Quick introduction to reverse engineering for beginners

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.text:3011E91B DD 1E fstp qword ptr [esi] If to set breakpoint on it, we may see result: PID=32852|TID=36488|(0) 0x2f40e91b (Excel.exe!BASE+0x11e91b) EAX=0x00598006 EBX=0x00598018 ECX=0x00000001 EDX=0x00000001 ESI=0x00598000 EDI=0x00294804 EBP=0x026CF93C ESP=0x026CF8F8 EIP=0x2F40E91B FLAGS=PF IF FPU ControlWord=IC RC=NEAR PC=64bits PM UM OM ZM DM IM FPU StatusWord=C1 P FPU ST(0): 0.333333 Also, as a practical joke, we can modify it on-fly: gt -l:excel.exe bpx=excel.exe!base+0x11E91B,set(st0,666) PID=36540|TID=24056|(0) 0x2f40e91b (Excel.exe!BASE+0x11e91b) EAX=0x00680006 EBX=0x00680018 ECX=0x00000001 EDX=0x00000001 ESI=0x00680000 EDI=0x00395404 EBP=0x0290FD9C ESP=0x0290FD58 EIP=0x2F40E91B FLAGS=PF IF FPU ControlWord=IC RC=NEAR PC=64bits PM UM OM ZM DM IM FPU StatusWord=C1 P FPU ST(0): 0.333333 Set ST0 register to 666.000000 Excel showing 666 in that cell what finally convincing us we find the right place. Figure 3.1: Practical joke worked If to try the same Excel version, but x64, we’ll see there are only 12 FDIV instructions, and the one we looking for — third. gt64.exe -l:excel.exe bpx=excel.exe!base+0x1B7FCC,set(st0,666) It seems, a lot of division operations of float and double types, compiler replaced by SSE-instructions like DIVSD (DIVSD present here 268 in total). 3.5 Suspicious code patterns Modern compilers do not emit LOOP and RCL instructions. On the other hand, these instructions are wellknown to coders who like to code in straight assembler. If you spot these, it can be said, with a big probability, this piece of code is hand-written. 123
Chapter 4 Tasks There are two questions almost for every task, if otherwise isn’t specified: 1) What this function does? Answer in one-sentence form. 2) Rewrite this function into C/C++. Hints and solutions are in the appendix of this brochure. 4.1 Easy level 4.1.1 Task 1.1 This is standard C library function. Source code taken from OpenWatcom. Compiled in MSVC 2010. _TEXT SEGMENT _input$ = 8 ; size = 1 _f PROC push ebp mov ebp, esp movsx eax, BYTE PTR _input$[ebp] cmp eax, 97 ; 00000061H jl SHORT $LN1@f movsx ecx, BYTE PTR _input$[ebp] cmp ecx, 122 ; 0000007aH jg SHORT $LN1@f movsx edx, BYTE PTR _input$[ebp] sub edx, 32 ; 00000020H mov BYTE PTR _input$[ebp], dl $LN1@f: mov al, BYTE PTR _input$[ebp] pop ebp ret 0 _f ENDP _TEXT ENDS It is the same code compiled by GCC 4.4.1 with -O3 option (maximum optimization): _f proc near input = dword ptr 8 push ebp mov ebp, esp movzx eax, byte ptr [ebp+input] lea edx, [eax-61h] 124
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Quick introduction to reverse engin
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1.16 C++ classes . . . . . . . . .
