KAuth

Execute actions as privileged user.

Using the Module

Using a KDE module's C++ API requires linking against the module library. The preferred way of consuming the library is through CMake.

Building with CMake

Use the find_package() command to locate the needed module component in the KF6 package:

 find_package(KF6 REQUIRED COMPONENTS Auth)
 target_link_libraries(mytarget PRIVATE KF6::AuthCore)

For more details, see the Build with CMake overview.

Introduction

The KDE Authorization API allows developers to write desktop applications that run high-privileged tasks in an easy, secure and cross-platform way. Previously, if an application had to do administrative tasks, it had to be run as root, using mechanisms such as sudo or graphical equivalents, or by setting the executable's setuid bit. This approach has some drawbacks. For example, the whole application code, including GUI handling and network communication, had to be done as root. More code that runs as root means more possible security holes.

The solution is the caller/helper pattern. With this pattern, the privileged code is isolated in a small helper tool that runs as root. This tool includes only the few lines of code that actually need to be run with privileges, not the whole application logic. All the other parts of the application are run as a normal user, and the helper tool is called when needed, using a secure mechanism that ensures that the user is authorized to do so. This pattern is not very easy to implement, because the developer has to deal with a lot of details about how to authorize the user, how to call the helper with the right privileges, how to exchange data with the helper, etc.. This is where the new KDE Authorization API becomes useful. Thanks to this new library, every developer can implement the caller/helper pattern to write application that require high privileges, with a few lines of code in an easy, secure and cross-platform way.

Not only: the library can also be used to lock down some actions in your application without using a helper but just checking for authorization and verifying if the user is allowed to perform it.

The KDE Authorization library uses different backends depending on the system where it's built. As far as the user authorization is concerned, it currently uses polkit-1 on linux and Authorization Services on Mac OSX, and a Windows backend will eventually be written, too. At the communication layer, the library uses D-Bus on every supported platform.

Concepts

There are a few concepts to understand when using the library. Much of those are carried from underlying APIs such as polkit-1, so if you know something about them there shouldn't be problems.

An action is a single task that needs to be done by the application. You refer to an action using an action identifier, which is a string in reverse domain name syntax (to avoid duplicates). For example, if the date/time control center module needs to change the date, it would need an action like "org.kde.datatime.change". If your application has to perform more than one privileged task, you should configure more than one action. This allows system administrators to fine tune the policies that allow users to perform your actions.

The authorization is the process that is executed to decide if a user can perform an action or not. In order to execute the helper as root, the user has to be authorized. For example, on linux, che policykit backend will look at the policykit policy database to see what requirements the user has to meet in order to execute the action you requested. The policy set for that action could allow or deny that user, or could say the user has to authenticate in order to gain the authorization.

The authentication is the process that allows the system to know that the person is in front of the console is who he says to be. If an action can be allowed or not depending on the user's identity, it has to be proved by entering a password or any other identification data the system requires.

A typical session with the authorization API is like this:

• The user want to perform some privileged task
• The application asks the system if the user is authorized.
• The system asks the user to authenticate, if needed, and reply the application.
• The application uses some system-provided mechanism to execute the helper's code as the root user. Previously, you had to set the setuid bit to do this, but we have something cool called "D-Bus activation" that doesn't require the setuid bit and is much more flexible.
• The helper code, immediately after starting, checks if the caller is authorized to do what it asks. If not the helper immediately exits!
• If the caller is authorized, the helper executes the task and exits.
• The application receives data back from the helper.

All these steps are managed by the library. Following sections will focus on how to write the helper to implement your actions and how to call the helper from the application.

Writing the helper tool

The first thing you need to do before writing anything is to decide what actions you need to implement. Every action needs to be identified by a string in the reverse domain name syntax. This helps to avoid duplicates. An example of action id is "org.kde.datetime.change" or "org.kde.ksysguard.killprocess". Action names can only contain lowercase letters and dots (not as the first or last char). You also need an identifier for your helper. An application using the KDE auth api can implement and use more than one helper, implementing different actions. An helper is uniquely identified in the system context with a string. It, again, is in reverse domain name syntax to avoid duplicates. A common approach is to call the helper like the common prefix of your action names. For example, the Date/Time kcm module could use a helper called "org.kde.datetime", to perform actions like "org.kde.datetime.changedate" and "org.kde.datetime.changetime". This naming convention simplifies the implementation of the helper.

