResourceAccessRec │the section called “Resource  │
│                       │                      │Access”                       │
├───────────────────────┼──────────────────────┼──────────────────────────────┤
│XACE_DEVICE_ACCESS     │XaceDeviceAccessRec   │the section called “Device    │
│                       │                      │Access”                       │
├───────────────────────┼──────────────────────┼──────────────────────────────┤
│XACE_PROPERTY_ACCESS   │XacePropertyAccessRec │the section called “Property  │
│                       │                      │Access”                       │
├───────────────────────┼──────────────────────┼──────────────────────────────┤
│XACE_SEND_ACCESS       │XaceSendAccessRec     │the section called “Send      │
│                       │                      │Access”                       │
├───────────────────────┼──────────────────────┼──────────────────────────────┤
│XACE_RECEIVE_ACCESS    │XaceReceiveAccessRec  │the section called “Receive   │
│                       │                      │Access”                       │
├───────────────────────┼──────────────────────┼──────────────────────────────┤
│XACE_CLIENT_ACCESS     │XaceClientAccessRec   │the section called “Client    │
│                       │                      │Access”                       │
├───────────────────────┼──────────────────────┼──────────────────────────────┤
│XACE_EXT_ACCESS        │XaceExtAccessRec      │the section called “Extension │
│                       │                      │Access”                       │
├───────────────────────┼──────────────────────┼──────────────────────────────┤
│XACE_SERVER_ACCESS     │XaceServerAccessRec   │the section called “Server    │
│                       │                      │Access”                       │
├───────────────────────┼──────────────────────┼──────────────────────────────┤
│XACE_SELECTION_ACCESS  │XaceSelectionAccessRec│the section called “Selection │
│                       │                      │Access”                       │
├───────────────────────┼──────────────────────┼──────────────────────────────┤
│XACE_SCREEN_ACCESS     │XaceScreenAccessRec   │the section called “Screen    │
│                       │                      │Access”                       │
├───────────────────────┼──────────────────────┼──────────────────────────────┤
│XACE_SCREENSAVER_ACCESS│XaceScreenAccessRec   │the section called “Screen    │
│                       │                      │Saver Access”                 │
├───────────────────────┼──────────────────────┼──────────────────────────────┤
│                       │                      │the section called            │
│XACE_AUTH_AVAIL        │XaceAuthAvailRec      │“Authorization Availability   │
│                       │                      │Hook”                         │
├───────────────────────┼──────────────────────┼──────────────────────────────┤
│XACE_KEY_AVAIL         │XaceKeyAvailRec       │the section called “Keypress  │
│                       │                      │Availability Hook”            │
├───────────────────────┼──────────────────────┼──────────────────────────────┤
│XACE_AUDIT_BEGIN       │XaceAuditRec          │the section called “Auditing  │
│                       │                      │Hooks”                        │
├───────────────────────┼──────────────────────┼──────────────────────────────┤
│XACE_AUDIT_END         │XaceAuditRec          │the section called “Auditing  │
│                       │                      │Hooks”                        │
└───────────────────────┴──────────────────────┴──────────────────────────────┘


In the descriptions that follow, it is helpful to have a listing of Xext/
xacestr.h available for reference.

Core Dispatch

This hook allows security extensions to examine all incoming core protocol
requests before they are dispatched. The hook argument is a pointer to a
structure of type XaceCoreDispatchRec. This structure contains a client field
of type ClientPtr and a status field of type int.

The client field refers to the client making the incoming request. Note that
the complete request is accessible via the requestBuffer member of the client
structure. The REQUEST family of macros, located in include/dix.h, are useful
in verifying and reading from this member.

The status field may be set to a nonzero X protocol error code. In this event,
the request will not be processed further and the error code will be returned
to the client.

Extension Dispatch

This hook allows security extensions to examine all incoming extension protocol
requests before they are dispatched. The hook argument is a pointer to a
structure of type XaceExtAccessRec. This structure contains a client field of
type ClientPtr, a ext field of type ExtensionEntry*, a access_mode field of
type Mask, and a status field of type int.

The client field refers to the client making the incoming request. Note that
the complete request is accessible via the requestBuffer member of the client
structure. The REQUEST family of macros, located in include/dix.h, are useful
in verifying and reading from this member.

