Reverse Engineering and Program Understanding
- Into the House of Logic
- Should Reverse Engineering Be Illegal?
- Reverse Engineering Tools and Concepts
- Approaches to Reverse Engineering
- Methods of the Reverser
- Writing Interactive Disassembler (IDA) Plugins
- Decompiling and Disassembling Software
- Decompilation in Practice: Reversing helpctr.exe
- Automatic, Bulk Auditing for Vulnerabilities
- Writing Your Own Cracking Tools
- Building a Basic Code Coverage Tool
Most people interact with computer programs at a surface level, entering input and eagerly (impatiently?!) awaiting a response. The public façade of most programs may be fairly thin, but most programs go much deeper than they appear at first glance. Programs have a preponderance of guts, where the real fun happens. These guts can be very complex. Exploiting software usually requires some level of understanding of software guts.
The single most important skill of a potential attacker is the ability to unravel the complexities of target software. This is called reverse engineering or sometimes just reversing. Software attackers are great tool users, but exploiting software is not magic and there are no magic software exploitation tools. To break a nontrivial target program, an attacker must manipulate the target software in unusual ways. So although an attack almost always involves tools (disassemblers, scripting engines, input generators), these tools tend to be fairly basic. The real smarts remain the attacker's prerogative.
When attacking software, the basic idea is to grok the assumptions made by the people who created the system and then undermine those assumptions. (This is precisely why it is critical to identify as many assumptions as possible when designing and creating software.) Reverse engineering is an excellent approach to ferreting out assumptions, especially implicit assumptions that can be leveraged in an attack. 
Into the House of Logic
In some sense, programs wrap themselves around valuable data, making and enforcing rules about who can get to the data and when. The very edges of the program are exposed to the outside world just the way the interior of a house has doors at its public edges. Polite users go through these doors to get to the data they need that is stored inside. These are the entry points into software. The problem is that the very doors used by polite company to access software are also used by remote attackers.
Consider, for example, a very common kind of Internet-related software door, the TCP/IP port. Although there are many types of doors in a typical program, many attackers first look for TCP/IP ports. Finding TCP/IP ports is simple using a port-scanning tool. Ports provide public access to software programs, but finding the door is only the beginning. A typical program is complex, like a house made up of many rooms. The best treasure is usually found buried deep in the house. In all but the most trivial of exploits, an attacker must navigate complicated paths through public doors, journeying deep into the software house. An unfamiliar house is like a maze to an attacker. Successful navigation through this maze renders access to data and sometimes complete control over the software program itself.
Software is a set of instructions that determines what a general-purpose computer will do. Thus, in some sense, a software program is an instantiation of a particular machine (made up of the computer and its instructions). Machines like this obviously have explicit rules and well-defined behavior. Although we can watch this behavior unfold as we run a program on a machine, looking at the code and coming to an understanding of the inner workings of a program sometimes takes more effort. In some cases the source code for a program is available for us to examine; other times, it is not. Therefore, attack techniques must not always rely on having source code. In fact, some attack techniques are valuable regardless of the availability of source code. Other techniques can actually reconstruct the source code from the machine instructions. These techniques are the focus of this chapter.
Reverse engineering is the process of creating a blueprint of a machine to discern its rules by looking only at the machine and its behavior. At a high level, this process involves taking something that you may not completely understand technically when you start, and coming to understand completely its function, its internals, and its construction. A good reverse engineer attempts to understand the details of software, which by necessity involves understanding how the overall computing machinery that the software runs on functions. A reverse engineer requires a deep understanding of both the hardware and the software, and how it all works together.
Think about how external input is handled by a software program. External "user" input can contain commands and data. Each code path in the target involves a number of control decisions that are made based on input. Sometimes a code path will be wide and will allow any number of messages to pass through successfully. Other times a code path will be narrow, closing things down or even halting if the input isn't formatted exactly the right way. This series of twists and turns can be mapped if you have the right tools. Figure 3-1 illustrates code paths as found in a common FTP server program. In this diagram, a complex subroutine is being mapped. Each location is shown in a box along with the corresponding machine instructions.
Figure 3-1 This graph illustrates control flow through a subroutine in a common FTP server. Each block is a set of instructions that runs as a group, one instruction after the other. The lines between boxes illustrate the ways that control in the code connects boxes. There are various "branches" between the boxes that represent decision points in the control flow. In many cases, a decision regarding how to branch can be influenced by data supplied by an attacker.
Generally speaking, the deeper you go as you wander into a program, the longer the code path between the input where you "start" and the place where you end up. Getting to a particular location in this house of logic requires following paths to various rooms (hopefully where the valuables are). Each internal door you pass through imposes rules on the kinds of messages that may pass. Wandering from room to room thus involves negotiating multiple sets of rules regarding the input that will be accepted. This makes crafting an input stream that can pass through lots of doors (both external and internal) a real challenge. In general, attack input becomes progressively more refined and specific as it digs deeper into a target program. This is precisely why attacking software requires much more than a simple brute-force approach. Simply blasting a program with random input almost never traverses all the code paths. Thus, many possible paths through the house remain unexplored (and unexploited) by both attackers and defenders.
Why Reverse Engineer?
Reverse engineering allows you to learn about a program's structure and its logic. Reverse engineering thus leads to critical insights regarding how a program functions. This kind of insight is extremely useful when you exploit software. There are obvious advantages to be had from reverse engineering. For example, you can learn the kind of system functions a target program is using. You can learn the files the target program accesses. You can learn the protocols the target software uses and how it communicates with other parts of the target network.
The most powerful advantage to reversing is that you can change a program's structure and thus directly affect its logical flow. Technically this activity is called patching, because it involves placing new code patches (in a seamless manner) over the original code, much like a patch stitched on a blanket. Patching allows you to add commands or change the way particular function calls work. This enables you to add secret features, remove or disable functions, and fix security bugs without source code. A common use of patching in the computer underground involves removing copy protection mechanisms.
Like any skill, reverse engineering can be used for good and for bad ends.