Access Control in PHP5 CMS - Part 2

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by Martin Brampton | January 2009 | MySQL Content Management Open Source PHP

In Part 1, we had a look at the Discussion and Considerations of highly flexible role-based access control system (RBAC). In this part of the article by Martin Brampton, we will look at the database implementation. Also we will discuss the code for administering RBAC, and consider in outline how questions about access can be answered.

Framework Solution

The implementation of access control falls into three classes. One is the class that is asked questions about who can do what. Closely associated with this is another class that caches general information applicable to all users. It is made a separate class to aid implementation of the split of cache between general and user specific. The third class handles administration operations. Before looking at the classes, though, let's figure out the database design.

Database for RBAC

All that is required to implement basic RBAC is two tables. A third table is required to extend to a hierarchical model. An optional extra table can be implemented to hold role descriptions. Thinking back to the design considerations, the first need is for a way to record the operations that can be done on the subjects, that is the permissions. They are the targets for our access control system. You'll recall that a permission consists of an action and a subject, where a subject is defined by a type, and an identifier. For ease of handling, a simple auto-increment ID number is added. But we also need a couple of other things.

To make our RBAC system general, it is important to be able to control not only the actual permissions, but also who can grant those permissions, and whether they can grant that right to others. So an extra control field is added with one bit for each of those three possibilities. It therefore becomes possible to grant the right to access something with or without the ability to pass on that right.

The other extra data item that is useful is a "system" flag. It is used to make some permissions incapable of deletion. Although not being a logical requirement, this is certainly a practical requirement. We want to give administrators a lot of power over the configuration of access rights, but at the same time, we want to avoid any catastrophes. The sort of thing that would be highly undesirable would be for the top level administrator to remove all of their own rights to the system. In practice, most systems will have a critical central structure of rights, which should not be altered even by the highest administrator.

So now the permissions table can be seen to be as shown in the following screenshot:

Access Control in PHP5 CMS - Part 2

Note that the character strings for role, action, and subject_type are given generous lengths of 60, which should be more than adequate. The subject ID will often be quite short, but to avoid constraining generality, it is made a text field, so that the RBAC system can still handle very complex identifiers, if required. Of course, there will be some performance penalties if this field is very long, but it is better to have a design trade-off than a limitation. If we restricted the subject ID to being a number, then more complex identifiers would be a special case. This would destroy the generality of our scheme, and might ultimately reduce overall efficiency. In addition to the auto-increment primary key ID, two indices are created, as shown in the following screenshot. They involve overhead during update operations but are likely to speed access operations. Since far more accesses will typically be made than updates, this makes sense. If for some reason an index does not give a benefit, it is always possible to drop it. Note that the index on the subject ID has to be constrained in length to avoid breaking limits on key size. The value chosen is a compromise between efficiency through short keys, and efficiency through the use of fine grained keys. In a heavily used system, it would be worth reviewing the chosen figure carefully, and perhaps modifying it in the light of studies into actual data.

Access Control in PHP5 CMS - Part 2

The other main database table is even simpler, and holds information about assignment of accessors to roles. Again, an auto-increment ID is added for convenience. Apart from the ID, the only fields required are the role, the accessor type, and the accessor ID. This time a single index, additional to the primary key, is sufficient. The assignment table is shown in the following screenshot, and its index is shown in the screenshot after that:

Access Control in PHP5 CMS - Part 2

Access Control in PHP5 CMS - Part 2

Adding hierarchy to RBAC requires only a very simple table, where each row contains two fields: a role, and an implied role. Both fields constitute the primary key, neither field on its own being necessarily unique. An index is not required for efficiency, since the volume of hierarchy information is assumed to be small, and whenever it is needed, the whole table is read. But it is still a good principle to have a primary key, and it also guarantees that there will not be redundant entries. For the example given earlier, a typical entry might have consultant as the role, and doctor as the implied role. At present, Aliro implements hierarchy only for backwards compatibility, but it is a relatively easy development to make hierarchical relationships generally available.

Optionally, an extra table can be used to hold a description of the roles in use. This has no functional purpose, and is simply an option to aid administrators of the system. The table should have the role as its primary key. As it does not affect the functionality of the RBAC at all, no further detail is given here.