From the code point of view, the helper is implemented as a QObject subclass. Every action is implemented by a public slot. In the example/ directory in the source code tree you find a complete example. Let's look at that. The helper.h file declares the class that implements the helper. It looks like:

 #include <KAuth/ActionReply>
 #include <KAuth/HelperSupport>

 using namespace KAuth;

 class MyHelper : public QObject
 {
     Q_OBJECT
 public Q_SLOTS:
     ActionReply read(const QVariantMap &args);
     ActionReply write(const QVariantMap &args);
     ActionReply longaction(const QVariantMap &args);
 };

The slot names are the last part of the action name, without the helper's ID if it's a prefix, with all the dots replaced by underscores. In this case, the helper ID is "org.kde.kf6auth.example", so those three slots implement the actions "org.kde.kf6auth.example.read", "org.kde.kf6auth.example.write" and "org.kde.kf6auth.example.longaction". The helper ID doesn't have to appear at the beginning of the action name, but it's good practice. If you want to extend MyHelper to implement also a different action like "org.kde.datetime.changetime", since the helper ID doesn't match you'll have to implement a slot called org_kde_datetime_changetime().

The slot's signature is fixed: the return type is ActionReply, a class that allows you to return results, error codes and custom data to the application when your action has finished to run.

Let's look at the read action implementation. Its purpose is to read files:

 ActionReply MyHelper::read(const QVariantMap &args)
 {
     ActionReply reply;
     QString filename = args["filename"].toString();
     QFile file(filename);
     if (!file.open(QIODevice::ReadOnly)) {
         reply = ActionReply::HelperErrorReply();
         reply.setErrorDescription(file.errorString());
         return reply;
     }
     QTextStream stream(&file);
     QString contents;
     stream >> contents;
     reply.addData("contents", contents);
     return reply;
 }

First, the code creates a default reply object. The default constructor creates a reply that reports success. Then it gets the filename parameter from the argument QVariantMap, that has previously been set by the application, before calling the helper. If it fails to open the file, it creates an ActionReply object that notifies that some error has happened in the helper, then set the error code to that returned by QFile and returns. If there is no error, it reads the file. The contents are added to the reply.

Because this class will be compiled into a standalone executable, we need a main() function and some code to initialize everything: you don't have to write it. Instead, you use the KAUTH_HELPER_MAIN() macro that will take care of everything. It's used like this:

 KAUTH_HELPER_MAIN("org.kde.kf6auth.example", MyHelper)

The first parameter is the string containing the helper identifier. Please note that you need to use this same string in the application's code to tell the library which helper to call, so please stay away from typos, because we don't have any way to detect them. The second parameter is the name of the helper's class. Your helper, if complex, can be composed of a lot of source files, but the important thing is to include this macro in at least one of them.

To build the helper, KDE macros provide a function named kauth_install_helper_files(). Use it in your cmake file like this:

 add_executable(<helper_target> your sources...)
 target_link_libraries(<helper_target> your libraries...)
 install(TARGETS <helper_target> DESTINATION ${KAUTH_HELPER_INSTALL_DIR})

 kauth_install_helper_files(<helper_target> <helper_id> <user>)

As locale is not inherited, the auth helper will have the text codec explicitly set to use UTF-8.

The first argument is the cmake target name for the helper executable, which you have to build and install separately. Make sure to INSTALL THE HELPER IN ${KAUTH_HELPER_INSTALL_DIR}, otherwise kauth_install_helper_files will not work. The second argument is the helper id. Please be sure to don't misspell it, and to not quote it. The user parameter is the user that the helper has to be run as. It usually is root, but some actions could require less strict permissions, so you should use the right user where possible (for example the user apache if you have to mess with apache settings). Note that the target created by this macro already links to libkauth and QtCore.

