Application framework/model (AppFw)

The AGL application framework consists of several inter-working parts:

  • SMACK: The kernel level LSM (Linux Security Module) that performs extended access control of the system.
  • Cynara: the native gatekeeper daemon used for policy handling, updating to the database and policy checking.
  • Security Manager: a master service, through which all security events are intended to take place.
  • Several native application framework utilities: afm-main-binding, afm-user-daemon, afm-system-daemon.

The application framework manages:

  • The applications and services management: Installing, Uninstalling, Listing, …
  • The life cycle of applications: Start -> (Pause, Resume) -> Stop.
  • Events and signals propagation.
  • Privileges granting and checking.
  • API for interaction with applications.
  • The security model refers to the security model used to ensure security and to the tools that are provided for implementing that model. It’s an implementation detail that should not impact the layers above the application framework.

  • The security model refers to how DAC (Discretionary Access Control), MAC (Mandatory Access Control) and Capabilities are used by the system to ensure security and privacy. It also includes features of reporting using audit features and by managing logs and alerts.

The AppFw uses the security model to ensure the security and the privacy of the applications that it manages. It must be compliant with the underlying security model. But it should hide it to the applications.

Domain Object Recommendations
Platform-AGLFw-AppFw-1 Security model Use the AppFw as Security model.

See AGL AppFw Privileges Management and AGL - Application Framework Documentation for more information.

The Security Manager communicates policy information to Cynara, which retains information in its own database in the format of a text file with comma-separated values (CSV). There are provisions to retain a copy of the CSV text file when the file is being updated.

Runtime checking occurs through Cynara. Each application that is added to the framework has its own instantiation of a SMACK context and D-bus bindings. The afb_daemon and Binder form a web-service that is communicated to through http or a websocket from the application-proper. This http or websocket interface uses a standard unique web token for API communication.

Application Framework Flow

Cynara

There’s a need for another mechanism responsible for checking applicative permissions: Currently in AGL, this task depends on a policy-checker service (Cynara).

  • Stores complex policies in databases.
  • “Soft” security (access is checked by the framework).

Cynara interact with D-Bus in order to deliver this information.

Cynara consists of several parts:

  • Cynara: a daemon for controlling policies and responding to access control requests.
  • Database: a spot to hold policies.
  • Libraries: several static and dynamic libraries for communicating with Cynara.

The daemon communicates to the libraries over Unix domain sockets. The database storage format is a series of CSV-like files with an index file.

There are several ways that an attacker can manipulate policies of the Cynara system:

  • Disable Cynara by killing the process.
  • Tamper with the Cynara binary on-disk or in-memory.
  • Corrupt the database controlled by Cynara.
  • Tamper with the database controlled by Cynara.
  • Highjack the communication between Cynara and the database.

The text-based database is the weakest part of the system and although there are some consistency mechanisms in place (i.e. the backup guard), these mechanisms are weak at best and can be countered by an attacker very easily.

Domain Object Recommendations
Platform-AGLFw-Cynara-1 Permissions Use Cynara as policy-checker service.

Policies

  • Policy rules:

    • Are simple - for pair [application context, privilege] there is straight answer (single Policy Type): [ALLOW / DENY / …].
    • No code is executed (no script).
    • Can be easily cached and managed.
  • Application context (describes id of the user and the application credentials) It is build of:

    • UID of the user that runs the application.
    • SMACK label of application.

Holding policies

Policies are kept in buckets. Buckets are set of policies which have additional a property of default answer, the default answer is yielded if no policy matches searched key. Buckets have names which might be used in policies (for directions).

Attack Vectors

The following attack vectors are not completely independent. While attackers may have varying levels of access to an AGL system, experience has shown that a typical attack can start with direct access to a system, find the vulnerabilities, then proceed to automate the attack such that it can be invoked from less accessible standpoint (e.g. remotely). Therefore, it is important to assess all threat levels, and protect the system appropriately understanding that direct access attacks are the door-way into remote attacks.

