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Understand android security The next generation of open operating systems wont be on desktops or mainframes but on the small mobile devices we carry every day. The openness of these new environments will lead to new applications and markets and will enable greater integration with existing online services. However, as the importance of the data and services our cell phones support increases, so too do the opportunities for vulnerability. Its essential that this next generation of platforms provide a comprehensive and usable security infrastructure.Developed by the Open Handset Alliance (visibly led by Google), Android is a widely anticipated open source operating system for mobile devices that provides a base operating system, an application middleware layer, a Java software development kit (SDK), and a collection of system applications. Although the Android SDK has been available since late 2007, the frst publicly available Android-ready “G1” phone debuted in late October 2008. Since then, Androids growth has been phenomenal: TMobiles G1 manufacturer HTC estimates shipment volumes of more than 1 million phones by the end of 2008, and industry insiders expect public adoption to increase steeply in 2009. Many other cell phone providers have either promised or plan to support it in the near future.A large community of developers has organized around Android, and many new products and applications are now available for it. One of Androids chief selling points is that it lets developers seamlessly extend online services to phones. The most visible example of this feature isunsurprisinglythe tight integration of Googles Gmail, Calendar, and Contacts Web applications with system utilities. Android users simply supply a username and password, and their phones automatically synchronize with Google services. Other vendors are rapidly adapting their existing instant messaging, social networks, and gaming services to Android, and many enterprises are looking for ways to integrate their own internal operations (such as inventory management, purchasing, receiving, and so forth) into it as well.Traditional desktop and server operating systems have struggled to securely integrate such personal and business applications and services on a single platform; although doing so on a mobile platform such as Android remains nontrivial, many researchers hope it provides a clean slate devoid of the complications that legacy software can cause. Android doesnt officially support applications eloped for other platforms: applications execute on top of a Java middleware layer running on an embedded Linux kernel, so developers wishing to port their application to Android must use its custom user interface environment. Additionally, Android restricts application interaction to its special APIs by running each application as its own user identity. Although this controlled interaction has several benefcial security features, our experiences developing Android applications have revealed that designing secure forward. Android uses a simple permission label assignment model to restrict access to resources and other applications, but for reasons of necessity and convenience, its designers have added several potentially confusing refinements as the system has evolved.This article attempts to unmask the complexity of Android security and note some possible development pitfalls that occur when defining an applications security. We conclude by attempting to draw some lessons and identify opportunities for future enhancements that should aid in clarity and correctness.Android Applications The Android application framework forces a structure on developers. It doesnt have a main() function or single entry point for executioninstead, developers must design applications in terms of components. Example Application.We developed a pair of applications to help describe how Android applications operate. Interested readers can download the source code from our web.(site：http:/siis.cse.psu.edu/android_sec_tutorial.html).Lets consider a location-sensitive social networking application for mobile phones in which users can discover their friendslocations. We split the functionality into two applications: one for tracking friends and one for viewing them. As Figure 1 shows, the FriendTracker application consists of components specific to tracking friend locations (for example, via a Web service), storing geographic coordinates, and sharing those coordinates with other applications. The user then uses the FriendViewer application to retrieve the stored geographic coordinates and view friends on a map.Both applications contain multiple components for performing their respective tasks; the components themselves are classified by their component types. An Android developer chooses from predefined component types depending on the components purpose (such as interfacing with a user or storing data).Component TypesAndroid defnes four component types:Activity.This components define an applications user interface. Typically, an application developer defines one activity per “screen.” Activities start each other, possibly passing and returning values. Only one activity on the system has keyboard and pocessing focus at a time; all others are suspended.Service.This components perform background processing. When an activity needs to perform some operation that must continue after the user interface disappears (such as download a fle or play music), it commonly starts a service specifcally designed for that action. The developer can also use services as application-specific daemons, possibly starting on boot. Services often define an interface for Remote Procedure Call (RPC) that other system components can use to send commands and retrieve data, as well as register callbacks. Content provider.This components store and share data using a relational database interface. Each content provider has an associated “authority” describing the content it contains. Other components use the authority name as a handle to perform SQL queries (such as SELECT, INSERT, or DELETE) to read and write content. Although content providers typically store values in database records, data retrieval is implementation-specificfor example, files are also shared through content provider interfaces.Broadcast receiver components act as mailboxes for messages from other applications. Commonly, application code broadcasts messages to an implicit destination. Broadcast receivers.This components act