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Computing checksums on code fragments or on entire executables in runtime can make for a fairly powerful antidebugging technique, because debuggers must modify the code in order to install breakpoints. The general idea is to precalculate a checksum for functions within the program (this trick could be reserved for particularly sensitive functions), and have the function randomly
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check that the function has not been modified. This method is not only effective against debuggers, but also against code patching (see 11), but has the downside that constantly recalculating checksums is a relatively expensive operation. There are several workarounds for this problem; it all boils down to employing a clever design. Consider, for example, a program that has 10 highly sensitive functions that are called while the program is loading (this is a common case with protected applications). In such a case, it might make sense to have each function verify its own checksum prior to returning to the caller. If the checksum doesn t match, the function could take an inconspicuous (so that reversers don t easily spot it) detour that would eventually lead to the termination of the program or to some kind of unusual program behavior that would be very difficult for the attacker to diagnose. The benefit of this approach is that it doesn t add much execution time to the program because only the specific functions that are considered to be sensitive are affected. Note that this technique doesn t detect or prevent hardware breakpoints, because such breakpoints don t modify the program code in any way.
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Fooling disassemblers as a means of preventing or inhibiting reversers is not a particularly robust approach to antireversing, but it is popular none the less. The strategy is quite simple. In processor architectures that use variable-length instructions, such as IA-32 processors, it is possible to trick disassemblers into incorrectly treating invalid data as the beginning of an instruction. This causes the disassembler to lose synchronization and disassemble the rest of the code incorrectly until it resynchronizes. Before discussing specific techniques, I would like to briefly remind you of the two common approaches to disassembly (discussed in 4). A linear sweep is the trivial approach that simply disassembles instruction sequentially in the entire module. Recursive traversal is the more intelligent approach whereby instructions are analyzed by traversing instructions while following the control flow instructions in the program, so that when the program branches to a certain address, disassembly also proceeds at that address. Recursive traversal disassemblers are more reliable and are far more tolerant of various antidisassembly tricks. Let s take a quick look at the reversing tools discussed in this book and see which ones actually use recursive traversal disassemblers. This will help you predict the effect each technique is going to have on the most common tools. Table 10.1 describes the disassembly technique employed in the most common reversing tools.
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Table 10.1 Common Reversing Tools and Their Disassembler Architectures. DISSASEMBLY METHOD Recursive traversal Linear sweep Linear sweep Recursive traversal Recursive traversal
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DISASSEMBLER/DEBUGGER NAME OllyDbg NuMega SoftICE Microsoft WinDbg IDA Pro PEBrowse Professional (including the interactive version)
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Linear Sweep Disassemblers
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Let s start experimenting with some simple sequences that confuse disassemblers. We ll initially focus exclusively on linear sweep disassemblers, which are easier to trick, and later proceed to more involved sequences that attempt to confuse both types of disassemblers. Consider for example the following inline assembler sequence:
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_asm { Some code... jmp After _emit 0x0f After: mov eax, [SomeVariable] push eax call AFunction }
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When loaded in OllyDbg, the preceding code sequence is perfectly readable, because OllyDbg performs a recursive traversal on it. The 0F byte is not disassembled, and the instructions that follow it are correctly disassembled. The following is OllyDbg s output for the previous code sequence.
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0040101D 0040101F 00401020 00401023 00401024 EB 01 0F 8B45 FC 50 E8 D7FFFFFF JMP SHORT disasmtest.00401020 DB 0F MOV EAX,DWORD PTR SS:[EBP-4] PUSH EAX CALL disasmtest.401000
In contrast, when fed into NuMega SoftICE, the code sequence confuses its disassembler somewhat, and outputs the following:
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