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4.13 利用 _IO_FILE 结构
FILE 结构
FILE 结构体的利用是一种通用的控制流劫持技术。攻击者可以覆盖堆上的 FILE 指针使其指向一个伪造的结构，利用结构中一个叫做 vtable 的指针，来执行任意代码。
我们知道 FILE 结构被一系列流操作函数（fopen()、fread()、fclose()等）所使用，大多数的 FILE 结构体保存在堆上（stdin、stdout、stderr除外，位于libc数据段），其指针动态创建并由 fopen() 返回。在 glibc（2.23） 中，这个结构体是 _IO_FILE_plout，包含了一个 _IO_FILE 结构体和一个指向 _IO_jump_t 结构体的指针：
// libio/libioP.h
struct _IO_jump_t
{
JUMP_FIELD(size_t, __dummy);
JUMP_FIELD(size_t, __dummy2);
JUMP_FIELD(_IO_finish_t, __finish);
JUMP_FIELD(_IO_overflow_t, __overflow);
JUMP_FIELD(_IO_underflow_t, __underflow);
JUMP_FIELD(_IO_underflow_t, __uflow);
JUMP_FIELD(_IO_pbackfail_t, __pbackfail);
/* showmany */
JUMP_FIELD(_IO_xsputn_t, __xsputn);
JUMP_FIELD(_IO_xsgetn_t, __xsgetn);
JUMP_FIELD(_IO_seekoff_t, __seekoff);
JUMP_FIELD(_IO_seekpos_t, __seekpos);
JUMP_FIELD(_IO_setbuf_t, __setbuf);
JUMP_FIELD(_IO_sync_t, __sync);
JUMP_FIELD(_IO_doallocate_t, __doallocate);
JUMP_FIELD(_IO_read_t, __read);
JUMP_FIELD(_IO_write_t, __write);
JUMP_FIELD(_IO_seek_t, __seek);
JUMP_FIELD(_IO_close_t, __close);
JUMP_FIELD(_IO_stat_t, __stat);
JUMP_FIELD(_IO_showmanyc_t, __showmanyc);
JUMP_FIELD(_IO_imbue_t, __imbue);
#if 0
get_column;
set_column;
#endif
};
/* We always allocate an extra word following an _IO_FILE.
This contains a pointer to the function jump table used.
This is for compatibility with C++ streambuf; the word can
be used to smash to a pointer to a virtual function table. */
struct _IO_FILE_plus
{
_IO_FILE file;
const struct _IO_jump_t *vtable;
};
extern struct _IO_FILE_plus *_IO_list_all;
vtable 指向的函数跳转表其实是一种兼容 C++ 虚函数的实现。当程序对某个流进行操作时，会调用该流对应的跳转表中的某个函数。
// libio/libio.h
struct _IO_FILE {
int _flags; /* High-order word is _IO_MAGIC; rest is flags. */
#define _IO_file_flags _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;
#if 0
int _blksize;
#else
int _flags2;
#endif
_IO_off_t _old_offset; /* This used to be _offset but it's too small. */
#define __HAVE_COLUMN /* temporary */
/* 1+column number of pbase(); 0 is unknown. */
unsigned short _cur_column;
signed char _vtable_offset;
char _shortbuf[1];
/* char* _save_gptr; char* _save_egptr; */
_IO_lock_t *_lock;
#ifdef _IO_USE_OLD_IO_FILE
};
struct _IO_FILE_complete
{
struct _IO_FILE _file;
#endif
#if defined _G_IO_IO_FILE_VERSION && _G_IO_IO_FILE_VERSION == 0x20001
_IO_off64_t _offset;
# if defined _LIBC || defined _GLIBCPP_USE_WCHAR_T
/* Wide character stream stuff. */
struct _IO_codecvt *_codecvt;
struct _IO_wide_data *_wide_data;
struct _IO_FILE *_freeres_list;
void *_freeres_buf;
# else
void *__pad1;
void *__pad2;
void *__pad3;
void *__pad4;
# endif
size_t __pad5;
int _mode;
/* Make sure we don't get into trouble again. */
char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)];
#endif
};
extern struct _IO_FILE_plus _IO_2_1_stdin_;
extern struct _IO_FILE_plus _IO_2_1_stdout_;
extern struct _IO_FILE_plus _IO_2_1_stderr_;