Remote Attacks

The local web server interface used for applications is the first point of attack, as web service APIs are well understood and easily intercepted. The local web server could potentially be exploited by redirecting web requests through the local service and exploiting the APIs. While there is the use of a security token on the web service API, this is weak textual matching at best. This will not be difficult to spoof. It is well known that API Keys do not provide any real security.

It is likely that the architectural inclusion of an http / web-service interface provided the most flexibility for applications to be written natively or in HTML5. However, this flexibility may trade-off with security concerns. For example, if a native application were linked directly to the underlying framework services, there would be fewer concerns over remote attacks coming through the web-service interface.

Leaving the interface as designed, mitigations to attacks could include further securing the interface layer with cryptographic protocols: e.g. encrypted information passing, key exchange (e.g. Elliptic-Curve Diffie-Hellman).

User-level Native Attacks

  • Modifying the CSV data-base
  • Modifying the SQLite DB
  • Tampering with the user-level binaries
  • Tampering with the user daemons
  • Spoofing the D-bus Interface
  • Adding executables/libraries

With direct access to the device, there are many security concerns on the native level. For example, as Cynara uses a text file data-base with comma-separated values (CSV), an attacker could simply modify the data-base to escalate privileges of an application. Once a single application has all the privileges possible on the system, exploits can come through in this manner. Similarly the SQLite database used by the Security Manager is not much different than a simple text file. There are many tools available to add, remove, modify entries in an SQLite data-base.

On the next level, a common point of attack is to modify binaries or daemons for exploiting functionality. There are many Linux tools available to aid in this regard, including: IDA Pro, and radare2. With the ability to modify binaries, an attacker can do any number of activities including: removing calls to security checks, redirecting control to bypass verification functionality, ignoring security policy handling, escalating privileges, etc.

Additionally, another attack vector would be to spoof the D-bus interface. D-bus is a message passing system built upon Inter-Process Communication (IPC), where structured messages are passed based upon a protocol. The interface is generic and well documented. Therefore, modifying or adding binaries/libraries to spoof this interface is a relatively straight-forward process. Once the interface has been spoofed, the attacker can issue any number of commands that lead into control of low-level functionality.

Protecting a system from native attacks requires a methodical approach. First, the system should reject processes that are not sanctioned to run. Signature-level verification at installation time will help in this regard, but run-time integrity verification is much better. Signatures need to originate from authorized parties, which is discussed further in a later section on the Application Store.

On the next level, executables should not be allowed to do things where they have not been granted permission. DAC and SMACK policies can help in this regard. On the other hand, there remain concerns with memory accesses, system calls, and other process activity that may go undetected. For this reason, a secure environment which monitors all activity can give indication of all unauthorized activity on the system.

Finally, it is very difficult to catch attacks of direct tampering in a system. These types of attacks require a defense-in-depth approach, where complementary software protection and hardening techniques are needed. Tamper-resistance and anti-reverse-engineering technologies include program transformations/obfuscation, integrity verification, and white-box cryptography. If applied in a mutually-dependent fashion and considering performance/security tradeoffs, the approach can provide an effective barrier to direct attacks to the system. Furthermore, the use of threat monitoring provides a valuable telemetry/analytics capability and the ability to react and renew a system under attack.

Root-level Native Attacks

  • Tampering the system daemon
  • Tampering Cynara
  • Tampering the security manager
  • Disabling SMACK
  • Tampering the kernel

Once root-level access (i.e. su) has been achieved on the device, there are many ways to compromise the system. The system daemon, Cynara, and the security manager are vulnerable to tampering attacks. For example, an executable can be modified in memory to jam a branch, jump to an address, or disregard a check. This can be as simple as replacing a branch instruction with a NOP, changing a memory value, or using a debugger (e.g. gdb, IDA) to change an instruction. Tampering these executables would mean that policies can be ignored and verification checks can be bypassed.

