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What Are Linux File Permissions?

Setuid, Setgid and Sticky Bits are special types of Unix/Linux file permission sets that permit certain users to run specific programs with elevated privileges. Ultimately the permissions that are set on a file determine what users can read, write or execute the file. Linux provides more advanced file permissions that allow you to do more specific things with a file, or directory.  Typically, these file permissions are used to allow a user to do certain tasks with elevated privileges (allow them to do things they normally are not permitted to do). This is accomplished with three distinct permission settings.  They are setuid, setgid, and the sticky bit.

Illustrated Linux Permission Structure

This article is intended for a user or administrator who already has a good working knowledge of the basic Linux file permission system. If you are unfamiliar with this topic, we have an excellent tutorial on Linux file permissions. Let’s begin by reviewing each of these permission sets and how they interact with our system.

Symbolic Permission Notation

--- no permission
--x execute
-w- write
-wx write and execute
r-- read
r-x read and execute
rw- read and write
rwx read, write and execute

Numeric Permission Notation

The numeric notation system uses the digits 1 through 7, with each corresponding to a different symbolic permission set:

0 --- no permission
1 --x execute
2 -w- write
3 -wx write and execute
4 r-- read
5 r-x read and execute
6 rw- read and write
7 rwx read, write and execute

What Each Type Controls

Read: Display the contents of a file. You may be able to open it in text editors like Vim, but you will not be able to save any changes to the file unless you also have write permissions.

Write: Make changes to a file or folder/directory.

Execute: Execute (a.k.a. Run) a file. Examples include compiled binaries and shell scripts.

What is Setuid?

Setuid is a Linux file permission setting that allows a user to execute that file or program with the permission of the owner of that file. This is primarily used to elevate the privileges of the current user. If a file is “setuid” and is owned by the user “root” then a user that has the ability to execute that program will do so as the user root instead of themselves. The most common example of this in Linux is ‘sudo’. In this example, the user ‘test’ located the executable ‘sudo’ and did a full listing of it with the ‘ls -l’ command.

root@host [~]# id
uid=1002(test) gid=1002(test) groups=1002(test)

root@host [~]# which sudo

root@host [~]# ls -l /usr/bin/sudo
-rwsr-xr-x 1 root root 136808 Jan 31 13:37 /usr/bin/sudo

root@host [~]#  

If you look at the permissions level of the ‘sudo’ executable, you can see the ‘s’ in the permissions for the user where normally there would be an ‘x’.  Also, notice that this file is owned by the user ‘root’ (the super-user) and that the file is executable by the world (the last ‘x’ in the permissions).  This indicates that when a user executes this program, the operating system will execute that file not as the user ‘test’, but as the user ‘root'. In the matter of using the ‘sudo’ command, this allows a normal user to perform elevated system functions without having to log in as the root user.

How Do I Set Up Setuid?

Setting the ‘setuid’ permission is as simple as setting any other permission in Linux. The file ownership is modified using the command. An example command to set this would be as follows.

root@host [~]# chmod u+s <filename>

In this example, we will create a file called ‘myfile’ using the command ‘touch’ and then we will examine its permissions with the ‘ls -l' command.

root@host [~]# touch myfile
root@host [~]# ls -l myfile
-rw-rw-r-- 1 test test 0 Mar 2 17:59 myfile
root@host [~]# 

Notice that the file does not have the execute permissions for user, group, or world. We will add the setuid bit as seen below.

root@host [~]# chmod u+s myfile 
root@host [~]# ls -l myfile
-rwSrw-r-- 1 test test 0 Mar 2 17:59 myfile
root@host [~]# 

This output looks a little different from what we were expecting. The lowercase ‘s’ we were looking for is the now a capital ‘S.’ This signifies that the setuid IS set, but the user that owns the file does not have execute permissions. We can add that permission using the ‘chmod u+x’ command.

root@host [~]#  chmod u+x myfile 
root@host [~]#  ls -l 
total 0
-rwsrw-r-- 1 test test 0 Mar 2 17:59 myfile
root@host [~]#  

What is Setgid?

Setgid, when used on files, is very similar to setuid. A process, when executed, will run as the group that owns the file. A typical example of a file that uses this is the ‘crontab’ command.

root@host [~]#  which crontab
root@host [~]#  ls -l /usr/bin/crontab
-rwxr-sr-x 1 root crontab 36080 Apr 5 2016 /usr/bin/crontab
root@host [~]#  

How Do I Set Up Setgid?