进程中的 FILE 结构会通过 _chain 域构成一个链表，链表头部用全局变量 _IO_list_all 表示。
下面我们来看几个函数的实现。
fopen
// libio/iofopen.c
_IO_FILE *
__fopen_internal (const char *filename, const char *mode, int is32)
{
struct locked_FILE
{
struct _IO_FILE_plus fp;
#ifdef _IO_MTSAFE_IO
_IO_lock_t lock;
#endif
struct _IO_wide_data wd;
} *new_f = (struct locked_FILE *) malloc (sizeof (struct locked_FILE)); // 为 FILE 结构分配空间
if (new_f == NULL)
return NULL;
#ifdef _IO_MTSAFE_IO
new_f->fp.file._lock = &new_f->lock;
#endif
#if defined _LIBC || defined _GLIBCPP_USE_WCHAR_T
_IO_no_init (&new_f->fp.file, 0, 0, &new_f->wd, &_IO_wfile_jumps);
#else
_IO_no_init (&new_f->fp.file, 1, 0, NULL, NULL);
#endif
_IO_JUMPS (&new_f->fp) = &_IO_file_jumps; // 设置 vtable = &_IO_file_jumps
_IO_file_init (&new_f->fp); // 调用 _IO_file_init 函数进行初始化
#if !_IO_UNIFIED_JUMPTABLES
new_f->fp.vtable = NULL;
#endif
if (_IO_file_fopen ((_IO_FILE *) new_f, filename, mode, is32) != NULL) // 打开目标文件
return __fopen_maybe_mmap (&new_f->fp.file);
_IO_un_link (&new_f->fp);
free (new_f);
return NULL;
}
_IO_FILE *
_IO_new_fopen (const char *filename, const char *mode)
{
return __fopen_internal (filename, mode, 1);
}
// libio/fileops.c
# define _IO_new_file_init _IO_file_init
void
_IO_new_file_init (struct _IO_FILE_plus *fp)
{
/* POSIX.1 allows another file handle to be used to change the position
of our file descriptor. Hence we actually don't know the actual
position before we do the first fseek (and until a following fflush). */
fp->file._offset = _IO_pos_BAD;
fp->file._IO_file_flags |= CLOSED_FILEBUF_FLAGS;
_IO_link_in (fp); // 调用 _IO_link_in 函数将 fp 放进链表
fp->file._fileno = -1;
}
// libio/genops.c
void
_IO_link_in (struct _IO_FILE_plus *fp)
{
if ((fp->file._flags & _IO_LINKED) == 0)
{
fp->file._flags |= _IO_LINKED;
#ifdef _IO_MTSAFE_IO
_IO_cleanup_region_start_noarg (flush_cleanup);
_IO_lock_lock (list_all_lock);
run_fp = (_IO_FILE *) fp;
_IO_flockfile ((_IO_FILE *) fp);
#endif
fp->file._chain = (_IO_FILE *) _IO_list_all; // fp 放到链表头部
_IO_list_all = fp; // 链表头 _IO_list_all 指向 fp
++_IO_list_all_stamp;
#ifdef _IO_MTSAFE_IO
_IO_funlockfile ((_IO_FILE *) fp);
run_fp = NULL;
_IO_lock_unlock (list_all_lock);
_IO_cleanup_region_end (0);
#endif
}
}
fread
// libio/iofread.c
_IO_size_t
_IO_fread (void *buf, _IO_size_t size, _IO_size_t count, _IO_FILE *fp)
{
_IO_size_t bytes_requested = size * count;
_IO_size_t bytes_read;
CHECK_FILE (fp, 0);
if (bytes_requested == 0)
return 0;
_IO_acquire_lock (fp);
bytes_read = _IO_sgetn (fp, (char *) buf, bytes_requested); // 调用 _IO_sgetn 函数
_IO_release_lock (fp);
return bytes_requested == bytes_read ? count : bytes_read / size;
}
// libio/genops.c
_IO_size_t
_IO_sgetn (_IO_FILE *fp, void *data, _IO_size_t n)
{
/* FIXME handle putback buffer here! */
return _IO_XSGETN (fp, data, n); // 调用宏 _IO_XSGETN
}
// libio/libioP.h
#define _IO_JUMPS_FILE_plus(THIS) \