With the database design settled, let's look at the classes. The simplest is the administration class, so we'll start there.

Administering RBAC

The administration of the system could be done by writing directly to the database, since that is what most of the operations involve. There are strong reasons not to do so. Although the operations are simple, it is vital that they be handled correctly. It is generally a poor principle to allow access to the mechanisms of a system rather than providing an interface through class methods. The latter approach ideally allows the creation of a robust interface that changes relatively infrequently, while details of implementation can be modified without affecting the rest of the system.

The administration class is kept separate from the classes handling questions about access because for most CMS requests, administration will not be needed, and the administration class will not load at all. As a central service, the class is implemented as a standard singleton, but it is not cached because information generally needs to be written immediately to the database. In fact, the administration class frequently requests the authorization cache class to clear its cache so that the changes in the database can be effective immediately. The class starts off:

class aliroAuthorisationAdmin 
{
private static $instance = __CLASS__;
private $handler = null;
private $authoriser = null;
private $database = null;
private function __construct()
{
$this->handler =& aliroAuthoriserCache::getInstance();
$this->authoriser =& aliroAuthoriser::getInstance();
$this->database = aliroCoreDatabase::getInstance();
}
private function __clone()
{
// Enforce singleton
}
public static function getInstance()
{
return is_object(self::$instance) ? self::$instance : (self::$instance = new self::$instance());
}
private function doSQL($sql, $clear=false)
{
$this->database->doSQL($sql);
if ($clear) $this->clearCache();
}
private function clearCache()
{
$this->handler->clearCache();
}

Apart from the instance property that is used to implement the singleton pattern, the other private properties are related objects that are acquired in the constructor to help other methods. Getting an instance operates in the usual fashion for a singleton, with the private constructor, and clone methods enforcing access solely via getInstance.

The doSQL method also simplifies other methods by combining a call to the database with an optional clearing of cache through the class's clearCache method. Clearly the latter is simple enough that it could be eliminated. But it is better to have the method in place so that if changes were made to the implementation such that different actions were needed when any relevant cache is to be cleared, the changes would be isolated to the clearCache method. Next we have a couple of useful methods that simply refer to one of the other RBAC classes:

public function getAllRoles($addSpecial=false) 
{
return $this->authoriser->getAllRoles($addSpecial);
}
public function getTranslatedRole($role)
{
return $this->authoriser->getTranslatedRole($role);
}

Again, these are provided so as to simplify the future evolution of the code so that implementation details are concentrated in easily identified locations. The general idea of getAllRoles is obvious from the name, and the parameter determines whether the special roles such as visitor, registered, and nobody will be included. Since those roles are built into the system in English, it would be useful to be able to get local translations for them. So the method getTranslatedRole will return a translation for any of the special roles; for other roles it will return the parameter unchanged, since roles are created dynamically as text strings, and will therefore normally be in a local language from the outset. Now we are ready to look at the first meaty method:

public function permittedRoles ($action, $subject_type, $subject_id) 
{
$nonspecific = true;
foreach ($this->permissionHolders ($subject_type, $subject_id) as $possible)
{
if ('*' == $possible->action OR $action == $possible->action)
{
$result[$possible->role] = $this->getTranslatedRole ($possible->role);
if ('*' != $possible->subject_type AND '*' != $possible_subject_id) $nonspecific = false;
}
}
if (!isset($result))
{
if ($nonspecific) $result = array('Visitor' => $this->getTranslatedRole('Visitor'));
else return array();
}
return $result;
}
private function &permissionHolders ($subject_type, $subject_id)
{
$sql = "SELECT DISTINCT role, action, control, subject_type, subject_id FROM #__permissions";
if ($subject_type != '*') $where[] = "(subject_type='$subject_type' OR subject_type='*')";
if ($subject_id != '*') $where[] = "(subject_id='$subject_id' OR subject_id='*')";
if (isset($where)) $sql .= " WHERE ".implode(' AND ', $where);
return $this->database->doSQLget($sql);
}

Any code that is providing an RBAC administration function for some part of the CMS is likely to want to know what roles already have a particular permission so as to show this to the administrator in preparation for any changes. The private method permissionHolders uses the parameters to create a SQL statement that will obtain the minimum relevant permission entries. This is complicated by the fact that in most contexts, asterisk can be used as a wild card.