Action registration

To be able to authorize the actions, they have to be added to the policy database. To do this in a cross-platform way, we provide a cmake macro. It looks like:

 kauth_install_actions(<helper_id> <actions definition file>)

The action definition file describes which actions are implemented by your code and which default security options they should have. It is a common text file in ini format, with one section for each action and some parameters. The definition for the read action is:

 [org.kde.kf6auth.example.read]
 Name=Read action
 Description=Read action description
 Policy=auth_admin
 Persistence=session

The name parameter is a text describing the action for who reads the file. The description parameter is the message shown to the user in the authentication dialog. It should be a finite phrase. The policy attribute specify the default rule that the user must satisfy to be authorized. Possible values are:

• yes: the action should be always allowed
• no: the action should be always denied
• auth_self: the user should authenticate as itself
• auth_admin: the user should authenticate as an administrator user

The persistence attribute is optional. It says how long an authorization should be retained for that action. The values could be:

• session: the authorization persists until the user logs-out
• always: the authorization will persist indefinitely

If this attribute is missing, the authorization will be queried every time.

Calling the helper from the application

Once the helper is ready, we need to call it from the main application. In examples/client.cpp you can see how this is done. To create a reference to an action, an object of type Action has to be created. Every Action object refers to an action by its action id. Two objects with the same action id will act on the same action. With an Action object, you can authorize and execute the action. To execute an action you need to retrieve an ExecuteJob, which is a standard KJob that you can run synchronously or asynchronously. See the KJob documentation (from KCoreAddons) for more details.

The piece of code that calls the action of the previous example is:

 QVariantMap args;
 args["filename"] = filename;
 Action readAction("org.kde.kf6auth.example.read");
 readAction.setHelperId("org.kde.kf6auth.example");
 readAction.setArguments(args);
 ExecuteJob *job = readAction.execute();
 if (!job->exec()) {
     qDebug() << "KAuth returned an error code:" << job->error();
 } else {
     QString contents = job->data()["contents"].toString();
     qDebug() << "KAuth succeeded. Contents: " << contents;
 }

First of all, it creates the action object specifying the action id. Then it loads the filename (we want to read a forbidden file) into the arguments() QVariantMap, which will be directly passed to the helper in the read() slot's parameter. This example code uses a synchronous call to execute the action and retrieve the reply. If the reply succeeded, the reply data is retrieved from the returned QVariantMap object. Please note that you have to explicitly set the helper ID to the action: this is done for added safety, to prevent the caller from accidentally invoking a helper, and also because KAuth actions may be used without a helper attached (the default).

Please note that if your application is calling the helper multiple times it must do so from the same thread.

Asynchronous calls, data reporting, and action termination

For a more advanced example, we look at the action "org.kde.kf6auth.example.longaction" in the example helper. This is an action that takes a long time to execute, so we need some features:

• The helper needs to regularly send data to the application, to inform about the execution status.
• The application needs to be able to stop the action execution if the user stops it or close the application.

The example code follows:

 ActionReply MyHelper::longaction(const QVariantMap &)
 {
     for (int i = 1; i <= 100; i++) {
         if (HelperSupport::isStopped()) {
             break;
         }
         HelperSupport::progressStep(i);
         QThread::usleep(250000);
     }
     return ActionReply::SuccessReply();
 }

In this example, the action is only waiting a "long" time using a loop, but we can see some interesting line. The progress status is sent to the application using the HelperSupport::progressStep(int) and HelperSupport::progressStep(const QVariantMap &) methods. When those methods are called, the HelperProxy associated with this action will emit the HelperProxy::progressStep(const QString &, int) and HelperProxy::progressStepData(const QString &, const QVariantMap &) signals, respectively, reporting back the data to the application. The method that takes an integer argument is the one used here. Its meaning is application dependent, so you can use it as a sort of percentage. If you want to report custom data back to the application, you can use the other method that takes a QVariantMap object which is directly passed to the app.

In this example code, the loop exits when the HelperSupport::isStopped() returns true. This happens when the application calls the HelperProxy::stopAction() method on the corresponding action object. The stopAction() method, this way, asks the helper to stop the action execution. It's up to the helper to obbey to this request, and if it does so, it should return from the slot, _not_ exit.

Other features

It doesn't happen very frequently that you code something that doesn't require some debugging, and you'll need some tool, even a basic one, to debug your helper code as well. For this reason, the KDE Authorization library provides a message handler for the Qt debugging system. This means that every call to qDebug() & co. will be reported to the application, and printed using the same qt debugging system, with the same debug level. If, in the helper code, you write something like:

 qDebug() << "I'm in the helper";

You'll see something like this in the application's output:

 Debug message from the helper: I'm in the helper

Remember that the debug level is preserved, so if you use qFatal() you won't only abort the helper (which isn't suggested anyway), but also the application.

API Reference

• KAuth C++ Classes