Similar to ‘setuid,’ ‘setgid’ is inserted with the ‘chmod g+s’ command. Let's create a new file called ‘myfile2’.

root@host [~]#  touch myfile2
root@host [~]#  ls -l myfile2
-rw-rw-r-- 1 test test 0 Mar 2 19:30 myfile2
root@host [~]# 

Now we will run the 'chmod g+s' command and review the results.

root@host [~]#  chmod g+s myfile2
root@host [~]#  ls -l myfile2
-rw-rwSr-- 1 test test 0 Mar 2 19:30 myfile2
root@host [~]# 

Again we see the capital ‘S’ is set, but we can modify that.

root@host [~]#  chmod g+x myfile2
root@host [~]#  ls -l
total 0
-rwsrw-r-- 1 test test 0 Mar 2 17:59 myfile
-rw-rwsr-- 1 test test 0 Mar 2 19:30 myfile2
root@host [~]#  

Setgid on Directories

Applying the setgid permission on a directory has as different behavior. A directory that has ‘setgid’ on it will cause all files that are created in that directory to be owned by the group of the directory as opposed to the group of the owner. First, we create a directory.

root@host [~]#  mkdir mydir
root@host [~]#  ls -ld mydir
drwxrwxr-x 2 test test 4096 Mar 2 19:36 mydir
root@host [~]#  

Then we change the group ownership of the directory by using the 'chgrp' command, and then we can add the ‘setgid’ permission like before.

root@host [~]#  chgrp test2 mydir/
root@host [~]#  chmod g+s mydir
root@host [~]#  ls -ld mydir/
drwxrwsr-x 2 test test2 4096 Mar 2 19:36 mydir/
root@host [~]# 

Let's test it out by creating a file in that directory. All other files in this tutorial were created this way and had ‘test’ as the group. Because ‘setgid’ is set on the directory and it is owned by group ‘test2’, this file will get ‘test2’ as its group.

root@host [~]#  touch mydir/myfile3
root@host [~]#  ls -l mydir/myfile3
-rw-rw-r-- 1 test test2 0 Mar 2 19:59 mydir/myfile3
root@host [~]#

What Is A Sticky Bit?

The final special permission is the ‘sticky bit.’ When this is set on a directory, the files in that directory can only be removed by the owner. A typical use of this is ‘/tmp/.’ The /tmp directory can be written to by any user, but other users cannot delete the files of others. 

root@host [~]#  ls -ld /tmp
drwxrwxrwt 8 root root 4096 Mar 2 20:17 /tmp
root@host [~]#

Notice that /tmp can be written to by everyone but has the ‘t’ in place of the ‘x’ at the end of the permissions list. This means it has the sticky bit.

How Do I Set Up A Sticky Bit?

The sticky bit is set with ‘chmod +t’ command.

root@host [~]#  mkdir mydir2
root@host [~]#  ls -ld mydir2
drwxrwxr-x 2 test test 4096 Mar 2 20:17 mydir2
root@host [~]#  chmod +t mydir2 
root@host [~]#  ls -ld mydir2
drwxrwxr-t 2 test test 4096 Mar 2 20:17 mydir2
root@host [~]#

Setting Special Permissions With Number Notation

You may remember from the definitions above that permissions can be set with a series of three numbers. The numbers represent the permissions for owner, group, and world, respectively. To determine the number you want to set, you can use x=1, w=2, and r=4. You add the numbers together to get the permission number. If we wanted to have read, write, and execute permissions, we would use 7. Read and write would be 6. Just read is 4. An example to set the file to read, write, and execute for owner, read and execute for group and world would look like this:

root@host [~]#  chmod 755 myfile
root@host [~]#  ls -l myfile
-rwxr-xr-x 1 test test 0 Mar 2 17:59 myfile
root@host [~]#  

For the special permissions, you prepend these numbers with another number where 4 is setuid, 2 is setgid, and 1 is the sticky bit. The following commands are all the same (assuming the file has the permissions we set above).

root@host [~]#  chmod 4755 myfile
root@host [~]#  chmod u+s myfile
root@host [~]#  ls -l myfile 
-rwsr-xr-x 1 test test 0 Mar 2 17:59 myfile
root@host [~]#

root@host [~]#  chmod 2755 myfile
root@host [~]#  chmod g+s myfile
root@host [~]#  ls -l myfile
-rwxr-sr-x 1 test test 0 Mar 2 17:59 myfile
root@host [~]#  

root@host [~]#  chmod 1755 mydir
root@host [~]#  chmod +t mydir
root@host [~]#  ls -ld mydir
drwxr-sr-t 2 test test2 4096 Mar 2 19:59 mydir
root@host [~]#  

Removing Special Permissions

To remove special permissions, we can use the same chmod commands with a ‘-’ instead of a ‘+.’

root@host [~]#  chmod u-s myfile
root@host [~]#  chmod g-s mydir
root@host [~]#  chmod -t mydir2
root@host [~]#  ls -l
total 8
drwxr-xr-x 2 test test2 4096 Mar 2 19:59 mydir
drwxrwxr-x 2 test test 4096 Mar 2 20:17 mydir2
-rwxr-xr-t 1 test test 0 Mar 2 17:59 myfile
root@host [~]#  


All in all, these special permissions are very useful for separating the ability of a user or group to read, write or execute a file or affect a change on a folder.

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About the Author: Matt Holtz

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