The public method permittedRoles uses the private method to obtain relevant database rows from the permissions table. These are checked against the action parameter to see which of them are relevant. If there are no results, or if none of the results refer specifically to the subject, without the use of wild cards, then it is assumed that all visitors can access the subject, so the special role of visitor is added to the results. When actual permission is to be granted we need the following methods:

public function permit ($role, $control, $action, $subject_type, $subject_id) 
{
$sql = $this->permitSQL($role, $control, $action, $subject_type, $subject_id);
$this->doSQL($sql, true);
}
private function permitSQL ($role, $control, $action, $subject_type, $subject_id)
{
$this->database->setQuery("SELECT id FROM #__permissions WHERE role='$role' AND action='$action' AND subject_type='$subject_type' AND subject_id='$subject_id'");
$id = $this->database->loadResult();
if ($id) return "UPDATE #__permissions SET control=$control WHERE id=$id";
else return "INSERT INTO #__permissions (role, control, action, subject_type, subject_id) VALUES ('$role', '$control',
'$action', '$subject_type', '$subject_id')";
}

The public method permit grants permission to a role. The control bits are set in the parameter $control. The action is part of permission, and the subject of the action is identified by the subject type and identity parameters. Most of the work is done by the private method that generates the SQL; it is kept separate so that it can be used by other methods. Once the SQL is obtained, it can be passed to the database, and since it will normally result in changes, the option to clear the cache is set.

 

The SQL generated depends on whether there is already a permission with the same parameters, in which case only the control bits are updated. Otherwise an insertion occurs. The reason for having to do a SELECT first, and then decide on INSERT or UPDATE is that the index on the relevant fields is not guaranteed to be unique, and also because the subject ID is allowed to be much longer than can be included within an index. It is therefore not possible to use ON DUPLICATE KEY UPDATE.

Wherever possible, it aids efficiency to use the MySQL option for ON DUPLICATE KEY UPDATE. This is added to the end of an INSERT statement, and if the INSERT fails by virtue of the key already existing in the table, then the alternative actions that follow ON DUPLICATE KEY UPDATE are carried out. They consist of one or more assignments, separated by commas, just as in an UPDATE statement. No WHERE is permitted since the condition for the assignments is already determined by the duplicate key situation.

A simple method allows deletion of all permissions for a particular action and subject:

public function dropPermissions ($action, $subject_type, $subject_id) 
{
$sql = "DELETE FROM #__permissions WHERE action='$action' AND subject_type='$subject_type'AND subject_id='$subject_id' AND system=0";
$this->doSQL($sql, true);
}

The final set of methods relates to assigning accessors to roles. Two of them reflect the obvious need to be able to remove all roles from an accessor (possibly preparatory to assigning new roles) and the granting of a role to an accessor. Where the need is to assign a whole set of roles, it is better to have a method especially for the purpose. Partly this is convenient, but it also provides an extra operation, minimization of the set of roles. The method is:

public function assign ($role, $access_type, $access_id, $clear=true) 
{
if ($this->handler->barredRole($role)) return false;
$this->database->setQuery("SELECT id FROM #__assignments WHERE
role='$role' AND access_type='$access_type' AND access_id='$access_id'");
if ($this->database->loadResult()) return true;
$sql = "INSERT INTO #__assignments (role, access_type, access_id) VALUES ('$role', '$access_type', '$access_id')";
$this->doSQL($sql, $clear);
return true;
}
public function assignRoleSet ($roleset, $access_type, $access_id)
{
$this->dropAccess ($access_type, $access_id);
$roleset = $this->authoriser->minimizeRoleSet($roleset);
foreach ($roleset as $role) $this->assign ($role, $access_type, $access_id, false);
$this->clearCache();
}
public function dropAccess ($access_type, $access_id)
{
$sql = "DELETE FROM #__assignments WHERE access_type='$access_type' AND access_id='$access_id'";
$this->doSQL($sql, true);
}

The method assign links a role to an accessor. It checks for barred roles first, these are simply the special roles discussed earlier, which cannot be allocated to any accessor. As with the permitSQL method, it is not possible to use ON DUPLICATE KEY UPDATE because the full length of the accessor ID is not part of an index, so again the existence of an assignment is checked first. If the role assignment is already in the database, there is nothing to do. Otherwise a row is inserted, and the cache is cleared.