The ext field refers to the extension being accessed. This is required
information since extensions are not associated with any particular major
number.

The access_mode field is set to DixUseAccess when this hook is exercised.

The status field may be set to a nonzero X protocol error code. In this event,
the request will not be processed further and the error code will be returned
to the client.

Resource Access

This hook allows security extensions to monitor all resource lookups. The hook
argument is a pointer to a structure of type XaceResourceAccessRec. This
structure contains a client field of type ClientPtr, a id field of type XID, a
rtype field of type RESTYPE, a res field of type pointer, a ptype field of type
RESTYPE, a parent field of type pointer, a access_mode field of type Mask, and
a status field of type int.

The client field refers to the client on whose behalf the lookup is being
performed. Note that this may be serverClient for server lookups.

The id field is the resource ID being looked up.

The rtype field is the type of the resource being looked up.

The res field is the resource itself: the result of the lookup.

The ptype field is the type of the parent resource or RT_NONE if not set.

The parent field is the parent resource itself or NULL if not set. The parent
resource is set only when two conditions are met: The resource in question is
being created at the time of the call (in which case the access_mode will
include DixCreateAccess) and the resource in question has a defined parent
object. Table 3, “Resource access hook parent objects.” lists the resources
that support parent objects. The purpose of these two fields is to provide
generic support for "parent" resources.

The access_mode field encodes the type of action being performed. The valid
mode bits are defined in include/dixaccess.h. The meaning of the bits depends
on the specific resource type. Tables for some common types can be found in
Table 2, “Resource access hook access modes.”. Note that the DixCreateAccess
access mode has special meaning: it signifies that the resource object is in
the process of being created. This provides an opportunity for the security
extension to initialize its security label information in the structure
devPrivates or otherwise. If the status field is set to an error code in this
case, the resource creation will fail and no entry will be made under the
specified resource ID.

The status field may be set to a nonzero X protocol error code. In this event,
the resource lookup will fail and an error (usually, but not always, the status
value) will be returned to the client.

Table 2. Resource access hook access modes.

