Quick introduction to reverse engineering for beginners

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.text:64411E0B .text:64411E0B loc_64411E0B: .text:64411E0B mov [ebx+3A4h], eax ; EBX is pointer to CNetwork object At this point we know that compression flag is stored in CNetwork class at this+0x3A4 address. Now let’s dig across SAPguilib.dll for 0x3A4 value. And here is the second occurence in CDwsGui::OnClientMessage (endless thanks for debugging information): .text:64406F76 loc_64406F76: .text:64406F76 mov ecx, [ebp+7728h+var_7794] .text:64406F79 cmp dword ptr [ecx+3A4h], 1 .text:64406F80 jnz compression_flag_is_zero .text:64406F86 mov byte ptr [ebx+7], 1 .text:64406F8A mov eax, [esi+18h] .text:64406F8D mov ecx, eax .text:64406F8F test eax, eax .text:64406F91 ja short loc_64406FFF .text:64406F93 mov ecx, [esi+14h] .text:64406F96 mov eax, [esi+20h] .text:64406F99 .text:64406F99 loc_64406F99: .text:64406F99 push dword ptr [edi+2868h] ; int .text:64406F9F lea edx, [ebp+7728h+var_77A4] .text:64406FA2 push edx ; int .text:64406FA3 push 30000 ; int .text:64406FA8 lea edx, [ebp+7728h+Dst] .text:64406FAB push edx ; Dst .text:64406FAC push ecx ; int .text:64406FAD push eax ; Src .text:64406FAE push dword ptr [edi+28C0h] ; int .text:64406FB4 call sub_644055C5 ; actual compression routine .text:64406FB9 add esp, 1Ch .text:64406FBC cmp eax, 0FFFFFFF6h .text:64406FBF jz short loc_64407004 .text:64406FC1 cmp eax, 1 .text:64406FC4 jz loc_6440708C .text:64406FCA cmp eax, 2 .text:64406FCD jz short loc_64407004 .text:64406FCF push eax .text:64406FD0 push offset aCompressionErr ; "compression error [rc = \%d]- program wi"... .text:64406FD5 push offset aGui_err_compre ; "GUI_ERR_COMPRESS" .text:64406FDA push dword ptr [edi+28D0h] .text:64406FE0 call SapPcTxtRead Let’s take a look into sub_644055C5. In it we can only see call to memcpy() and some other function named (by IDA 5) sub_64417440. And, let’s take a look inside sub_64417440. What we see is: .text:6441747C push offset aErrorCsrcompre ; "\nERROR: CsRCompress: invalid handle" .text:64417481 call eax ; dword_644F94C8 .text:64417483 add esp, 4 Voilà! We’ve found the function which actually compresses data. As I revealed in past, this function is used in SAP and also open-source MaxDB project. So it is available in sources. 195
Doing last check here: .text:64406F79 cmp dword ptr [ecx+3A4h], 1 .text:64406F80 jnz compression_flag_is_zero Replace JNZ here for unconditional JMP. Remove environment variable TDW_NOCOMPRESS. Voilà! In Wireshark we see that client messages are not compressed. Server responses, however, are compressed. So we found exact connection between environment variable and the point where data compression routine may be called or may be bypassed. 196
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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
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}; printf ("stepping=%d\n", tmp->st
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call ___printf_chk mov eax, esi shr
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mov edx, DWORD PTR _t$[ebp] or edx,
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1.16 C++ classes I placed a C++ cla
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push ecx mov DWORD PTR _this$[ebp],
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call _ZN1c4dumpEv lea eax, [esp+20h
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1.17 Unions 1.17.1 Pseudo-random nu
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xor eax, eax pop esi mov esp, ebp p
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Let’s compile it in MSVC 2010 (I
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comp() function: public comp comp p
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Some compilers can do vectorization
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mov ecx, [esp+10h+var_10] mov edx,
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GCC In all other cases, non-SSE2 co
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add ebx, edx lea esi, [esi+0] loc_8
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movdqa xmm1, XMMWORD PTR [ecx+16] a
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1.20 x86-64 It’s a 64-bit extensi
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} x40 = a3 | x31; x41 = x24 & ~x37;
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or ebx, DWORD PTR _x6$[esp+36] mov
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and r11, r8 and rsi, r8 and r10, r1
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pop rdi pop rsi ret 0 s1 ENDP Nothi
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2.1 LEA instruction LEA (Load Effec
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2.3 npad It’s an assembler macro
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2.4 Signed number representations T
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call function function: .. do somet
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3.2 String Debugging messages are o
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.text:3011E91B DD 1E fstp qword ptr
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loc_80483F2: pop ebp retn _f endp 4
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loc_8048444: loc_8048448: loc_80484
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public f2 f2 proc near push ebp mov
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loc_804840A: loc_804842E: loc_80484
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mov esi, DWORD PTR _k$[esp+16] push
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or eax, edx retn f endp 4.1.8 Task
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4.2 Middle level 4.2.1 Task 2.1 Wel
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mov [edx+eax], bl mov ebx, edi mov
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loc_80485AB: ; CODE XREF: f+1E0 ; f
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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
Page 178 and 179: This instruction clears bit, in oth
Page 180 and 181: .text:005410CD add esp, 40h .text:0
Page 182 and 183: }; for (i=0; i
Page 184 and 185: }; tmp[y][z]=get_bit (row, y, z); f
Page 186 and 187: }; fread (buf, flen, 1, f); fclose
Page 188 and 189: 7.2 SAP client network traffic comp
Page 190 and 191: .text:64413F68 .text:64413F68 loc_6
Page 192 and 193: .text:64405171 call dbg .text:64405
Page 194 and 195: .text:64404FF7 push offset aLogging
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Page 198 and 199: Now let’s dig deeper and find con
Page 202 and 203: Chapter 8 Other things 8.1 Compiler
Page 204 and 205: Chapter 9 Tasks solutions 9.1 Easy
Page 206 and 207: 9.1.5 Task 1.5 Hint #1: Keep in min
Page 208 and 209: } 9.1.8 Task 1.8 Solution: two 100*
Page 210 and 211: #define PUSH(low, high) ((void) ((t
Page 212 and 213: } } ignore one or both. Otherwise,
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