Getting rid of all role assignments for an accessor is a simple database deletion, and is implemented in the dropAccess method. The higher level method assignRoleSet uses dropAccess to clear out any existing assignments. The call to the authorizer object to minimize the role set reflects the implementation of a hierarchical model. Once there is a hierarchy, it is possible for one role to imply another as consultant implied doctor in our earlier example. This means that a role set may contain redundancy. For example, someone who has been allocated the role of consultant does not need to be allocated the role of doctor. The minimizeRoleSet method weeds out any roles that are superfluous. Once that has been done, each role is dealt with using the assign method, with the clearing of the cache saved until the very end.

The General RBAC Cache

As outlined earlier, the information needed to deal with RBAC questions is cached in two ways. The file system cache is handled by the aliroAuthoriserCache singleton class, which inherits from the cachedSingleton class. This means that the data of the singleton object will be automatically stored in the file system whenever possible, with the usual provisions for timing out an old cache, or clearing the cache when an update has occurred. It is highly desirable to cache the data both to avoid database operations and to avoid repeating the processing needed in the constructor. So the intention is that the constructor method will run only infrequently. It contains this code:

protected function __construct() 
{
// Making private enforces singleton
$database = aliroCoreDatabase::getInstance();
$database->setQuery("SELECT role, implied FROM #__role_link UNION SELECT DISTINCT role, role AS implied FROM #__assignments UNION SELECT DISTINCT role,role AS implied FROM #__permissions");
$links = $database->loadObjectList();
if ($links) foreach ($links as $link)
{
$this->all_roles[$link->role] = $link->role;
$this->linked_roles[$link->role][$link->implied] = 1;
foreach ($this->linked_roles as $role=>$impliedarray)
{
foreach ($impliedarray as $implied=>$marker)
{
if ($implied == $link->role OR $implied == $link->implied)
{
$this->linked_roles[$role][$link->implied] = 1;
if (isset($this->linked_roles[$link->implied])) foreach ($this->linked_roles[$link->implied] as $more=>$marker)
{
$this->linked_roles[$role][$more] = 1;
}
}
}
}
}
$database->setQuery("SELECT role, access_id FROM #__assignments WHERE access_type = 'aUser' AND (access_id = '*' OR access_id = '0')");
$user_roles = $database->loadObjectList();
if ($user_roles) foreach ($user_roles as $role) $this- >user_roles[$role->access_id][$role->role] = 1;
if (!isset($this->user_roles['0'])) $this->user_roles['0'] = array();
if (isset($this->user_roles['*'])) $this->user_roles['0'] = array_merge($this->user_roles['0'], $this->user_roles['*']);
}

All possible roles are derived by a UNION of selections from the permissions, assignments, and linked roles database tables. The union operation has overheads, so that alone is one reason for favoring the use of a cache. The processing of linked roles is also complex, and therefore worth running as infrequently as possible. Rather than working through the code in detail, it is more useful to describe what it is doing. The concept is much simpler than the detail! If we take an example from the backwards compatibility features of Aliro, there is a role hierarchy that includes the role Publisher, which implies membership of the role Editor. The role Editor also implies membership of the role Author. In the general case, it is unreasonable to expect the administrator to figure out the implied relationships. In this case, it is clear that the role Publisher must also imply membership of the role Editor. But these linked relationships can plainly become quite complex. The code in the constructor therefore assumes that only the least number of connections have been entered into the database, and it figures out all the implications.

The other operation where the code is less than transparent is the setting of the user_roles property. The Aliro RBAC system permits the use of wild cards for specification of identities within accessor, or subject types. An asterisk indicates any identity. For accessors whose accessor type is user, another wild card available is zero. This means any user who is logged in, and is not an unregistered visitor. Given the relatively small number of role assignments of this kind, it saves a good deal of processing if all of them are cached. Hence the user_roles processing is done in the constructor.

