03 Oct 09 Linux C setuid setgid tutorial
Here’s a very brief example of how to use setuid() and setgid() functions in your C program.
#include <sys/types.h>
#include <unistd.h>
int main(void)
{
int current_uid = getuid();
printf(“My UID is: %d. My GID is: %d\n”, current_uid, getgid());
system(“/usr/bin/id”);
if (setuid(0))
{
perror(“setuid”);
return 1;
}
//I am now root!
printf(“My UID is: %d. My GID is: %d\n”, getuid(), getgid());
system(“/usr/bin/id”);
//Time to drop back to regular user priviledges
setuid(current_uid);
printf(“My UID is: %d. My GID is: %d\n”, getuid(), getgid());
system(“/usr/bin/id”);
return 0;
}
The program above should be pretty self explainatory, now:
adam@staging:~$ ls -al setuid
-rwxr-xr-x 1 adam adam 9792 2009-10-03 18:09 setuid
adam@staging:~$
Trying to run the application now will result in:
My UID is: 1000. My GID is: 1000
uid=1000(adam) gid=1000(adam) groups=20(dialout),24(cdrom),25(floppy),29(audio),44(video),46(plugdev),1000(adam)
setuid: Operation not permitted
The setuid(0) call fails, as the application does not have permission to gain root access.
Password:
staging:~# cd /home/adam
staging:/home/adam# chown root.root setuid
staging:/home/adam# chmod +s setuid
staging:/home/adam# ls -al setuid
-rwsr-sr-x 1 root root 9792 2009-10-03 18:09 setuid
staging:/home/adam# exit
logout
adam@staging:~$
And now:
My UID is: 1000. My GID is: 1000
uid=1000(adam) gid=1000(adam) euid=0(root) egid=0(root) groups=20(dialout),24(cdrom),25(floppy),29(audio),44(video),46(plugdev),1000(adam)
My UID is: 0. My GID is: 1000
uid=0(root) gid=1000(adam) egid=0(root) groups=20(dialout),24(cdrom),25(floppy),29(audio),44(video),46(plugdev),1000(adam)
My UID is: 1000. My GID is: 1000
uid=1000(adam) gid=1000(adam) egid=0(root) groups=20(dialout),24(cdrom),25(floppy),29(audio),44(video),46(plugdev),1000(adam)
adam@staging:~$
Works just as expected!
Now:
We don’t want to leave things like that lying around…
Tags: C, gcc, Linux, setgid, setuid, System
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I'm Adam Palmer, and I'm a PHP Programmer, Web Security Consultant, and Linux Consultant.
I run 'APNIC Solutions Ltd' and spend most of my time consulting, managing server clusters, and writing PHP, whilst what little spare time I have is dedicated to playing with gadgets.
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