┌──────────────────┬──────────────────────────────┬───────────────────────────┐
│Access Mode Bit   │Meaning                       │Example Call Site          │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│                  │The primary data or contents  │                           │
│DixReadAccess     │of the object are being read  │GetImage, GetCursorImage,  │
│                  │(drawables, cursors,          │CreatePicture, QueryColors │
│                  │colormaps).                   │                           │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│                  │The primary data or contents  │PutImage, RenderTriFan,    │
│DixWriteAccess    │of the object are being       │ClearArea, StoreColors,    │
│                  │written (drawables, cursors,  │RecolorCursor              │
│                  │colormaps).                   │                           │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│                  │                              │CloseFont, DestroyWindow,  │
│DixDestroyAccess  │The object is being removed.  │FreePixmap, FreeCursor,    │
│                  │                              │RenderFreePicture          │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│DixCreateAccess   │The object is being created.  │CreateWindow, CreatePixmap,│
│                  │                              │CreateGC, CreateColormap   │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│                  │The object's attributes are   │GetWindowAttributes,       │
│DixGetAttrAccess  │being queried, or the object  │GetGeometry, QueryFont,    │
│                  │is being referenced.          │CopyGC, QueryBestSize      │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│                  │                              │SetWindowAttributes,       │
│DixSetAttrAccess  │The object's attributes are   │ChangeGC,                  │
│                  │being changed.                │SetClipRectangles,         │
│                  │                              │XFixesSetCursorName        │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│                  │User properties set on the    │                           │
│DixListPropAccess │object are being listed       │ListProperties             │
│                  │(windows).                    │                           │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│                  │A user property set on the    │GetProperty,               │
│DixGetPropAccess  │object is being read          │RotateProperties           │
│                  │(windows).                    │                           │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│                  │A user property set on the    │ChangeProperty,            │
│DixSetPropAccess  │object is being written       │RotateProperties,          │
│                  │(windows).                    │DeleteProperty             │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│                  │Child objects of the object   │QueryTree, MapSubwindows,  │
│DixListAccess     │are being listed out          │UnmapSubwindows            │
│                  │(windows).                    │                           │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│                  │A child object is being added │CreateWindow,              │
│DixAddAccess      │to the object (drawables,     │ReparentWindow, AllocColor,│
│                  │fonts, colormaps).            │RenderCreatePicture,       │
│                  │                              │RenderAddGlyphs            │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│                  │A child object is being       │DestroyWindow,             │
│DixRemoveAccess   │removed from object           │ReparentWindow, FreeColors,│
│                  │(drawables, fonts, colormaps).│RenderFreeGlyphs           │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│                  │Object is being unmapped or   │UnmapWindow,               │
│DixHideAccess     │hidden from view (drawables,  │XFixesHideCursor           │
│                  │cursor).                      │                           │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│DixShowAccess     │Object is being mapped or     │MapWindow, XFixesShowCursor│
│                  │shown (drawables, cursor).    │                           │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│                  │Drawable contents are being   │Background "None",         │
│DixBlendAccess    │mixed in a way that may       │CompositeRedirectWindow,   │
│                  │compromise contents.          │CompositeRedirectSubwindows│
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│DixGrabAccess     │Override-redirect bit on a    │CreateWindow,              │
│                  │window has been set.          │ChangeWindowAttributes     │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│DixInstallAccess  │Colormap is being installed.  │InstallColormap            │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│DixUninstallAccess│Colormap is being uninstalled.│UninstallColormap          │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│DixSendAccess     │An event is being sent to a   │SendEvent                  │
│                  │window.                       │                           │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│DixReceiveAccess  │A client is setting an event  │ChangeWindowAttributes,    │
│                  │mask on a window.             │XiSelectExtensionEvent     │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│                  │The object is being used      │CreateWindow, FillPoly,    │
│DixUseAccess      │without modifying it (fonts,  │GrabButton, ChangeGC       │
│                  │cursors, gc).                 │                           │
├──────────────────┼──────────────────────────────┼───────────────────────────┤
│                  │Window-manager type actions on│CirculateWindow,           │
│DixManageAccess   │a drawable.                   │ChangeSaveSet,             │
│                  │                              │ReparentWindow             │
└──────────────────┴──────────────────────────────┴───────────────────────────┘


Table 3. Resource access hook parent objects.

┌─────────────────┬───────────┬───────────────────────────────────────────────┐
│                 │Parent     │                                               │
│Resource Type    │Resource   │Notes                                          │
│                 │Type       │                                               │
├─────────────────┼───────────┼───────────────────────────────────────────────┤
│RT_WINDOW        │RT_WINDOW  │Contains the parent window. This will be NULL  │
│                 │           │for root windows.                              │
├─────────────────┼───────────┼───────────────────────────────────────────────┤
│RT_PIXMAP        │RT_WINDOW  │COMPOSITE extension only: the source window is │
│                 │           │passed as the parent for redirect pixmaps.     │
├─────────────────┼───────────┼───────────────────────────────────────────────┤
│RenderPictureType│RC_DRAWABLE│The source drawable is passed as the parent for│
│                 │           │Render picture objects.                        │
└─────────────────┴───────────┴───────────────────────────────────────────────┘


Device Access

This hook allows security extensions to restrict client actions on input
devices. The hook argument is a pointer to a structure of type 
XaceDeviceAccessRec. This structure contains a client field of type ClientPtr,
a dev field of type DeviceIntPtr, a access_mode field of type Mask, and a
status field of type int.

The client field refers to the client attempting to access the device
(keyboard). Note that this may be serverClient.

The dev field refers to the input device being accessed.

The access_mode field encodes the type of action being performed. The valid
mode bits are described in the table below.

The status field may be set to a nonzero X protocol error code. In this event,
the device operation will fail and an error (usually, but not always, the
status value) will be returned to the client.

Table 4. Device access hook access modes.