Without going so far as to tamper an executable, the SMACK system is also vulnerable to attack. For example, if the kernel is stopped and restarted with the security=none flag, then SMACK is not enabled. Furthermore, systemd starts the loading of SMACK rules during start-up. If this start-up process is interfered with, then SMACK will not run. Alternatively, new policies can be added with smackload allowing unforseen privileges to alternative applications/executables.

Another intrusion on the kernel level is to rebuild the kernel (as it is open-source) and replace it with a copy that has SMACK disabled, or even just the SMACK filesystem (smackfs) disabled. Without the extended label attributes, the SMACK system is disabled.

Root-level access to the device has ultimate power, where the entire system can be compromised. More so, a system with this level access allows an attacker to craft a simpler point-attack which can operate on a level requiring fewer privileges (e.g. remote access, user-level access).

Vulnerable Resources

Resource: afm-user-daemon

The afm-user-daemon is in charge of handling applications on behalf of a user. Its main tasks are:

  • Enumerate applications that the end user can run and keep this list available on demand.
  • Start applications on behalf of the end user, set user running environment, set user security context.
  • List current runnable or running applications.
  • Stop (aka pause), continue (aka resume), terminate a running instance of a given application.
  • Transfer requests for installation/uninstallation of applications to the corresponding system daemon afm-system-daemon.

The afm-user-daemon launches applications. It builds a secure environment for the application before starting it within that environment. Different kinds of applications can be launched, based on a configuration file that describes how to launch an application of a given kind within a given launching mode: local or remote. Launching an application locally means that the application and its binder are launched together. Launching an application remotely translates in only launching the application binder.

The UI by itself has to be activated remotely by a request (i.e. HTML5 homescreen in a browser). Once launched, running instances of the application receive a runid that identifies them. afm-user-daemon manages the list of applications that it has launched. When owning the right permissions, a client can get the list of running instances and details about a specific running instance. It can also terminate, stop or continue a given application. If the client owns the right permissions, afm-user-daemon delegates the task of installing and uninstalling applications to afm-system-daemon.

afm-user-daemon is launched as a systemd service attached to a user session. Normally, the service file is located at /usr/lib/systemd/user/afm-user-daemon.service.

Attacker goals:

  • Disable afm-user-daemon.
  • Tamper with the afm-user-daemon configuration.
    • /usr/lib/systemd/user/afm-user-daemon.service.
    • Application(widget) config.xml file.
    • /etc/afm/afm-launch.conf (launcher configuration).
  • Escalate user privileges to gain more access with afm-user-daemon.
  • Install malicious application (widget).
  • Tamper with afm-user-daemon on disk or in memory.

Resource: afm-system-daemon

The afm-system-daemon is in charge of installing applications on the AGL system. Its main tasks are:

  • Install applications and setup security framework for newly installed applications.
  • Uninstall applications.

afm-system-daemon is launched as a systemd service attached to system. Normally, the service file is located at /lib/systemd/system/afm-systemdaemon.service.

Attacker goals:

  • Disable afm-system-daemon.
  • Tamper with the afm-system-daemon configuration.
  • Tamper afm-system-daemon on disk or in memory.

Resource afb-daemon

afb-binder is in charge of serving resources and features through an HTTP interface. afb-daemon is in charge of binding one instance of an application to the AGL framework and AGL system. The application and its companion binder run in a secured and isolated environment set for them. Applications are intended to access to AGL system through the binder. afb-daemon binders serve files through HTTP protocol and offers developers the capability to expose application API methods through HTTP or WebSocket protocol.

Binder bindings are used to add APIs to afb-daemon. The user can write a binding for afb-daemon. The binder afb-daemon serves multiple purposes:

  1. It acts as a gateway for the application to access the system.
  2. It acts as an HTTP server for serving files to HTML5 applications.
  3. It allows HTML5 applications to have native extensions subject to security enforcement for accessing hardware resources or for speeding up parts of algorithm.

Attacker goals:

  • Break from isolation.
  • Disable afb-daemon.
  • Tamper afb-demon on disk or in memory.
  • Tamper capabilities by creating/installing custom bindings for afb-daemon.