緩沖區溢出 strcpy()

Question

我想知道我們必須溢出多少字節才能運行 shellcode？

int fun (char data[256]){
int i;
char *tmp;

strcpy(tmp,data);

}

眾所周知：

• 如果字符串鏈 *data 大於 *tmp 那么就會溢出。

• 否則不會有緩沖區溢出。

Answer 1

為編譯器提供通用方法。 這是計算機碩士的考試。 我們必須解釋兩種情況：

-when *tmp[200] 例如和

-當 *tmp[300] 即一個 case 或 *tmp 大於 *data（無溢出）和 *tmp 小於 *data（溢出）

如何知道代碼執行時不堪重負的字節數？

Answer 2

*tmp未初始化，因此您通常會出現分段錯誤。

一個更好的例子是改變char *tmp; 類似於char tmp[64]; 並將數據中的內容（在這種情況下超過 64 字節的內容）復制到 tmp。 要從那時起回答您的問題，您需要在更改代碼后啟動像 gdb 這樣的調試器，然后查看在覆蓋 RIP 之前可以寫出多遠。 在我的系統上是 78 個字節。

marshall@marshall-debian-testbed:~$ cat bof.c
int fun (char data[256]) {
int i;
char tmp[64];
strcpy(tmp,data);
}

int main (int argc, char *argv[]) {
fun(argv[1]);
return(0);
}
marshall@marshall-debian-testbed:~$ gcc bof.c -o bof
bof.c: In function ‘fun’:
bof.c:4:1: warning: implicit declaration of function ‘strcpy’ [-Wimplicit-function-declaration]
 strcpy(tmp,data);
 ^~~~~~
bof.c:4:1: warning: incompatible implicit declaration of built-in function ‘strcpy’
bof.c:4:1: note: include ‘<string.h>’ or provide a declaration of ‘strcpy’
marshall@marshall-debian-testbed:~$ ./bof AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
Segmentation fault
marshall@marshall-debian-testbed:~$ gdb ./bof
GNU gdb (Debian 7.12-6) 7.12.0.20161007-git
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License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
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and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
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<http://www.gnu.org/software/gdb/bugs/>.
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For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./bof...(no debugging symbols found)...done.
(gdb) disas main
Dump of assembler code for function main:
   0x00000000000006d2 <+0>:     push   %rbp
   0x00000000000006d3 <+1>:     mov    %rsp,%rbp
   0x00000000000006d6 <+4>:     sub    $0x10,%rsp
   0x00000000000006da <+8>:     mov    %edi,-0x4(%rbp)
   0x00000000000006dd <+11>:    mov    %rsi,-0x10(%rbp)
   0x00000000000006e1 <+15>:    mov    -0x10(%rbp),%rax
   0x00000000000006e5 <+19>:    add    $0x8,%rax
   0x00000000000006e9 <+23>:    mov    (%rax),%rax
   0x00000000000006ec <+26>:    mov    %rax,%rdi
   0x00000000000006ef <+29>:    callq  0x6b0 <fun>
   0x00000000000006f4 <+34>:    mov    $0x0,%eax
   0x00000000000006f9 <+39>:    leaveq
   0x00000000000006fa <+40>:    retq
End of assembler dump.
(gdb) disas fun
Dump of assembler code for function fun:
   0x00000000000006b0 <+0>:     push   %rbp
   0x00000000000006b1 <+1>:     mov    %rsp,%rbp
   0x00000000000006b4 <+4>:     sub    $0x50,%rsp
   0x00000000000006b8 <+8>:     mov    %rdi,-0x48(%rbp)
   0x00000000000006bc <+12>:    mov    -0x48(%rbp),%rdx
   0x00000000000006c0 <+16>:    lea    -0x40(%rbp),%rax
   0x00000000000006c4 <+20>:    mov    %rdx,%rsi
   0x00000000000006c7 <+23>:    mov    %rax,%rdi
   0x00000000000006ca <+26>:    callq  0x560 <strcpy@plt>
   0x00000000000006cf <+31>:    nop
   0x00000000000006d0 <+32>:    leaveq
   0x00000000000006d1 <+33>:    retq
End of assembler dump.
(gdb) r `perl -e 'print "A"x78;'`
Starting program: /home/marshall/bof `perl -e 'print "A"x78;'`

Program received signal SIGSEGV, Segmentation fault.
0x0000414141414141 in ?? ()
(gdb) info registers
rax            0x7fffffffdce0   140737488346336
rbx            0x0      0
rcx            0x4141414141414141       4702111234474983745
rdx            0x414141 4276545
rsi            0x7fffffffe140   140737488347456
rdi            0x7fffffffdd23   140737488346403
rbp            0x4141414141414141       0x4141414141414141
rsp            0x7fffffffdd30   0x7fffffffdd30
r8             0x555555554770   93824992233328
r9             0x7ffff7de99e0   140737351948768
r10            0x5b     91
r11            0x7ffff7b9ab28   140737349528360
r12            0x555555554580   93824992232832
r13            0x7fffffffde20   140737488346656
r14            0x0      0
r15            0x0      0
rip            0x414141414141   0x414141414141
eflags         0x10202  [ IF RF ]
cs             0x33     51
ss             0x2b     43
ds             0x0      0
es             0x0      0
fs             0x0      0
gs             0x0      0
(gdb)