┌─────────────────┬─────────────────────────────┬─────────────────────────────┐
│Access Mode Bit  │Meaning                      │Example Call Site            │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│                 │Attributes of the device are │GetKeyboardMapping,          │
│DixGetAttrAccess │being queried.               │XiGetKeyboardControl,        │
│                 │                             │XkbGetDeviceInfo             │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│DixReadAccess    │The state of the device is   │QueryPointer, QueryKeymap,   │
│                 │being polled.                │XkbGetState                  │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│                 │The state of the device is   │WarpPointer, XTestFakeInput, │
│DixWriteAccess   │being programmatically       │XiSendExtensionEvent         │
│                 │manipulated.                 │                             │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│                 │Per-client device            │                             │
│DixSetAttrAccess │configuration is being       │XkbPerClientFlags            │
│                 │performed.                   │                             │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│                 │Global device configuration  │ChangeKeyboardMapping,       │
│DixManageAccess  │is being performed.          │XiChangeDeviceControl,       │
│                 │                             │XkbSetControls               │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│DixUseAccess     │The device is being opened or│XiOpenDevice, XkbSelectEvents│
│                 │referenced.                  │                             │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│DixGrabAccess    │The device is being grabbed. │GrabPointer, GrabButton,     │
│                 │                             │GrabKey                      │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│                 │The state of the device is   │                             │
│DixFreezeAccess  │being frozen by a synchronous│GrabKeyboard, GrabPointer    │
│                 │grab.                        │                             │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│DixForceAccess   │The device cursor is being   │GrabPointer, GrabButton      │
│                 │overridden by a grab.        │                             │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│DixGetFocusAccess│The device focus is being    │GetInputFocus,               │
│                 │retrieved.                   │XiGetDeviceFocus             │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│DixSetFocusAccess│The device focus is being    │SetInputFocus,               │
│                 │set.                         │XiSetDeviceFocus             │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│DixBellAccess    │The device bell is being     │Bell, XiDeviceBell           │
│                 │rung.                        │                             │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│DixCreateAccess  │The device object has been   │XIChangeDeviceHierarchy,     │
│                 │newly allocated.             │XIAddMaster                  │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│DixDestroyAccess │The device is being removed. │XIChangeDeviceHierarchy,     │
│                 │                             │XIRemoveMaster               │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│DixAddAccess     │A slave device is being      │XIChangeDeviceHierarchy,     │
│                 │attached to the device.      │XIChangeAttachment           │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│DixRemoveAccess  │A slave device is being      │XIChangeDeviceHierarchy,     │
│                 │unattached from the device.  │XIChangeAttachment           │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│DixListPropAccess│Properties set on the device │ListDeviceProperties,        │
│                 │are being listed.            │XIListProperties             │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│DixGetPropAccess │A property set on the device │GetDeviceProperty,           │
│                 │is being read.               │XIGetProperty                │
├─────────────────┼─────────────────────────────┼─────────────────────────────┤
│DixSetPropAccess │A property set on the device │SetDeviceProperty,           │
│                 │is being written.            │XISetProperty                │
└─────────────────┴─────────────────────────────┴─────────────────────────────┘


Property Access

This hook allows security extensions to monitor all property accesses and
additionally to support polyinstantiation if desired. The hook argument is a
pointer to a structure of type XacePropertyAccessRec. This structure contains a
client field of type ClientPtr, a pWin field of type WindowPtr, a ppProp field
of type PropertyPtr*, a access_mode field of type Mask, and a status field of
type int.

The client field refers to the client which is accessing the property. Note
that this may be serverClient for server lookups.

The pWin field is the window on which the property is being accessed.

The ppProp field is a double-indirect pointer to the PropertyRec structure
being accessed. The extra level of indirection supports property
polyinstantiation; see below. If your extension does not use the
polyinstantiation feature, simply dereference the pointer to obtain a 
PropertyPtr for the property

The access_mode field encodes the type of action being performed. The valid
mode bits are described in the table below.

The status field may be set to a nonzero X protocol error code. In this event,
the property request will not be processed further and the error code will be
returned to the client. However, the BadMatch code has special meaning; see
below.

Table 5. Property access hook mode bits.

