3.4. File security

3.4.1. Access rights: Linux's first line of defense

The Linux security model is based on the one used on UNIX systems, and is as rigid, and in some cases even more, as the UNIX security model, which is already quite robust. On a Linux system, every file is owned by a user and a group user. There is also a third category of users, those that are not the user owner and don't belong to the group owning the file. For each category of users, read, write and execute permissions can be granted or denied.

We already used the long option to list files using the ls -l command, though for other reasons. This command also displays file permissions for these three user categories; they are indicated by the nine characters that follow the first character, which is the file type indicator at the beginning of the file properties line. As seen in the examples below, the first three characters in this series of nine display access rights for the actual user that owns the file. The next three are for the group owner of the file, the last three for other users. The permissions are always in the same order: read, write, execute for the user, the group and the others. Some examples:

marise:~>ls -l To_Do
-rw-rw-r--    1 marise  users      5 Jan 15 12:39 To_Do
marise:~>ls -l /bin/ls
-rwxr-xr-x    1 root    root   45948 Aug  9 15:01 /bin/ls*

The first file is a regular file (first dash). Users with user name marise or users belonging to the group users can read and write (change/move/delete) the file, but they can't execute it (second and third dash). All other users are only allowed to read this file, but they can't write or execute it (fourth and fifth dash).

The second example is an executable file, the difference: everybody can run this program, but you need to be root to change it.

The Info pages explain how the ls command handles display of access rights in detail, see the section What information is listed.

For easy use with commands, both access rights or modes and user groups have a code. See the tables below.

Table 3-7. Access mode codes

0 or -The access right that is supposed to be on this place is not granted.
4 or rread access is granted to the user category defined in this place
2 or wwrite permission is granted to the user category defined in this place
1 or xexecute permission is granted to the user category defined in this place

Table 3-8. User group codes

uuser permissions
ggroup permissions
opermissions for others

This straight forward scheme is applied very strictly, which allows a high level of security even without network security. Among other functions, the security scheme takes care of user access to programs, it can serve files on a need-to-know basis and protect sensitive data such as home directories and system configuration files.

You should know what your user name is. If you don't, it can be displayed using the id command, which also displays the default group you belong to and eventually other groups of which you are a member:

uid=504(tilly) gid=504(tilly) groups=504(tilly),100(users),2051(org)

Your user name is also stored in the environment variable USER:

tilly:~>echo $USER

3.4.2. The tools The chmod command

A normal consequence of applying strict file permissions, and sometimes a nuisance, is that access rights will need to be changed for all kinds of reasons. We use the chmod command to do this, and eventually to chmod has become an almost acceptable English verb, meaning the changing of the access mode of a file. The chmod command can be used with alphanumeric or numeric options, whatever you like best.

The example below uses alphanumeric options in order to solve a problem that commonly occurs with new users:

bash: ./hello: bad interpreter: Permission denied

asim:~>cat hello
echo "Hello, World"

asim:~>ls -l hello
-rw-rw-r--    1 asim    asim    32 Jan 15 16:29 hello

asim:~>chmod u+x hello

Hello, World

asim:~>ls -l hello
-rwx-rw-r--   1 asim    asim    32 Jan 15 16:29 hello*

The + and - operators are used to grant or deny a given right to a given group. Combinations separated by commas are allowed. The Info and man pages contain useful examples. Here's another one, which makes the file from the previous example a private file to user asim:

asim:~>chmod u+rwx,go-rwx hello

asim:~>ls -l hello
-rwx------    1 asim    asim    32 Jan 15 16:29 hello*

The kind of problem resulting in an error message saying that permission is denied somewhere is usually a problem with access rights in most cases. Also, comments like, "It worked yesterday," and "When I run this as root it works," are most likely caused by the wrong file permissions.

When using chmod with numeric arguments, the values for each granted access right have to be counted together per group. Thus we get a 3-digit number, which is the symbolic value for the settings chmod has to make. The following table lists the most common combinations:

Table 3-9. File protection with chmod

chmod 400 fileTo protect a file against accidental overwriting.
chmod 500 directoryTo protect yourself from accidentally removing, renaming or moving files from this directory.
chmod 600 fileA private file only changeable by the user who entered this command.
chmod 644 fileA publicly readable file that can only be changed by the issuing user.
chmod 660 fileUsers belonging to your group can change this files, others don't have any access to it at all.
chmod 700 fileProtects a file against any access from other users, while the issuing user still has full access.
chmod 755 directoryFor files that should be readable and executable by others, but only changeable by the issuing user.
chmod 775 fileStandard file sharing mode for a group.
chmod 777 fileEverybody can do everything to this file.

If you enter a number with less than three digits as an argument to chmod, omitted characters are replaced with zeros starting from the left. There is actually a fourth digit on Linux systems, that precedes the first three and sets special access modes. Everything about these and many more are located in the Info pages. The file mask

When a new file is saved somewhere, it is first subjected to the standard security procedure. Files without permissions don't exist on Linux. The standard file permission is determined by the mask for new file creation. The value of this mask can be displayed using the umask command:


Instead of adding the symbolic values to each other, as with chmod, for calculating the permission on a new file they need to be subtracted from the total possible access rights. In the example above, however, we see 4 values displayed, yet there are only 3 permission categories: user, group and other. The first zero is part of the special file attributes settings, which we will discuss in Section and Section 4.1.6. It might just as well be that this first zero is not displayed on your system when entering the umask command, and that you only see 3 numbers representing the default file creation mask.

