Negative Time - 170 (Game)

Writeup by r3ndom_

Created: 2015-12-8


In the game engine's FPS limitation algorithm, the current time is logged.

What if the current time were to become negative?


Game is Windows only. :)



Make the time variable go negative.

Methods: 1) Patch the check. 2) Patch the point when variable thats checked it set. 3) Decrypt the string.


This can be found in two ways, going to the next encrypted string or finding uses of the clock() function. The variable for fps limitation is set using two clock() calls then subtracting the two to determine the frame time. That variable is then referenced to decide whether to print the string.

I went with finding the usage of the encrypted string and found this code.

loc_4092CB:                             ; CODE XREF: sub_409190+A0j
                                        ; sub_409190+ADj
                cmp     word_568834, 0FF9Ch
                jge     loc_4093E4
                cmp     byte_568833, 0
                jnz     loc_4093E4
                mov     eax, dword_55C4FC
                sub     esp, 18h
                mov     esi, esp
                mov     dword_55C564, eax
                sub     esp, 18h
                mov     dword_55C568, ebx
                mov     ecx, esp        ; int
                mov     dword_55C56C, 2
                mov     byte_568833, 1
                push    24h             ; size_t
                mov     dword ptr [ecx+14h], 0Fh
                mov     dword ptr [ecx+10h], 0
                push    offset a322505102c1a28 ; "322505102c1a282e0d21242110023e192834"
                mov     byte ptr [ecx], 0
                call    sub_40A2D0
                mov     ecx, esi
                call    sub_408C20
                add     esp, 18h
                lea     ecx, [esp+48h+var_1C]
                call    sub_408D30
                mov     esi, eax
                add     esp, 18h
                cmp     esi, offset dword_55C628
                jz      short loc_4093CB
                cmp     dword_55C63C, 10h
                jb      short loc_409365
                push    dword_55C628    ; void *
                call    j__free
                add     esp, 4

Note the check at the top against 0xff9c, aka -100. If its less than that the code lets this string print.

// Where to return to.
dword g_dwRet = 0;
// My code to do a little comparison myself and set the flags I want.
__declspec(naked) void mfHook()
        mov dword ptr ds : [0x568834], -101
        cmp dword ptr ds :[0x568834], 0xff9c
        jmp dword ptr ds : [ g_dwRet ]

dword HookThread(LPVOID lpParam)
    g_dwRet = Detour((byte*)0x4092CB, (byte*)mfHook, 8);
    return 0;

#define COERCE_THREAD(a) reinterpret_cast<LPTHREAD_START_ROUTINE>(a)

    _In_ HINSTANCE hinstDLL,
    _In_ DWORD     fdwReason,
    _In_ LPVOID    lpvReserved
    if (fdwReason == DLL_PROCESS_ATTACH)
        //CreateThread(0, 0, COERCE_THREAD(HookThread), 0, 0, 0);
    return TRUE;

This is kind of a long way to do it, but its the first that came to mind. Again patching the compare with nops would have taken less time but I think it also has less finesse. Some games do hash checks on themselves and as such modifying the binary itself can be dangerous.



Alternative Methods (in brief)

Decrypting the string is possible as always.

There are also many possible modifications that all achieve the same goal of skipping the check, I chose that in particular because it was fairly quick.

The other code for setting the word that gets checked is at 0x408E33 in the binary. Its something like:

word begin = clock();
// Do something that I guess is drawing, it doesn't do any calls in here so I don't know what exactly to make of it
// without reversing more... Its just like a loop where it sets variables in a struct array.
word_568834 = begin - clock();

Which of course is trivial to modify. Also it should probably be clock() - begin to avoid people with trash frames from getting the flag... But to each his own code, I guess.