Suid root seems not to work - Getting SUIDcidal ;-)

Greetings!

As I'm using Kanotix (which doesn't seem to have such an extensive forum) and it's based on Knoppix, I figure I'll post this question here.

For a program which will use the parallel port for digital I/O, I'm trying to get access to the port WITHOUT being root (no sudo, either). For this, I've compiled a simple program in C, but it keeps complaining it has no permissions to access the port (/dev/parport0). Port permissions are as follows:

Code:

---

crw-rw-rw-  1 root root 99, 0 Feb  9 19:40 /dev/parport0

---

The program has been compiled using gcc -O1 -o test test.c, after which I've issued (as root):

Code:

---

chown root:users test
chmod u+s test

---

I figured it should work now, but it doesn't. Keeps complaining about permissions.

Furthermore I've tried creating a file foo.txt and done chmod 600 foo.txt. Then made an executable script bar.sh, which just cat's foo.txt. Then:

Code:

---

chown root:users bar.sh
chmod u+s bar.sh

---

Executing bar.sh also gives permission errors. So in other words: what gives? I'm stumped, but I've probably misunderstood how suid root works. Any help would be greatly appreciated.

Thanks in advance,

Marc.

Well, that wasn't too hard. After putting my program in a directory in the path, all seems well. OTOH, it doesn't work when it's in a directory that only belongs to my personal path. Does it have to be in the root's path somewhere?

That's weird, man. :shock:

Quote:

---

Originally Posted by mvandoornik

Well, that wasn't too hard. After putting my program in a directory in the path, all seems well. OTOH, it doesn't work when it's in a directory that only belongs to my personal path. Does it have to be in the root's path somewhere?

---

If Kanotix is similar Knoppix I guess the $PATH variable for root has a different (more restricted) set of paths to a normal user. This is partly for security.
This can be tested by

Code:

---

$PATH

---

followed by <enter> at a terminal window. Do this for root and for a normal user and see if this helps explain what has happened.

I think you meant:
echo $PATH

Quote:

---

Originally Posted by markpreston

If Kanotix is similar Knoppix I guess the $PATH variable for root has a different (more restricted) set of paths to a normal user. This is partly for security.
This can be tested by

Code:

---

$PATH

---

followed by <enter> at a terminal window. Do this for root and for a normal user and see if this helps explain what has happened.

---

> Executing bar.sh also gives permission errors.

The suid permission bit is ignored for scripts for security reasons. Suid for "true" executables only give the *effective* UID,
not the *real* UID. This can make a difference, e.g. as in cpio.

Hi Dave,
$PATH alone works as well for this.
mark@z:~$ $PATH
bash: /home/mark/.dist/bin:/sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin:/usr/local/sbin:/usr/local/bin:/usr/games:.: No such file or directory
mark@z:~$
mark@z:~$ su
Password:
root@z:/home/mark# $PATH
bash: /sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin:/usr/local/sbin:/usr/local/bin:/usr/games: No such file or directory
root@z:/home/mark#

Quote:

---

I think you meant:
echo $PATH

---

--Yes, but it's not proper *nix form to try and _execute_ your PATH variable. Somebody could attack you that way. :twisted:

Quote:

---

Originally Posted by markpreston

Hi Dave,
$PATH alone works as well for this.
mark@z:~$ $PATH
bash: /home/mark/.dist/bin:/sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin:/usr/local/sbin:/usr/local/bin:/usr/games:.: No such file or directory
mark@z:~$
mark@z:~$ su
Password:
root@z:/home/mark# $PATH
bash: /sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin:/usr/local/sbin:/usr/local/bin:/usr/games: No such file or directory
root@z:/home/mark#

Quote:

---

I think you meant:
echo $PATH

---

---

Quote:

---

Originally Posted by Dave_Bechtel

--Yes, but it's not proper *nix form to try and _execute_ your PATH variable. Somebody could attack you that way. :twisted:

---

Thanks Dave,
At least I now know what you were concerned about.
Regards,
Mark