Each UNIX-like system has a system function for creating new files, which is called each time a user uses a program that creates new files, for instance, when downloading a file from the Internet, when saving a new text document and so on. This function creates both new files and new directories. Full read, write and execute permission is granted to everybody when creating a new directory. When creating a new file, this function will grant read and write permissions for everybody, but set execute permissions to none for all user categories. This, before the mask is applied, a directory has permissions 777 or rwxrwxrwx, a plain file 666 or rw-rw-rw-.

The umask value is subtracted from these default permissions after the function has created the new file or directory. Thus, a directory will have permissions of 775 by default, a file 664, if the mask value is (0)002. This is demonstrated in the example below:

bert:~> mkdir newdir

bert:~> ls -ld newdir
drwxrwxr-x    2 bert    bert		4096 Feb 28 13:45 newdir/

bert:~> touch newfile

bert:~> ls -l newfile
-rw-rw-r--    1 bert    bert		   0 Feb 28 13:52 newfile

The root user usually has stricter default file creation permissions:

[root@estoban root]# umask

These defaults are set system-wide in the shell resource configuration files, for instance /etc/bashrc or /etc/profile. You can change them in your own shell configuration file, see Chapter 7 on customizing your shell environment. Changing user and group ownership

When a file is owned by the wrong user or group, the error can be repaired with the chown (change owner) and chgrp (change group) commands. Changing file ownership is a frequent task in environments where files need to be shared in a group. Both commands are very flexible, as you can find out by using the --help option.

The chown command can be applied to change both user and group ownership of a file, while chgrp only changes group ownership. Of course the system will check if the user issuing one of these commands has sufficient permissions on the file(s) she wants to change.

In order to only change the user ownership of a file, use this syntax:

chown newuser file

If you use a colon after the user name (see the Info pages), group ownership will be changed as well, to the primary group of the user issuing the command. On a Linux system, each user has his own group, so this form can be used to make files private:

uid=1304(jacky) gid=(1304) groups=1304(jacky),2034(pproject)

jacky:~>ls -l my_report
-rw-rw-r--  1 jacky   project       29387 Jan 15 09:34 my_report

jacky:~>chown jacky: my_report

jacky:~>chmod o-r my_report

jacky:~>ls -l my_report
-rw-rw----  1 jacky   jacky         29387 Jan 15 09:34 my_report

If jacky would like to share his file again, he can use the chgrp command:

jacky:~>ls -l report-20020115.xls
-rw-rw-rw- 1 jacky   jacky   45635 Jan 15 09:35 report-20020115.xls

jacky:~>chgrp project report-20020115.xls

jacky:~>chmod o= report-20020115.xls

jacky:~>ls -l report-20020115.xls
-rw-rw---- 1 jacky   project 45635 Jan 15 09:35 report-20020115.xls

Both chown and chgrp can be used to change ownership recursively, using the -R option. In that case, all underlying files and subdirectories of a given directory will belong to the given user and/or group. Special modes

For the system admin to not be bothered solving permission problems all the time, special access rights can be given to entire directories, or to separate programs. There are three special modes:

  • Sticky bit mode: After execution of a job, the command is kept in the system memory. Originally this was a feature used a lot to save memory, but these days memory is inexpensive, so it is not used anymore for its optimizing capabilities on single files. When applied to an entire directory, however, the sticky bit has a different meaning. In that case, a user can only change files in this directory when she is the user owner of the file or when the file has appropriate permissions. This feature is used on directories like /var/tmp, that have to be accessible for everyone, but where it is not appropriate for users to change or delete each other's data. The sticky bit is indicated by a t at the end of the file permission field:

mark:~>ls -ld /var/tmp
    drwxrwxrwt   19 root     root         8192 Jan 16 10:37 /var/tmp/

    The sticky bit is set using the command chmod o+t directory. The historic origin of the "t" is in UNIX' save Text access feature.

  • SUID (set user ID) and SGID (set group ID): represented by the character s in the user or group permission field. When this mode is set on an executable file, it will run with the user and group permissions on the file instead of with those of the user issuing the command, thus giving access to system resources. We will discuss this further in Chapter 4.

  • SGID (set group ID) on a directory: in this special case every file created in the directory will have the same group owner as the directory itself (while normal behavior would be that new files are owned by the users who create them). This way, users don't need to worry about file ownership when sharing directories:

mimi:~>ls -ld /opt/docs
    drwxrws---  4 root    users          4096 Jul 25 2001 docs/
    mimi:~>ls -l /opt/docs
    -rw-rw----  1 mimi    users        345672 Aug 30 2001-Council.doc

    This is the standard way of sharing files in UNIX.

    NoteExisting files are left unchanged!

    Files that are being moved to a SGID directory but were created elsewhere keep their original user and group owner. This may be confusing.