//csdn博客目前暂时不再更新了，有兴趣请访问我的技术博客-晓的博客：zhangxiaolong.org
写这个blog主要是对自己学习的内容进行一个总结和记录，希望对学习这些知识的同道之人有所帮助，第一次写博文，经验有所不足，我会慢慢练习。
实验操作我是参照了一位比较厉害的师兄，他的set_uid blog：http://blog.sina.com.cn/s/blog_70dd16910100pz8k.html。希望对大家有所帮助。
这个实验是信息安全实验的第一个实验，共有7个task，每一个task都有截图作为步骤。
1. Figure out why "passwd", "chsh", "su", and "sudo" commands need to be Set-UIDprograms. What will happen if they are not? If you are not familiar with these programs, you should first learn what
they can do by reading their manuals. Please copy these commands to your owndirectory; the copies will not be Set-UID programs. Run the copied programs, and observe whathappens.
实验截图如下：
图 1
图 2
由图1，图2所示：拷贝到/home/seed下的passwd程序，没有了root权限，这样就没有了修改密码的权限。同样chsh su等等同样的道理。
2. Run Set-UID shell programs in Linux, and describe and explain your observations.
(a) Login as root, copy /bin/zsh to /tmp, and make it a set-root-uid program with permission4755. Then login as a normal user, and run /tmp/zsh. Will you get root privilege? Please describe your observation. If you
cannot find /bin/zsh in your operating system, please use the following command to install it:_ Note: in our pre-built Ubuntu VM image, zsh is already installed.
su
Password: (enter root password)
yum install zsh
For Ubuntu
$ su
Password: (enter root password)
apt-get install zsh
图 3
由图3示，可以获得root权限
(b) Instead of copying /bin/zsh, this time, copy /bin/bash to /tmp, make it a set-root-uidprogram. Run /tmp/bash as a normal user. will you get root privilege? Please describe andexplain your observation.
图 4
由图4示获得不了root权限，，从实验中可以看出/bin/bash有某种内在的保护机制可以阻止Set-UID机制的滥用。
3. (Setup for the rest of the tasks) As you can find out from the previous task, /bin/bash has certainbuilt-in protection that prevent the abuse of the Set-UID mechanism. To see the life before such
aprotection scheme was implemented, we are going to use a different shell program called /bin/zsh.In some Linux distributions (such as Fedora and Ubuntu), /bin/sh is actually a symbolic linkto /bin/bash. To use zsh, we need to link /bin/sh to /bin/zsh. The
following instructionsdescribe how to change the default shell to zsh.
$ su
Password: (enter root password)
# cd /bin
# rm sh
# ln -s zsh sh
图 5
4. The PATH environment variable.The system(const char *cmd) library function can be used to execute a command withina program. The way system(cmd) works is to invoke the /bin/sh program, and then
let theshell program to execute cmd. Because of the shell program invoked, calling system() within aSet-UID program is extremely dangerous. This is because the actual behavior of the shell programcan be affected by environment variables, such as PATH; these
environment variables are under user’scontrol. By changing these variables, malicious users can control the behavior of the Set-UIDprogram.The Set-UID program below is supposed to execute the /bin/ls command; however, the programmeronly uses the relative path
for the ls command, rather than the absolute path:
int main()
{
system("ls");
return 0;
}
(a) Can you let this Set-UID program (owned by root) run your code instead of /bin/ls? If you can, is your code running with the root privilege? Describe and explain your observations.
图 6
可以获得，在root权限下：
（1）首先以root权限编译如下程序，并将该程序设置为SUID。
int main()
{
system("ls");
return 0;
}
在普通用户模式下：
（2）拷贝sh到/tmp，并命名为ls。
（3）修改环境变量为/tmp。
（4）运行root用户编写的SUID程序。
结果：用/usr/bin/id命令查询用户eid变为0。
(b) Now, change /bin/sh so it points back to /bin/bash, and repeat the above attack. Can youstill get the root privilege? Describe and explain your observations.
不能够获得root权限，因为bash存在某种内部的安全机制。
5. The difference between system() and execve(). Before you work on this task,please make sure that /bin/sh is pointed to /bin/zsh.Background: Bob works for an auditing agency, and he needs to investigate
a company for a suspectedfraud. For the investigation purpose, Bob needs to be able to read all the files in the company’sUnix system; on the other hand, to protect the integrity of the system, Bob should not be able tomodify any file. To achieve this goal,
Vince, the superuser of the system, wrote a special set-root-uidprogram (see below), and then gave the executable permission to Bob. This program requires Bob totype a file name at the command line, and then it will run /bin/cat to display the specified file.Since
the program is running as a root, it can display any file Bob specifies. However, since the programhas no write operations, Vince is very sure that Bob cannot use this special program to modify any file.