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Preface Here (will be) some of my n
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1.1 Hello, world! Let’s start wit
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1.2 Stack Stack — is one of the m
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(_snprintf() function works just li
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1.3 printf() with several arguments
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1.4 scanf() Now let’s use scanf()
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GCC replaced first printf() call to
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}; return 0; printf ("What you ente
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1.5 Passing arguments via stack Now
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1.6 One more word about results ret
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push OFFSET $SG741 ; ’a
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1.8 switch()/case/default 1.8.1 Few
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1.8.2 A lot of cases If switch() st
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loc_804840C: ; DATA XREF: .rodata:0
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et 0 _main ENDP Nothing very specia
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add esp, 1Ch xor eax, eax ; return
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C/C++ standard defines char type as
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eax=eax+ecx return eax ... and this
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mov eax, ecx imul edx sar edx, 1 mo
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; current stack state: ST(0) = resu
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; in local stack here 8 bytes still
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fld QWORD PTR _a$[esp-4] ; current
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In other words, fnstsw ax / sahf in
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1.13 Arrays Array is just a set of
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mov [esp+70h+var_70], eax call _pri
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jge SHORT $LN1@main mov ecx, DWORD
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ebp 0x15 0x15 esi 0x0 0 edi 0x0 0 e
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1.14 Bit fields A lot of functions
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So, let’s download Linux Kernel 2
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There are some redundant code prese
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y 10 just by dropping last digit (3
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_key$ = 8 ; size = 4 _len$ = 12 ; s
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1.15 Structures It can be defined t
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call DWORD PTR __imp__GetSystemTime
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1.15.4 Fields packing in structure
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}; int b; struct outer_struct { cha
Page 78 and 79: }; printf ("stepping=%d\n", tmp->st
Page 80 and 81: call ___printf_chk mov eax, esi shr
Page 82 and 83: mov edx, DWORD PTR _t$[ebp] or edx,
Page 84 and 85: 1.16 C++ classes I placed a C++ cla
Page 86 and 87: push ecx mov DWORD PTR _this$[ebp],
Page 88 and 89: call _ZN1c4dumpEv lea eax, [esp+20h
Page 90 and 91: 1.17 Unions 1.17.1 Pseudo-random nu
Page 92 and 93: xor eax, eax pop esi mov esp, ebp p
Page 94 and 95: Let’s compile it in MSVC 2010 (I
Page 96 and 97: comp() function: public comp comp p
Page 98 and 99: Some compilers can do vectorization
Page 100 and 101: mov ecx, [esp+10h+var_10] mov edx,
Page 102 and 103: GCC In all other cases, non-SSE2 co
Page 104 and 105: add ebx, edx lea esi, [esi+0] loc_8
Page 106 and 107: movdqa xmm1, XMMWORD PTR [ecx+16] a
Page 108 and 109: 1.20 x86-64 It’s a 64-bit extensi
Page 110 and 111: } x40 = a3 | x31; x41 = x24 & ~x37;
Page 112 and 113: or ebx, DWORD PTR _x6$[esp+36] mov
Page 114 and 115: and r11, r8 and rsi, r8 and r10, r1
Page 116 and 117: pop rdi pop rsi ret 0 s1 ENDP Nothi
Page 118 and 119: 2.1 LEA instruction LEA (Load Effec
Page 120 and 121: 2.3 npad It’s an assembler macro
Page 122 and 123: 2.4 Signed number representations T
Page 124 and 125: call function function: .. do somet
Page 126 and 127: 3.2 String Debugging messages are o
Page 130 and 131: loc_80483F2: pop ebp retn _f endp 4
Page 132 and 133: loc_8048444: loc_8048448: loc_80484
Page 134 and 135: public f2 f2 proc near push ebp mov
Page 136 and 137: loc_804840A: loc_804842E: loc_80484
Page 138 and 139: mov esi, DWORD PTR _k$[esp+16] push
Page 140 and 141: or eax, edx retn f endp 4.1.8 Task
Page 142 and 143: 4.2 Middle level 4.2.1 Task 2.1 Wel
Page 144 and 145: mov [edx+eax], bl mov ebx, edi mov
Page 146 and 147: loc_80485AB: ; CODE XREF: f+1E0 ; f
Page 148 and 149: mov [ecx+eax], dl inc eax cmp ebx,
Page 150 and 151: Chapter 5 Tools ∙ IDA as disassem
Page 152 and 153: Chapter 7 More examples 147
Page 154 and 155: .text:00541050 arg_0 = dword ptr 4
Page 156 and 157: .text:005410F3 .text:005410F3 loc_5
Page 158 and 159: .text:005411A9 pop ebx .text:005411
Page 160 and 161: .text:00541249 .text:00541249 next_
Page 162 and 163: .text:005412FC mov esi, offset cube
Page 164 and 165: .text:005413DC call _fwrite ; write
Page 166 and 167: .text:005414A7 push ecx ; Filename
Page 168 and 169: .text:005413A3 mov ecx, [esp+44h+pa
Page 170 and 171: .text:00541408 push offset aRb ; "r
Page 172 and 173: }; }; return; fseek (f, 0, SEEK_END
Page 174 and 175: .text:005411B5 mov ebp, eax Check f
Page 176 and 177: .text:0054124C inc ebp .text:005412
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This instruction clears bit, in oth
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.text:005410CD add esp, 40h .text:0
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}; for (i=0; i
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}; tmp[y][z]=get_bit (row, y, z); f
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}; fread (buf, flen, 1, f); fclose
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7.2 SAP client network traffic comp
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.text:64413F68 .text:64413F68 loc_6
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.text:64405171 call dbg .text:64405
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.text:64404FF7 push offset aLogging
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.text:64405113 loc_64405113: .text:
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Now let’s dig deeper and find con
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.text:64411E0B .text:64411E0B loc_6
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Chapter 8 Other things 8.1 Compiler
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Chapter 9 Tasks solutions 9.1 Easy
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9.1.5 Task 1.5 Hint #1: Keep in min
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} 9.1.8 Task 1.8 Solution: two 100*
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#define PUSH(low, high) ((void) ((t
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} } ignore one or both. Otherwise,
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