as mailboxes for messages from other applications. Commonly, application code broadcasts messages to an implicit destination. Broadcast receivers thus subscribe to such destinations to receive the messages sent to it. Application code can also address a broadcast receiver explicitly by including the namespace assigned to its containing application. Figure 1 shows the FriendTracker and FriendViewer applications containing the diferent component types. The developer specifies components using a manifest file (also used to defne policy as described later). There are no restrictions on the number of components an application defnes for each type, but as a convention, one component has the same name as the application. Frequently, this is an activity, as in the FriendViewer application. This activity usually indicates the primary activity that the system application launcher uses to start the user interface; however, the specifc activity chosen on launch is marked by meta information in the manifest. In the FriendTracker application, for example, the FriendTrackerControl activity is marked as the main user interface entry point. In this case, we reserved the name “FriendTracker” for the service component performing the core application logic.The FriendTracker application contains each of the four component types. The FriendTracker service polls an external service to discover friends locations. In our example code, we generate locations randomly, but extending the component to interface with a Web service is straightforward. The FriendProvider Content provider maintains the most recent geographic coordinates for friends, the FriendTrackerControl activity defines a user interface for starting and stopping the tracking functionality, and the BootReceiver broadcast receiver obtains a notification from the system once it boots (the application uses this to automatically start the FriendTracker service).The FriendViewer application is primarily concerned with showing information about friends locations. The FriendViewer activity lists all friends and their geographic coordinates, and the FriendMap activity displays them on a map. The FriendReceiver broadcast receiver waits for messages that indicate the physical phone is near a particular friend and displays a message to the user upon such an event. Although we could have placed these components within the FriendTracker application, we created a separate application to demonstrate cross-application communication. Additionally, by separating the tracking and user interface logic, we can create alternative user interfaces with different displays and featuresthat is, many applications can reuse the logic performed in FriendTracker.Component Interaction.The primary mechanism for component interaction is an intent, which is simply a message object containing a destination component address and data. The Android API defines methods that accept intents, and uses that information to start activities (startActivity(Intent), start services (startService (Intent), and broadcast messages (sendBroadcast(Intent). The invocation of these methods tells the Android framework to begin executing code in the target application. This process of intercomponent communication is known as an action. Simply put, an intent object defines the “intent” to perform an “action.”One of Androids most powerful features is the fexibility allowed by its intent-addressing mechanism. Although developers can uniquely address a target component using its applications namespace, they can also specify an implicit name. In the latter case, the system determines the best component for an action by considering the set of installed applications and user choices. The implicit name is called an action string because it specifes the type of requested actionfor example, if the “VIEW” action string is specifed in an intent with data felds pointing to an image fle, the system will direct the intent to the preferred image viewer. Developers also use action strings to broadcast a message to a group of broadcast receivers. On the receiving end, developers use an intent flter to subscribe to specifc action strings. Android includes additional destination resolution rules, but action strings with optional data types are the most common.Figure 2 shows the interaction between components in the FriendTracker and FriendViewer applications and with components in applications defined as part of the base Android distribution. In each case, one component initiates communication with another. For simplicity, we call this inter-component communication (ICC). In many ways, ICC is analogous to interprocess communication (IPC) in Unix-based systems. To the developer, ICC functions identically regardless of whether the target is in the same or diferent application, with the exception of the security rules defned later in this article.The available ICC actions depend on the target component.Each component type supports interaction specifc to its type for example, when FriendViewer starts FriendMap, the FriendMap activity appears on the screen. Service components support start, stop, and bind actions, so the FriendTrackerControl activity, for instance, can start and stop the FriendTracker service that runs in the background. The bind action establishes a connection between components, allowing the initiator to execute RPCs defned by the service. References1. J.P. Anderson, Computer Security Technology Planning Study, tech. report ESD-TR-73-51, Mitre, Oct. 1972.2. M.A. Harrison, W.L. Ruzzo, and J.D. Ullman, “Protection in Operating Systems,” Comm. ACM, vol. 19, no. 8, 1976, pp. 461471.3. L. Badger et al., “Practical Domain and Type Enforcement for UNIX,” Proc. IEEE Symp. Security and Privacy, IEEE CS Press, 1995, pp. 6677.4. J. Saltzer and M. Schroeder, “The Protection of Information in Computer Systems,” Proc. IEEE, vol. 63, no. 9, 1975, pp. 12781308.5. I. Krstic and S.L. Gar!nkel, “Bitfrost: The One Laptop per Child Security Model,” Proc. Symp. Usable Privacy and Security, ACM Press, 2007, pp. 132142.6. N. Li, B.N. Grosof, and J. Feigenbaum, “Delegation Logic: A Logic-Based Approach to Distributed Authorization,” ACM Trans. Information and System Security, vol. 6, no.1, 2003, pp. 128171.7. W. Enck, M. Ongtang, and P. McDaniel, Mitigating Android Software Misuse Before It Happens, tech. report NAS-TR-0094-2008, Network and Security Research Ctr., Dept. Computer Science and Eng., Pennsylvania State Univ., Nov. 2008知识改变命运3 / 3
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