Other methods in the cache class are simple enough to be mentioned rather than given in detail. They include the actual implementation of the getTranslatedRole method, which provides local translations for the special roles. Other actual implementations are getAllRoles with the option to include the special roles, getTranslatedRole, which translates a role if it turns out to be one of the special ones and barredRole, which in turn, tests to see if the passed role is in the special group. It may therefore not be assigned to an accessor.

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Asking RBAC Questions

Perhaps the most significant class is the one that actually answers questions about permitted access. The aliroAuthoriser class is once again a singleton with the usual mechanisms. For convenience, it has getAllRoles and getTranslatedRole methods, but these are really implemented in the cache class described above.

The constructor does some relatively simple setting, including looking for cached data in the PHP super-global $_SESSION:

private function __construct() 
{
// Make sure session started
aliroSessionFactory::getSession();
// Use session data as the source for cached user related data
foreach ($this->auth_vars as $one_var)
{
if (!isset($_SESSION['aliro_auth'][$one_var])) $_SESSION['aliro_auth'][$one_var] = array();
$this->$one_var =& $_SESSION['aliro_auth'][$one_var];
}
$this->handler = aliroAuthoriserCache::getInstance();
$this->linked_roles = $this->handler->getLinkedRoles();
$this->database = aliroCoreDatabase::getInstance();
}

Getting the current session, even though it is not used directly for anything, ensures that a session has been started so that $_SESSION will contain data, if there is any. Since Aliro always activates a session, and much RBAC data is specific to the current user, it makes good sense to cache as session data. The handler and database objects are found using the usual singleton access method, getInstance, and linked roles are obtained from the authorizer cache.

Many RBAC questions involve roles, and the option of a hierarchy means that one role can imply another. This relationship is stored in the linked_roles property. Having roles implied means that a set of roles may include entries that are not really needed. The minimizeRoleSet method eliminates them:

public function minimizeRoleSet ($roleset) 
{
if (0 == count($roleset)) return $roleset;
$first = array_shift($roleset);
foreach ($roleset as $key=>$role)
{
if (isset($this->linked_roles[$first][$role])) unset ($roleset[$key]);
if (isset($this->linked_roles[$role][$first])) return $this->minimizeRoleSet ($roleset);
}
array_unshift($roleset, $first);
return $roleset;
}

There are about a score of other methods, some public, and some private. In detail, the key ones become quite complex. This is partly because of the nature of RBAC, and partly to do with attempts at efficiency. Others are very simple, but this is because they are interfaces to the more substantial methods, but with simplified parameters, so as to provide a more usable interface. Because of the complexity, a selection of the remaining classes is discussed in outline rather than being reviewed in detail. The full code is downloadable from the Aliro website.

Permissions refer to actions on subjects, and it is very likely that multiple queries will arise around similar subjects. The private method getSubjectData is used to load permissions, based on a subject and an action, that is, a specific permission. This method always ensures that all relevant rows from the permission table will be loaded. The number of directly relevant rows will be the number of roles that have the given permission. But the method also tries to get more data than is strictly necessary. Depending on the number of records involved, the method may load all permission data relating to the type of subject specified, not merely to the specific subject. The precise number chosen is subject to optimization work. That is to say, all records where the subject type matches, not just those that match both subject type, and subject identifier. This is done because it is common for a question about rights to a particular subject is often followed by a question about another subject of the same kind. The permission data that is loaded is organized into array structures to maximize the efficiency of lookups, and it is also cached as session data.

The method getAccessorRoles is used both internally and externally. Its prototype is:

public function getAccessorRoles ($type, $id)

It also returns an array of roles. The processing is complicated by the storage of data in cache, something that is especially important for accessors since it is very likely that a number of questions will be asked about the current user. The parameters are the type of accessor (such as 'a User'), and the identifier (such as a user ID number). A private method, accessorPermissionOrControl, does the basic work of finding out whether a particular accessor has rights to a given subject for a stated action. The type of access is passed as a bit mask. This method is then used to create a series of very simple public methods. The most frequently used has a prototype:

public function checkPermission ($a_type, $a_id, $action, $s_type='*', $s_id='*')

The result is zero or one to indicate false or true respectively. The accessor type and ID together define the accessor. Action is self explanatory. Subject type and ID together define the subject. There are situations where wild cards are used. For example, when the action is to manage and the subjects are all users, then the subject ID will be the asterisk wild card. Other actions may have no subject at all, in which case both subject type and ID will be asterisks.