┌────────────────┬───────────────────────────────────────────┬────────────────┐
│Access Mode Bit │Meaning                                    │Example Call    │
│                │                                           │Site            │
├────────────────┼───────────────────────────────────────────┼────────────────┤
│                │The property object has been newly         │                │
│DixCreateAccess │allocated (this bit will always occur in   │ChangeProperty  │
│                │conjunction with DixWriteAccess).          │                │
├────────────────┼───────────────────────────────────────────┼────────────────┤
│DixWriteAccess  │The property data is being completely      │ChangeProperty, │
│                │overwritten with new data.                 │RotateProperties│
├────────────────┼───────────────────────────────────────────┼────────────────┤
│DixBlendAccess  │The property data is being appended or     │ChangeProperty  │
│                │prepended to.                              │                │
├────────────────┼───────────────────────────────────────────┼────────────────┤
│DixReadAccess   │The property data is being read.           │GetProperty     │
├────────────────┼───────────────────────────────────────────┼────────────────┤
│DixDestroyAccess│The property data is being deleted.        │DeleteProperty  │
├────────────────┼───────────────────────────────────────────┼────────────────┤
│DixGetAttrAccess│Existence of the property is being         │ListProperties  │
│                │disclosed.                                 │                │
├────────────────┼───────────────────────────────────────────┼────────────────┤
│                │Post-write call reflecting new contents    │                │
│DixPostAccess   │(this bit will always occur in conjunction │ChangeProperty  │
│                │with DixWriteAccess).                      │                │
└────────────────┴───────────────────────────────────────────┴────────────────┘


New in XACE Version 2.0, this hook supports the polyinstantiation of
properties. This means that more than one property may exist having the same
name, and the security extension can control which property object is seen by
which client. To perform property polyinstantiation, your security extension
should take the following steps:

  • When a property is being created (DixCreateAccess), the security extension
    should label it appropriately based on the client that is creating it. In
    this case, the ppProp field should not be modified.

  • When a property is being looked up, the ppProp field will refer to the
    first structure in the linked list with the given name. The security
    extension may change the ppProp field to a different property structure by
    traversing the linked list (using the PropertyRec next field) to find an
    alternate structure with the same property name.

  • Alternately, when a property is being looked up, the status may be set to
    BadMatch which will cause the DIX layer to treat the property as not
    existing. This may result in an additional property object with the same
    name being created (in which case the hook will be called again with the
    create access mode).

New in XACE Version 2.2, this hook allows security extensions to verify the
contents of properties after the client has written them. On a property change,
the property access hook will be called twice. The first call is unchanged from
previous versions. The second call will have the DixPostAccess bit together
with DixWriteAccess and the ppProp property pointer will contain the new data.
Setting the status field to something other than Success will cause the
previous property contents to be restored and the client to receive the status
code as an error.

Note that in the case of property creation (when DixCreateAccess is set), the
ppProp field already reflects the new data. Hence security extensions wishing
to validate property data should check for either DixPostAccess or
DixCreateAccess in conjunction with DixWriteAccess. If your extension does not
need this feature, simply ignore calls with the DixPostAccess bit set.

Send Access

This hook allows security extensions to prevent devices and clients from
posting X events to a given window. The hook argument is a pointer to a
structure of type XaceSendAccessRec. This structure contains a client field of
type ClientPtr, a dev field of type DeviceIntPtr, a pWin field of type 
WindowPtr, a events field of type events, a count field of type int, and a
status field of type int.

The client field refers to the client attempting a SendEvent request or other
synthetic event generation to the given window. This field may be NULL if the
dev field is set.

The dev field refers to the device attempting to post an event which would be
delivered to the given window. This field may be NULL if the client field is
set.

The pWin field refers to the target window.

The events field refers to the events that are being sent.

The count field contains the number of events in the events array.

The status field may be set to a nonzero X protocol error code. In this event,
the events will be dropped on the floor instead of being delivered.

Warning

This hook does not currently cover all instances of event delivery.

Receive Access

This hook allows security extensions to prevent a client from receiving X
events that have been delivered to a given window. The hook argument is a
pointer to a structure of type XaceReceiveAccessRec. This structure contains a
client field of type ClientPtr, a pWin field of type WindowPtr, a events field
of type events, a count field of type int, and a status field of type int.

The client field refers to the client to which the event would be delivered.

The pWin field refers to the window where the event has been sent.

The events field refers to the events that are being sent.

The count field contains the number of events in the events array.