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
char *v[3];
if(argc < 2) {
printf("Please type a file name.\n");
return 1;
}
v[0] = "/bin/cat"; v[1] = argv[1]; v[2] = 0;
/* Set q = 0 for Question a, and q = 1 for Question b */
int q = 0;
if (q == 0){
char *command = malloc(strlen(v[0]) + strlen(v[1]) + 2);
sprintf(command, "%s %s", v[0], v[1]);
system(command);
}
else execve(v[0], v, 0);
return 0 ;
}
(a) Set q = 0 in the program. This way, the program will use system() to invoke the command.Is this program safe? If you were Bob, can you compromise the integrity of the system? Fo rexample, can you remove any
file that is not writable to you? (Hint: remember that system()actually invokes /bin/sh, and then runs the command within the shell environment. We havetried the environment variable in the previous task; here let us try a different attack. Please pay attention
to the special characters used in a normal shell environment).
这个命令不安全，Bob可能会出去好奇或者个人利益驱使会阅读或者修改只有root用户才可以运行的一些文件，如图所示。
图 7
(b) Set q = 1 in the program. This way, the program will use execve() to invoke the command.Do your attacks in task (a) still work? Please describe and explain your observations.
不会有效，在(a)中之所以有效，是具有root权限的system在执行了cat file文件后，还会接着执行mv file file_new命令。而当令q=1, execve()函数会把file; mv file file_new 看成是一个文件名，系统会提示不存在这个文件。示意图如下：
图 8
6. The LD PRELOAD environment variable.
To make sure Set-UID programs are safe from the manipulation of the LD PRELOAD environmentvariable, the runtime linker (ld.so) will ignore this environment variable if the program is aSet-UID root program, except
for some conditions. We will figure out what these conditions are inthis task.
(a) Let us build a dynamic link library. Create the following program, and name it mylib.c. Itbasically overrides the sleep() function in libc:
#include <stdio.h>
void sleep (int s)
{
printf("I am not sleeping!\n");
}
(b) We can compile the above program using the following commands (in the -W1 argument, thethird character is one, not `; in the -lc argment, the second character is `):
% gcc -fPIC -g -c mylib.c
% gcc -shared -W1,-soname,libmylib.so.1 \
-o libmylib.so.1.0.1 mylib.o –lc
(c) Now, set the LD PRELOAD environment variable:% export LD_PRELOAD=./libmylib.so.1.0.1
(d) Finally, compile the following program myprog (put this program in the same directory as libmylib.so.1.0.1):
/* myprog.c */
int main()
{
sleep(1);
return 0;
}
Please run myprog under the following conditions, and observe what happens. Based on your observations,tell us when the runtime linker will ignore the LD PRELOAD environment variable, andexplain why.
_ Make myprog a regular program, and run it as a normal user.
在这种情况下，忽略LD_PRELOAD环境变量，不重载sleep函数，使用系统自带的sleep函数。
图 9
_ Make myprog a Set-UID root program, and run it as a normal user.
在这种情况下，使用LD_PRELOAD环境变量，使用重载的sleep函数。
图 10
_ Make myprog a Set-UID root program, and run it in the root account.
在这种情况下，忽略LD_PRELOAD环境变量。
图 11
图 12
图 13
_ Make myprog a Set-UID user1 program (i.e., the owner is user1, which is another user account),and run it as a different user (not-root user).
7. Relinquishing privileges and cleanup.
To be more secure, Set-UID programs usually call setuid() system call to permanently relinquishtheir root privileges. However, sometimes, this is not enough. Compile the following program,and make the program a set-root-uid
program. Run it in a normal user account, and describe what youhave observed. Will the file /etc/zzz be modified? Please explain your observation.
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
void main()
{ int fd;
/* Assume that /etc/zzz is an important system file,
and it is owned by root with permission 0644 */
fd = open("/etc/zzz", O_RDWR | O_APPEND);
/* Simulate the tasks conducted by the program */
sleep(1);
/* After the task, the root privileges are no longer needed,
it’s time to relinquish the root privileges permanently. */
setuid(getuid()); /* getuid() returns the real uid */
if (fork()) { /* In the parent process */
close (fd);
exit(0);
} else { /* in the child process */
/* Now, assume that the child process is compromised, malicious
attackers have injected the following statements
into this process */
write (fd, "Malicious Data", 14);
close (fd);
}
}
图 14
图 15
图 16