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Security Warrior by Anton Chuvakin, Cyrus Peikari

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Chapter 4. Windows CE Reverse Engineering

In the previous chapters, we covered reverse engineering on traditional platforms such as Win32 and Linux. However, what about the little guys? Can you reverse engineer software on embedded operating systems? Why would you want to?

Many embedded operating systems are stripped-down microversions of their big brothers. An embedded operating system brings the power of a complete OS to small devices such as mobile phones or watches, which suffer from severely restricted processing and memory resources. However, as embedded devices continue to increase in sophistication, their vulnerability to attack increases as well. Already the first computer viruses have hit embedded platforms, as we describe in Chapter 17. Corporate spyware will likely follow soon. With hundreds of millions of "smart" consumer appliances on the horizon, the potential for abuse keeps increasing.

Embedded RCE is still in its infancy. In this chapter, we introduce embedded OS architecture and how to crack the applications that run on it. For our example, we have chosen Windows CE, which powers many Windows Mobile OS flavors such as PocketPC and Smartphone. Windows CE is a semi-open, scalable, 32-bit, true-multitasking operating system that has been designed to run with maximum power on minimum resources. This OS is actually a miniature version of Windows 2000/XP that can run on appliances as small as a watch.

Why have we chosen Windows CE for our reverse engineering research, instead of friendly, open source, and free embedded Linux? For better or worse, CE is set to become one of the most prevalent operating systems of all time, thanks to aggressive marketing tactics by Microsoft. In addition, because of their closed nature, Windows platforms usually see the majority of viruses and unethical corporate spyware. Thus, the need to reverse engineer embedded Windows applications is more pressing. Download the free eMbedded Visual Tools (MVT) package from Microsoft.com and get cracking—literally.

Windows CE Architecture

Windows CE is the basis of all Windows Mobile PocketPC and Smartphone devices. In addition, using the CE Platform Builder, any programmer can create her own miniature operating system based on Windows CE. Consequently, CE is starting to control a vast array of consumer devices, ranging from toasters to exercise bicycles. Because of its growing prevalence, if you want to become proficient at reverse engineering applications on mobile devices it is important to understand the basics of how this operating system works. This segment briefly covers the Windows CE architecture, with a deeper look at topics important to understand when reversing.

Processors

In the world of miniature gadgets, physics is often the rate-limiting step. For example, the intense heat generated by high-speed processors in notebook PCs has been shown to be hot enough to fry eggs. In fact, News.com reported that one unfortunate man inadvertently burned his genitals with a laptop computer (http://www.news.com.au/common/story_page/0,4057,5537960%255E1702,00.html)!

Windows CE devices are likewise limited in their choice of processors. The following is a list of processors supported by Windows CE:

ARM

Supported processors include ARM720T, ARM920T, ARM1020T, StrongARM, and XScale. ARM-based processors are by far the most common choice of CE devices at the time of this writing.

MIPS

Supported processors include MIPS II/32 w/FP, MIPS II/32 w/o FP, MIPS16, MIPS IV/64 w/FP, and MIPS IV/64 w/o FP.

SHx

Supported processors include SH-3, SH-3 DSP, and SH-4.

x86

Supported processors include 486, 586, Geode, and Pentium I/II/III/IV.

If heat dissipation is a serious issue, the best choice is one of the non-x86 processors that uses a reduced level of power. The reduction in power consumption reduces the amount of heat created during processor operation, but it also limits the processor speed.

Kernel, Processes, and Threads

The kernel is the key component of a Windows CE OS. It handles all the core functions of the OS, such as processes, threads, and memory management. It also handles scheduling and interrupts. However, it is important to understand that Windows CE uses parts from its big brother—i.e., desktop Windows software. This means its threading, processing, and virtual memory models are similar to those of traditional Windows platforms.

While CE has a lot in common with traditional Windows, there are several items that distinguish it. These differences center on the use of memory and the simple fact that there is no hard drive (as discussed in the next section). In addition, dynamic link libraries (DLLs) in Windows CE are not implemented as they are in other Windows operating systems. Instead, they are used in such a way as to maximize the available memory. Integrating them into the core operating system means that DLLs don't take up precious space when they are executed. This is an important concept to understand before trying to reverse a program in Windows CE. Due to this small difference, attempting to break a program while it is executing a system DLL is not allowed by Microsoft's MVT.

A process in Windows CE represents an executing program. The number of processes is limited to 32, but each process can execute a theoretically unlimited number of threads. Each thread has a 64K memory block assigned to it, in addition to an ID and a set of registers. It is important to understand this concept because when debugging a program, you will be monitoring the execution of a particular thread, its registers, and the allotted memory space. In the process, you will be able to deduce hidden passwords, serial numbers, and more.

Processes can run in two modes: kernel and user. A kernel process has direct access to the OS and the hardware. This gives it more power, but a crash in a kernel process often crashes the whole OS. A user process, on the other hand, operates outside the kernel memory—but a crash only kills the running program, not the whole OS. In Windows CE, any third-party program will operate in user mode, which means it is protected. In other words, if you crash a program while reversing it, the whole OS will not crash (though you still may need to reboot the device).

There are two other important points to understand. First, one process cannot affect the data of another process. While related threads can interact with each other, a process is restricted to its own memory slot. The second point to remember is that each existing thread is continuously being stopped and restarted by a scheduler (discussed next). This is how multitasking is actually performed. While it may appear that more than one program is running at a time, the truth is that only one thread may execute at any one time on single-processor devices.

The scheduler is responsible for managing the thread process times. It does this by giving each thread a chance to use the processor. By continuously moving from thread to thread, the scheduler ensures that each gets a turn. Three key features for adjusting processor time are built into the scheduler.