The status field may be set to a nonzero X protocol error code. In this event,
the events will not be delivered to the client.

Warning

This hook does not currently cover all instances of event delivery.

Client Access

This hook allows security extensions to prevent clients from manipulating other
clients directly. This hook applies to a small set of protocol requests such as
KillClient. The hook argument is a pointer to a structure of type 
XaceClientAccessRec. This structure contains a client field of type ClientPtr,
a target field of type ClientPtr, a access_mode field of type Mask, and a
status field of type int.

The client field refers to the client making the request.

The target field refers to the client being manipulated.

The access_mode field encodes the type of action being performed. The valid
mode bits are described in the table below.

The status field may be set to a nonzero X protocol error code. In this event,
the request will fail and an error (usually, but not always, the status value)
will be returned to the client.

Table 6. Client access hook mode bits.

┌────────────────┬───────────────────────────────────────────┬────────────────┐
│Access Mode Bit │Meaning                                    │Example Call    │
│                │                                           │Site            │
├────────────────┼───────────────────────────────────────────┼────────────────┤
│DixGetAttrAccess│Attributes of the client are being queried.│SyncGetPriority │
├────────────────┼───────────────────────────────────────────┼────────────────┤
│DixSetAttrAccess│Attributes of the client are being set.    │SyncSetPriority │
├────────────────┼───────────────────────────────────────────┼────────────────┤
│                │The client's close-down-mode (which affects│                │
│DixManageAccess │global server resource management) is being│SetCloseDownMode│
│                │set.                                       │                │
├────────────────┼───────────────────────────────────────────┼────────────────┤
│DixDestroyAccess│The client is being killed.                │KillClient      │
└────────────────┴───────────────────────────────────────────┴────────────────┘


Extension Access

This hook allows security extensions to approve or deny requests involving
which extensions are supported by the server. This allows control over which
extensions are visible. The hook argument is a pointer to a structure of type 
XaceExtAccessRec. This structure contains a client field of type ClientPtr, a
ext field of type ExtensionEntry*, a access_mode field of type Mask, and a
status field of type int.

The client field refers to the client making the incoming request, which is
typically QueryExtension or ListExtensions.

The ext field refers to the extension being accessed. This is required
information since extensions are not associated with any particular major
number.

The access_mode field is set to DixGetAttrAccess when this hook is exercised.

The status field may be set to a nonzero X protocol error code. In this event,
the extension will be reported as not supported (QueryExtensions) or omitted
from the returned list (ListExtensions).

Warning

If this hook is used, an extension dispatch hook should also be installed to
make sure that clients cannot circumvent the check by guessing the major
opcodes of extensions.

Server Access

This hook allows security extensions to approve or deny requests that affect
the X server itself. The hook argument is a pointer to a structure of type 
XaceServerAccessRec, which contains a client field of type ClientPtr, a
access_mode field of type Mask, and a status field of type int.

The client field refers to the client making the request.

The access_mode field encodes the type of action being performed. The valid
mode bits are described in the table below.

The status field may be set to a nonzero X protocol error code. In this event,
the request will fail and an error (usually, but not always, the status value)
will be returned to the client.

Table 7. Server access hook mode bits.

┌────────────────┬────────────────────────────────┬───────────────────────────┐
│Access Mode Bit │Meaning                         │Example Call Site          │
├────────────────┼────────────────────────────────┼───────────────────────────┤
│DixGetAttrAccess│Attributes of the server are    │GetFontPath                │
│                │being queried.                  │                           │
├────────────────┼────────────────────────────────┼───────────────────────────┤
│DixSetAttrAccess│Attributes of the server are    │SetFontPath                │
│                │being set.                      │                           │
├────────────────┼────────────────────────────────┼───────────────────────────┤
│DixManageAccess │Server management is being      │ChangeAccessControl,       │
│                │performed.                      │ListHosts                  │
├────────────────┼────────────────────────────────┼───────────────────────────┤
│DixGrabAccess   │A server grab is being          │GrabServer                 │
│                │performed.                      │                           │
├────────────────┼────────────────────────────────┼───────────────────────────┤
│DixReadAccess   │The server's actions are being  │Record, XEVIE extensions   │
│                │recorded.                       │                           │
├────────────────┼────────────────────────────────┼───────────────────────────┤
│DixDebugAccess  │Server debug facilities are     │XTest extension,           │
│                │being used.                     │XkbSetDebuggingFlags       │
└────────────────┴────────────────────────────────┴───────────────────────────┘