_IO_CAST_FIELD_ACCESS ((THIS), struct _IO_FILE_plus, vtable)
#if _IO_JUMPS_OFFSET
# define _IO_JUMPS_FUNC(THIS) \
(*(struct _IO_jump_t **) ((void *) &_IO_JUMPS_FILE_plus (THIS) \
+ (THIS)->_vtable_offset))
# define _IO_vtable_offset(THIS) (THIS)->_vtable_offset
#else
# define _IO_JUMPS_FUNC(THIS) _IO_JUMPS_FILE_plus (THIS)
# define _IO_vtable_offset(THIS) 0
#endif
#define JUMP2(FUNC, THIS, X1, X2) (_IO_JUMPS_FUNC(THIS)->FUNC) (THIS, X1, X2)
#define _IO_XSGETN(FP, DATA, N) JUMP2 (__xsgetn, FP, DATA, N)
所以 _IO_XSGETN 宏最终会调用 vtable 中的函数，即：
// libio/fileops.c
_IO_size_t
_IO_file_xsgetn (_IO_FILE *fp, void *data, _IO_size_t n)
{
fwrite
// libio/iofwrite.c
_IO_size_t
_IO_fwrite (const void *buf, _IO_size_t size, _IO_size_t count, _IO_FILE *fp)
{
_IO_size_t request = size * count;
_IO_size_t written = 0;
CHECK_FILE (fp, 0);
if (request == 0)
return 0;
_IO_acquire_lock (fp);
if (_IO_vtable_offset (fp) != 0 || _IO_fwide (fp, -1) == -1)
written = _IO_sputn (fp, (const char *) buf, request); // 调用 _IO_sputn 函数
_IO_release_lock (fp);
/* We have written all of the input in case the return value indicates
this or EOF is returned. The latter is a special case where we
simply did not manage to flush the buffer. But the data is in the
buffer and therefore written as far as fwrite is concerned. */
if (written == request || written == EOF)
return count;
else
return written / size;
}
// libio/libioP.h
#define _IO_XSPUTN(FP, DATA, N) JUMP2 (__xsputn, FP, DATA, N)
#define _IO_sputn(__fp, __s, __n) _IO_XSPUTN (__fp, __s, __n)
_IO_XSPUTN 最终将调用下面的函数：
// libio/fileops.c
_IO_size_t
_IO_new_file_xsputn (_IO_FILE *f, const void *data, _IO_size_t n)
{
fclose
// libio/iofclose.c
int
_IO_new_fclose (_IO_FILE *fp)
{
int status;
CHECK_FILE(fp, EOF);
#if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
/* We desperately try to help programs which are using streams in a
strange way and mix old and new functions. Detect old streams
here. */
if (_IO_vtable_offset (fp) != 0)
return _IO_old_fclose (fp);
#endif
/* First unlink the stream. */
if (fp->_IO_file_flags & _IO_IS_FILEBUF)
_IO_un_link ((struct _IO_FILE_plus *) fp); // 将 fp 从链表中取出
_IO_acquire_lock (fp);
if (fp->_IO_file_flags & _IO_IS_FILEBUF)
status = _IO_file_close_it (fp); // 关闭目标文件
else
status = fp->_flags & _IO_ERR_SEEN ? -1 : 0;
_IO_release_lock (fp);
_IO_FINISH (fp);
if (fp->_mode > 0)
{
#if _LIBC
/* This stream has a wide orientation. This means we have to free
the conversion functions. */
struct _IO_codecvt *cc = fp->_codecvt;
__libc_lock_lock (__gconv_lock);
__gconv_release_step (cc->__cd_in.__cd.__steps);
__gconv_release_step (cc->__cd_out.__cd.__steps);
__libc_lock_unlock (__gconv_lock);
#endif
}
else
{
if (_IO_have_backup (fp))
_IO_free_backup_area (fp);
}
if (fp != _IO_stdin && fp != _IO_stdout && fp != _IO_stderr)
{
fp->_IO_file_flags = 0;
free(fp); // 释放 FILE 结构体
}
return status;
}
