VMProtect, Part 3: Optimization and Code Generation

Originally published on August 6th, 2008 on OpenRCE

The reader should inspect this unoptimized IR listing before continuing.  In an attempt to keep this entry from becoming unnecessarily long, the example snippets will be small, but for completeness a more thorough running example is linked throughout the text.

We begin by removing the stack machine features of the IR.  Since VMProtect operates on disassembled x86 code, and x86 itself is not a stack machine, this aspect of the protection is unnatural and easily removed.  Here is a 15-line fragment of VMProtect IR.

push Dword(-88)
push esp
push Dword(4)
pop t3
pop t4
t5 = t3 + t4
push t5
push flags t5
pop DWORD Scratch:[Dword(52)]
pop t6
pop t7
t8 = t6 + t7
push t8
push flags t8
pop DWORD Scratch:[Dword(12)]
pop esp

All but two instructions are pushes or pops, and the pushes can be easily matched up with the pops.  Tracking the stack pointer, we see that, for example, t3 = Dword(4).  A simple analysis allows us to "optimize away" the push/pop pairs into assignment statements.  Simply iterate through each instruction in a basic block and keep a stack describing the source of each push.  For every pop, ensure that the sizes match and record the location of the corresponding push.  We wish to replace the pop with an assignment to the popped expression from the pushed expression, as in

t3 = Dword(4)
t4 = esp 
t7 = Dword(-88)

With the stack aspects removed, we are left with a more conventional listing containing many assignment statements.  This optimization substantially reduces the number of instructions in a given basic block (~40% for the linked example) and opens the door for other optimizations.  The newly optimized code is eight lines, roughly half of the original:

t3 = Dword(4)
t4 = esp
t5 = t3 + t4
DWORD Scratch:[Dword(52)] = flags t5
t6 = t5
t7 = Dword(-88)
t8 = t6 + t7
DWORD Scratch:[Dword(12)] = flags t8
esp = t8

A complete listing of the unoptimized IR versus the one with the stack machine features removed is here, which should be perused before proceeding.

Now we turn our attention to the temporary variables and the scratch area.  Recall that the former were not part of the pre-protected x86 code, nor the VMProtect bytecode -- they were introduced in order to ease the IR translation.  The latter is part of the VMProtect bytecode, but was not part of the original pre-protected x86 code.  Since these are not part of the languages we are modelling, we shall eliminate them wholesale.  On a high level, we treat each temporary variable, each byte of the scratch space, and each register as being a variable defined within a basic block, and then eliminate the former two via the compiler optimizations previously discussed.

Looking again at the last snippet of IR, we can see several areas for improvement.  First, consider the variable t6.  It is clearly just a copy of t5, neither of which are redefined before the next use in the assignment to t8.  Copy propagation will replace variable t6 with t5 and eliminate the former.  More generally, t3, t4, and t7 contain either constants or values that are not modified between their uses.  Constant and copy propagation will substitute the assignments to these variables in for their uses and eliminate them.

The newly optimized code is a slender three lines compared to the original 15; we have removed 80% of the IR for the running example.

DWORD Scratch:[Dword(52)] = flags Dword(4) + esp
esp = Dword(4) + esp + Dword(-88)
DWORD Scratch:[Dword(12)] = flags Dword(4) + esp + Dword(-88)

The side-by-side comparison can be found here.

The IR now looks closer to x86, with the exception that the results of computations are being stored in the scratch area, not into registers.  As before, we apply dead-store elimination, copy and constant propagation to the scratch area, removing dependence upon it entirely in the process.  See here for a comparison with the last phase.

Here is a comparison of the final, optimized code against the original x86:

push ebp                          push ebp
ebp = esp                         mov ebp, esp
push Dword(-1)                    push 0FFFFFFFFh
push Dword(4525664)               push 450E60h
push Dword(4362952)               push offset sub_4292C8
eax = DWORD FS:[Dword(0)]         mov eax, large fs:0
push eax                          push eax
DWORD FS:[Dword(0)] = esp         mov large fs:0, esp
eflags = flags esp + Dword(-88)
esp = esp + Dword(-88)            add esp, 0FFFFFFA8h
push ebx                          push ebx
push esi                          push esi
push edi                          push edi
DWORD SS:[Dword(-24) + ebp] = esp mov [ebp-18h], esp
call DWORD [Dword(4590300)]       call dword ptr ds:unk_460ADC
vmreturn Dword(0) + Dword(4638392)

Code generation is an afterthought.