Selection Access

This hook allows security extensions to monitor all selection accesses and
additionally to support polyinstantiation if desired. The hook argument is a
pointer to a structure of type XaceSelectionAccessRec. This structure contains
a client field of type ClientPtr, a ppSel field of type Selection**, a
access_mode field of type Mask, and a status field of type int.

The client field refers to the client which is accessing the property. Note
that this may be serverClient for server lookups.

The ppSel field is a double-indirect pointer to the Selection structure being
accessed. The extra level of indirection supports selection polyinstantiation;
see below. If your extension does not use the polyinstantiation feature, simply
dereference the pointer to obtain a SelectionRec * for the selection.

The access_mode field encodes the type of action being performed. The valid
mode bits are described in the table below.

The status field may be set to a nonzero X protocol error code. In this event,
the property request will not be processed further and the error code will be
returned to the client. However, the BadMatch code has special meaning; see
below.

Table 8. Selection access hook mode bits.

┌────────────────┬──────────────────────────────────────────┬─────────────────┐
│Access Mode Bit │Meaning                                   │Example Call Site│
├────────────────┼──────────────────────────────────────────┼─────────────────┤
│                │The selection object has been newly       │                 │
│DixCreateAccess │allocated (this bit will always occur in  │SetSelectionOwner│
│                │conjunction with DixSetAttrAccess).       │                 │
├────────────────┼──────────────────────────────────────────┼─────────────────┤
│DixSetAttrAccess│The selection owner is being set.         │SetSelectionOwner│
├────────────────┼──────────────────────────────────────────┼─────────────────┤
│DixGetAttrAccess│The selection owner is being queried.     │GetSelectionOwner│
├────────────────┼──────────────────────────────────────────┼─────────────────┤
│DixReadAccess   │A convert operation is being requested on │ConvertSelection │
│                │the selection.                            │                 │
└────────────────┴──────────────────────────────────────────┴─────────────────┘


This hook supports the polyinstantiation of selections. This means that more
than one selection may exist having the same name, and the security extension
can control which selection object is seen by which client. To perform
selection polyinstantiation, your security extension should take the following
steps:

  • When selection ownership is being established (DixSetAttrAccess), the
    security extension should label it appropriately based on the client that
    is taking ownership. In this case, the ppSel field should not be modified.

  • When a selection is being looked up, the ppProp field will refer to the
    first structure in the linked list with the given name. The security
    extension may change the ppSel field to a different selection structure by
    traversing the linked list (using the Selection next field) to find an
    alternate structure with the same selection name.

  • Alternately, when a selection is being looked up, the status may be set to
    BadMatch which will cause the DIX layer to treat the selection as not
    existing. This may result in an additional selection object with the same
    name being created (in which case the hook will be called again with the
    create access mode).

Screen Access

This hook allows security extensions to approve or deny requests that
manipulate screen objects The hook argument is a pointer to a structure of type
XaceScreenAccessRec. This structure contains a client field of type ClientPtr,
a screen field of type ScreenPtr, a access_mode field of type Mask, and a
status field of type int.

The client field refers to the client making the request.

The screen field refers to the screen object being referenced.

The access_mode field encodes the type of action being performed. The valid
mode bits are described in the table below.

The status field may be set to a nonzero X protocol error code. In this event,
the request will not be processed further and the error code will be returned
to the client.

Table 9. Screen access hook mode bits.

