c - 缓冲区溢出 strcpy()

Question

我想知道我们需要溢出多少字节才能运行 shellcode？

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

strcpy(tmp,data);

}

据了解:

• 如果字符串链*data大于*tmp则会溢出。

• 否则不会出现缓冲区溢出。

Answer 1

*tmp 未初始化，因此通常无论如何都会出现段错误。

一个更好的例子是将 char *tmp; 更改为 char tmp[64]; 并包含一些内容(在本例中超过 64 字节的内容)将数据复制到 tmp。要回答您的问题，您需要在更改代码后启动 gdb 之类的调试器，然后查看在覆盖 RIP 之前可以写出多远。在我的系统上是 78 字节。

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
Copyright (C) 2016 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 "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
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)