FSOP
FSOP（File Stream Oriented Programming）是一种劫持 _IO_list_all（libc.so中的全局变量） 来伪造链表的利用技术，通过调用 _IO_flush_all_lockp() 函数来触发，该函数会在下面三种情况下被调用：
- libc 检测到内存错误时
- 执行 exit 函数时
- main 函数返回时
当 glibc 检测到内存错误时，会依次调用这样的函数路径：malloc_printerr -> _libc_message -> abort -> _IO_flush_all_lockp。
// libio/genops.c
int
_IO_flush_all_lockp (int do_lock)
{
int result = 0;
struct _IO_FILE *fp;
int last_stamp;
#ifdef _IO_MTSAFE_IO
__libc_cleanup_region_start (do_lock, flush_cleanup, NULL);
if (do_lock)
_IO_lock_lock (list_all_lock);
#endif
last_stamp = _IO_list_all_stamp;
fp = (_IO_FILE *) _IO_list_all; // 将其覆盖为伪造的链表
while (fp != NULL)
{
run_fp = fp;
if (do_lock)
_IO_flockfile (fp);
if (((fp->_mode <= 0 && fp->_IO_write_ptr > fp->_IO_write_base) // 条件
#if defined _LIBC || defined _GLIBCPP_USE_WCHAR_T
|| (_IO_vtable_offset (fp) == 0
&& fp->_mode > 0 && (fp->_wide_data->_IO_write_ptr
> fp->_wide_data->_IO_write_base))
#endif
)
&& _IO_OVERFLOW (fp, EOF) == EOF) // 将其修改为 system 函数
result = EOF;
if (do_lock)
_IO_funlockfile (fp);
run_fp = NULL;
if (last_stamp != _IO_list_all_stamp)
{
/* Something was added to the list. Start all over again. */
fp = (_IO_FILE *) _IO_list_all;
last_stamp = _IO_list_all_stamp;
}
else
fp = fp->_chain; // 指向我们指定的区域
}
#ifdef _IO_MTSAFE_IO
if (do_lock)
_IO_lock_unlock (list_all_lock);
__libc_cleanup_region_end (0);
#endif
return result;
}
于是对 _IO_OVERFLOW(fp, EOF) 的调用会变成对 system('/bin/sh') 的调用。
防御机制
但是在 libc-2.24 中加入了对 vtable 指针的检查。这个 commit 新增了两个函数：IO_validate_vtable 和 _IO_vtable_check。
// libio/libioP.h
/* 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;
const char *ptr = (const char *) vtable;
uintptr_t offset = ptr - __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;
}
// libio/vtables.c
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");
}
所有的 libio vtables 被放进了专用的只读的 __libc_IO_vtables 段，以使它们在内存中连续。在任何间接跳转之前，vtable 指针将根据段边界进行检查，如果指针不在这个段，则调用函数 _IO_vtable_check() 做进一步的检查，并且在必要时终止进程。
新的利用技术
在防御机制下通过修改虚表的利用技术已经用不了了，但同时出现了新的利用技术。既然无法将 vtable 指针指向 __libc_IO_vtables 以外的地方，那么就在 __libc_IO_vtables 里面找些有用的东西。比如 _IO_str_jumps：
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)
};
这个 vtable 中包含了一个叫做 _IO_str_overflow 的函数，该函数中存在相对地址的引用（可伪造）：
int
_IO_str_overflow (_IO_FILE *fp, int c)
{
int flush_only = c == EOF;
_IO_size_t pos;
if (fp->_flags & _IO_NO_WRITES)
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))
{
if (fp->_flags & _IO_USER_BUF) /* not allowed to enlarge */
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; // new_size 的计算方法
if (new_size < old_blen)
return EOF;
new_buf
= (char *) (*((_IO_strfile *) fp)->_s._allocate_buffer) (new_size); // fp 上的相对地址
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;
}
构造的条件如下：
- _flags = 0
- _IO_buf_base = 0
- _IO_buf_end = (bin_sh - 100) / 2
- _IO_write_ptr = 0x7fffffffffffffff
- _IO_write_base = 0
CTF 实例
请查看章节 6.1.24、6.1.25 和 6.1.26。另外在章节 3.1.8 中也有相关内容。