┌────────────────┬──────────────────────────────────┬─────────────────────────┐
│Access Mode Bit │Meaning                           │Example Call Site        │
├────────────────┼──────────────────────────────────┼─────────────────────────┤
│DixGetAttrAccess│Attributes of the screen object   │ListInstalledColormaps,  │
│                │are being queried.                │QueryBestSize            │
├────────────────┼──────────────────────────────────┼─────────────────────────┤
│DixSetAttrAccess│Attributes of the screen object   │InstallColormap          │
│                │are being set.                    │                         │
├────────────────┼──────────────────────────────────┼─────────────────────────┤
│DixHideAccess   │The cursor on the screen is being │XFixesHideCursor         │
│                │globally hidden.                  │                         │
├────────────────┼──────────────────────────────────┼─────────────────────────┤
│DixShowAccess   │The cursor on the screen is being │XFixesShowCursor         │
│                │globally unhidden.                │                         │
└────────────────┴──────────────────────────────────┴─────────────────────────┘


Screen Saver Access

This hook allows security extensions to approve or deny requests that
manipulate the screensaver. The hook argument is a pointer to a structure of
type XaceScreenAccessRec. This structure contains a client field of type 
ClientPtr, a screen field of type ScreenPtr, a access_mode field of type Mask,
and a status field of type int.

The client field refers to the client making the request.

The screen field refers to the screen object being referenced.

The access_mode field encodes the type of action being performed. The valid
mode bits are described in the table below.

The status field may be set to a nonzero X protocol error code. In this event,
the request will not be processed further and the error code will be returned
to the client.

Table 10. Screen saver access hook mode bits.

┌────────────────┬───────────────────────────────────┬────────────────────────┐
│Access Mode Bit │Meaning                            │Example Call Site       │
├────────────────┼───────────────────────────────────┼────────────────────────┤
│DixGetAttrAccess│Attributes of the screen saver are │GetScreenSaver,         │
│                │being queried.                     │ScreenSaverQueryInfo    │
├────────────────┼───────────────────────────────────┼────────────────────────┤
│DixSetAttrAccess│Attributes of the screen saver are │SetScreenSaver,         │
│                │being set.                         │ScreenSaverSelectInput  │
├────────────────┼───────────────────────────────────┼────────────────────────┤
│DixHideAccess   │The screen saver is being          │ForceScreenSaver,       │
│                │programmatically activated.        │DPMSEnable              │
├────────────────┼───────────────────────────────────┼────────────────────────┤
│DixShowAccess   │The screen saver is being          │ForceScreenSaver,       │
│                │programmatically deactivated.      │DPMSDisable             │
└────────────────┴───────────────────────────────────┴────────────────────────┘


Authorization Availability Hook

This hook allows security extensions to examine the authorization associated
with a newly connected client. This can be used to set up client security state
depending on the authorization method that was used. The hook argument is a
pointer to a structure of type XaceAuthAvailRec. This structure contains a
client field of type ClientPtr, and a authId field of type XID.

The client field refers to the newly connected client.

The authId field is the resource ID of the client's authorization.

This hook has no return value.

Note

This hook is called after the client enters the initial state and before the
client enters the running state. Keep this in mind if your security extension
uses the ClientStateCallback list to keep track of clients.

This hook is a legacy of the APPGROUP Extension. In the future, this hook may
be phased out in favor of a new client state, ClientStateAuthenticated.

Keypress Availability Hook

This hook allows security extensions to examine keypresses outside of the
normal event mechanism. This could be used to implement server-side hotkey
support. The hook argument is a pointer to a structure of type XaceKeyAvailRec.
This structure contains a event field of type xEventPtr, a keybd field of type 
DeviceIntPtr, and a count field of type int.

The event field refers to the keyboard event, typically a KeyPress or
KeyRelease.

The keybd field refers to the input device that generated the event.

The count field is the number of repetitions of the event (not 100\% sure of
this at present, however).

This hook has no return value.

Auditing Hooks

Two hooks provide basic auditing support. The begin hook is called immediately
before an incoming client request is dispatched and before the dispatch hook is
called (refer to the section called “Core Dispatch”). The end hook is called
immediately after the processing of the request has finished. The hook argument
is a pointer to a structure of type XaceKeyAvailRec. This structure contains a
client field of type ClientPtr, and a requestResult field of type int.

The client field refers to client making the request.

The requestResult field contains the result of the request, either Success or
one of the protocol error codes. Note that this field is significant only in
the end hook.

These hooks have no return value.

Protocol

Requests

XACE does not define any X protocol.

Events

XACE does not define any X protocol.

Errors

XACE does not define any X protocol.