The first feature is a method that is used to increase the amount of processor time. The secret is found in multithreading an application. Since the scheduler assigns processor time at the thread level, a process with 10 threads will get 10 times the processor time of a process with one thread.

Another method for gaining more processor time is to increase the process priority; but it's not encouraged unless necessary. Changing priority levels can cause serious problems in other programs, and it affects the speed of the computing device as a whole. The THREAD_PRIORITY_TIME_CRITICAL priority is important; it forces the processor to complete the critical thread.

The final interesting feature of the scheduler deals with a problem that can arise when priority threading is used. If a low-priority thread is executing and it ties up a resource needed by a higher-priority thread, the system could become unstable. In short, a paradox is created in which the high thread waits for the low thread to finish, which in turn waits on the high to complete. To prevent this situation from occurring, the scheduler will detect such a paradox and boost the lower-priority thread to a higher level, thus allowing it to finish.

Note that all of these problems are issues that every Windows OS must deal with. A Windows Mobile device may seem different, but it is still a Microsoft product, and as such it is limited by those products' common constraints.

Memory Architecture

One of the unique properties of most devices running Windows CE is the lack of a disc hard drive. Instead of spinning discs, pocket PCs use old-fashioned RAM (Random Access Memory) and ROM (Read Only Memory) to store data. While this may seem like a step back in technology, the use of static memory like ROM is on the rise and will eventually make moving storage devices obsolete. The next few paragraphs explain how memory in a Windows CE device is used to facilitate program execution.

In a Windows CE device, the entire operating system is stored in ROM. This type of memory is typically read-only and is not used to store temporary data that can be deleted. On the other hand, data in RAM is constantly being updated and changed. This memory is used to hold all files and programs that are loaded into the Windows CE-based device.

RAM is also used to execute programs. When a third-party game is executed, it is first copied into RAM and is executed from there. This is why a surplus of RAM is important in a Windows CE device. However, the real importance of RAM is that its data can be written to and accessed by an address. This is necessary because a program will often have to move data around. Since each program is allotted a section of RAM to run in when it is executed, it must be able to write directly to its predefined area.

While ROM is typically only used as a static storage area, in Windows CE it can be used to execute programs. This process is known as Execute In Place (XIP). In other words, RAM is not required to hold the ROM's data as a program executes. This freedom allows RAM to be used for other important applications. However, it only works with ROM data that is not compressed. While compression allows more data to be stored in ROM, the decompression will force any execution to be done via RAM.

RAM usage on a Windows CE device is divided between two functions. The first is the object store , which is used to hold files and data that are used by the programs but are not stored in ROM. In particular, the object store holds compressed program files, user files, database files, and the infamous Windows registry file. Although this data is stored in RAM, it remains intact when the device is turned off, because the RAM is kept charged by the power supply. This is the reason it is very important to never let the charge on a Pocket PC device completely die. If this happens, the RAM loses power and resets. It dumps all installed programs and wipes everything on the device except what is stored in ROM. This is referred to as a hard reboot when dealing with a Pocket PC device.

The second function of the RAM is to facilitate program execution. As previously mentioned, when a program is running, it needs to store the information it is using—this is the same function that RAM serves on a typical desktop PC. Any data passing through a program, such as a password or serial number, will be written to the RAM at one time or another.

Windows CE does have a limit on the RAM size. In Windows CE 3.0 it is 256 MB with a 32 MB limit on each file, but in Windows CE .NET this value has been increased to a rather large 4 GB. In addition, there is a limit to the number of files that can be stored in RAM (4 million) and to the number of programs that can operate at the same time. This brings us to multitasking.

Windows CE was designed to be a true multitasking operating system. Just like other modern Windows operating systems, it allows more than one program to be open at a time. In other words, you can listen to an MP3 while taking notes and checking out sites on the Internet. Without multitasking, you would be forced to close one program before opening another. However, you must be careful not to open too many programs on a Windows CE device. Since you are limited by the amount RAM in the device, and each open program takes up a chunk of the RAM, you can quickly run out of memory.

Finally, the limitation of RAM in a pocket PC also affects the choice of operating system. Since Windows CE devices may only have 32-128 MB of internal RAM, they do not make good platforms for operating systems that use a lot of memory, such as embedded Windows XP. In this OS, the minimum footprint for a program is 5 MB. On the other hand, Windows CE only requires 200K; this is a 2500% difference.

Graphics, Windowing, and Event Subsystem (GWES)

This part of the Windows CE architecture is responsible for handling all the input (e.g., stylus) and output (e.g., screen text and images). Since every program uses windows to receive messages, it is a very important part of Windows CE. It is one of the areas you need to understand to successfully reverse a program.

Without going into too much detail, you should know that every Windows CE process is assigned its own windows messaging queue. The queue is similar to a stack of papers that is added to and read from. This queue is created when the program calls GetMessage, which is very common in Windows CE programs. While the program executes and interacts with the user, messages are placed in and removed from the queue. The following is a list and explanation of the common commands that you will see while reverse engineering:

PostMessage

Places message on queue of target thread, which is returned immediately to the process/thread

SendMessage

Places message on queue, but does not return until it is processed

SendThreadMessage

Sends messages directly to thread instead of to queue

These Message commands, and others, act as bright, virtual flares when reversing a program. For example, if a "Sorry, wrong serial number" warning is flashed on the screen, you can bet some Message command was used. By looking for the use of this command in a disassembler, you can find the part of the program that needs further research.

We've given you a quick inside look at how Windows CE operates. This information is required reading for the rest of the chapter. Understanding processing, memory architecture, and how Windows CE uses messages to communicate with the executing program will make it easier for you to understand how CE cracking works. Just as a doctor must understand the entire human body before diagnosing even a headache, a reverse engineer must thoroughly understand the platform he is dissecting to be successful in making a patch or deciphering a serial number.

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