For ease of development, an alternative to checkPermission is the method with prototype:

public function checkUserPermission ($action, $s_type='*', $s_id='*')

It assumes that the accessor is the current user, whose details can be obtained from a standard class in the CMS, so only the action and the subject need be specified. Similar methods to the last two also exist to handle the granting of rights.

While the link between accessors, and subjects via roles can often be kept under the covers and handled within the authorizer class, in some cases it is needed explicitly. It is therefore possible to ask whether a particular role can access a subject for a particular action:

public function checkRolePermission ($role, $action, $s_type, $s_id)

When it comes to deciding questions of access to objects that are generally managed by another piece of software, the most effective query is to find out which items are not available. Let's return to our example of a file repository, where roles are given access to download from specific folders. A folder is identified by its subject type, say remosFolder and an identifier, which in this case, is an ID number. Because we have a rule saying that anything that does not have any specific permissions set is available to all, it is possible to identify a list of all the folders where there are permissions of some kind. For some of those, the user for whom we are asking may have been granted access, via their roles. So those folders are removed from the list. If any folders are left, they are the ones where access is not allowed. The method used to support these queries is:

public function getRefusedList ($a_type, $a_id, $s_type, $actionlist)

It returns an array of ID numbers, given an accessor defined by type, and ID along with a subject type, and an action list. The action list may be a single action, but for convenience, it is allowed to be a comma separated list of actions. The result is the ID numbers for all folders where the accessor is denied permission to carry out any of the actions.

Again to provide a more useful interface, an extended version of the method is available:

public function getRefusedListSQL ($a_type, $a_id, $s_type, $actionlist, $keyname)

It returns a fragment of SQL. Taking an example, if we call getRefusedListSQL('aUser', 47, 'remosFolder', 'download', 'id') we might get back a string containing CAST(id AS CHAR) NOT IN ('5', '14', '27'). This can be used as part of a SQL statement to select folders where the user with ID 47 is allowed to download. So, supposing we want to get a list of the repository container names that are available to our sample user, the full SQL will be constructed using SELECT name FROM #__downloads_containers WHERE followed by the partial SQL provided by getRefusedListSQL. The final sample SQL is then SELECT name FROM #__downloads_containers WHERE CAST(id AS CHAR) NOT IN ('5', '14', '27').

Summary

In this two part article we got the outline of a highly flexible role-based access control system. The principles are established, using standard notions of RBAC. Specific details, such as the way accessors and subjects are identified are adapted to the particular situation of a CMS framework.

The implementation in the database has been established in detail. We've studied the code for administering RBAC, and considered in outline how questions about access can be answered. Further details are available by downloading the Aliro implementation.

 

PHP 5 CMS Framework Development Expert insight and practical guidance to creating an efficient, flexible, and robust framework for a PHP 5-based content management system
Published: June 2008
eBook Price: $39.99
Book Price: $49.99
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Select your format and quantity:

About the Author :


Martin Brampton

Now primarily a software developer and writer, Martin Brampton started out studying mathematics at Cambridge University. He then spent a number of years helping to create the so-called legacy, which remained in use far longer than he ever expected. He worked on a variety of major systems in areas like banking and insurance, spiced with occasional forays into technical areas such as cargo ship hull design and natural gas pipeline telemetry.

After a decade of heading IT for an accountancy firm, a few years as a director of a leading analyst firm, and an MA degree in Modern European Philosophy, Martin finally returned to his interest in software, but this time transformed into web applications. He found PHP5, which fits well with his prejudice in favor of programming languages that are interpreted and strongly object oriented.

Utilizing PHP, Martin took on development of useful extensions for the Mambo (and now also Joomla!) systems, then became leader of the team developing Mambo itself. More recently, he has written a complete new generation CMS named Aliro, many aspects of which are described in this book. He has also created a common API to enable add-on applications to be written with a single code base for Aliro, Joomla (1.0 and 1.5) and Mambo.

All in all, Martin is now interested in many aspects of web development and hosting; he consequently has little spare time. But his focus remains on object oriented software with a web slant, much of which is open source. He runs Black Sheep Research, which provides software, speaking and writing services, and also manages web servers for himself and his clients.

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