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NOTE: CentOS Enterprise Linux 5 is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux 5 is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux 5.

Chapter 44. Working With SELinux

SELinux presents both a new security paradigm and a new set of practices and tools for administrators and some end-users. The tools and techniques discussed in this chapter focus on standard operations performed by end-users, administrators, and analysts.

44.1. End User Control of SELinux

In general, end users have little interaction with SELinux when Red Hat Enterprise Linux is running the targeted policy. This is because users are running in the domain of unconfined_t along with the rest of the system except the targeted daemons.

In most situations, standard DAC controls prevent you from performing tasks for which you do not have the required access or permissions before SELinux is consulted. Consequently, it is likely that you will never generate an avc: denied message.

The following sections cover the general tasks and practices that an end user might need to perform on a Red Hat Enterprise Linux system. These tasks apply to users of all privilege levels, not only to end users.

44.1.1. Moving and Copying Files

In file system operations, security context must now be considered in terms of the label of the file, the process accessing it, and the directories where the operation is happening. Because of this, moving and copying files with mv and cp may have unexpected results.

Copying Files: SELinux Options for cp

Unless you specify otherwise, cp follows the default behavior of creating a new file based on the domain of the creating process and the type of the target directory. Unless there is a specific rule to set the label, the file inherits the type from the target directory.

Use the -Z user:role:type option to specify the required label for the new file.

The -p (or --preserve=mode,ownership,timestamps) option preserves the specified attributes and, if possible, additional attributes such as links.

touch bar foo
ls -Z bar foo
-rw-rw-r--  auser   auser   user_u:object_r:user_home_t   bar
-rw-rw-r--  auser   auser   user_u:object_r:user_home_t   foo

If you use the cp command without any additional command-line arguments, a copy of the file is created in the new location using the default type of the creating process and the target directory. In this case, because there is no specific rule that applies to cp and /tmp, the new file has the type of the parent directory:

cp bar /tmp
ls -Z /tmp/bar
-rw-rw-r--  auser   auser   user_u:object_r:tmp_t   /tmp/bar

The type tmp_t is the default type for temporary files.

Use the -Z option to specify the label for the new file:

cp -Z user_u:object_r:user_home_t foo /tmp
ls -Z /tmp/foo
-rw-rw-r--  auser   auser   user_u:object_r:user_home_t   /tmp/foo
Moving Files: SELinux Options for mv

Moving files with mv retains the original type associated with the file. Care should be taken using this command as it can cause problems. For example, if you move files with the type user_home_t into ~/public_html, then the httpd daemon is not able to serve those files until you relabel them. Refer to Section 44.1.3, “Relabeling a File or Directory” for more information about file labeling.

Command Behavior
mv The file retains its original label. This may cause problems, confusion, or minor insecurity. For example, the tmpwatch program running in the sbin_t domain might not be allowed to delete an aged file in the /tmp directory because of the file's type.
cp Makes a copy of the file using the default behavior based on the domain of the creating process (cp) and the type of the target directory.
cp -p Makes a copy of the file, preserving the specified attributes and security contexts, if possible. The default attributes are mode, ownership, and timestamps. Additional attributes are links and all.
cp -Z <user:role:type> Makes a copy of the file with the specified labels. The -Z option is synonymous with --context.

Table 44.1. Behavior of mv and cp Commands

44.1.2. Checking the Security Context of a Process, User, or File Object

Checking a Process ID

In Red Hat Enterprise Linux, the -Z option is equivalent to --context, and can be used with the ps, id, ls, and cp commands. The behavior of the cp command with respect to SELinux is explained in Table 44.1, “Behavior of mv and cp Commands”.

The following example shows a small sample of the output of the ps command. Most of the processes are running in the unconfined_t domain, with a few exceptions.

[[email protected] ~]$ ps auxZ
LABEL                           USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
system_u:system_r:init_t        root         1  0.0  0.1   2032   620 ?        Ss   15:09   0:00 init [5]
system_u:system_r:kernel_t      root         2  0.0  0.0      0     0 ?        S    15:09   0:00 [migration/0]
system_u:system_r:kernel_t      root         3  0.0  0.0      0     0 ?        SN   15:09   0:00 [ksoftirqd/0]

user_u:system_r:unconfined_t    user     3122  0.0  0.6   6908  3232 ?        S    16:47   0:01 /usr/libexec/gconfd-2 5
user_u:system_r:unconfined_t    user     3125  0.0  0.1   2540   588 ?        S    16:47   0:00 /usr/bin/gnome-keyring-daemon
user_u:system_r:unconfined_t    user     3127  0.0  1.4  33612  6988 ?        Sl   16:47   0:00 /usr/libexec/gnome-settings-daemon
user_u:system_r:unconfined_t    user     3144  0.1  1.4  16528  7360 ?        Ss   16:47   0:01 metacity --sm-client-id=default1
user_u:system_r:unconfined_t    user     3148  0.2  2.9  79544 14808 ?        Ss   16:47   0:03 gnome-panel --sm-client-id default2

Checking a User ID

You can use the -Z option with the id command to determine a user's security context. Note that with this command you cannot combine -Z with other options.

[[email protected] ~]# id -Z
user_u:system_r:unconfined_t

Note that you cannot use the -Z option with the id command to inspect the security context of a different user. That is, you can only display the security context of the currently logged-in user:

[[email protected] ~]$ id
uid=501(user) gid=501(user) groups=501(user) context=user_u:system_r:unconfined_t
[[email protected] ~]$ id root
uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel)
[[email protected] ~]$ id -Z root
id: cannot display context when selinux not enabled or when displaying the id
of a different user
Check a File ID

You can use the -Z option with the ls command to group common long-format information. You can display mode, user, group, security context, and filename information.

cd /etc
ls -Z h* -d
drwxr-xr-x  root root  system_u:object_r:etc_t        hal
-rw-r--r--  root root  system_u:object_r:etc_t        host.conf
-rw-r--r--  root root  user_u:object_r:etc_t          hosts
-rw-r--r--  root root  system_u:object_r:etc_t        hosts.allow
-rw-r--r--  root root  system_u:object_r:etc_t        hosts.canna
-rw-r--r--  root root  system_u:object_r:etc_t        hosts.deny
drwxr-xr-x  root root  system_u:object_r:hotplug_etc_t  hotplug
drwxr-xr-x  root root  system_u:object_r:etc_t        hotplug.d
drwxr-xr-x  root root  system_u:object_r:httpd_sys_content_t htdig
drwxr-xr-x  root root  system_u:object_r:httpd_config_t httpd

44.1.3. Relabeling a File or Directory

You may need to relabel a file when moving or copying into special directories related to the targeted daemons, such as ~/public_html directories, or when writing scripts that work in directories outside of /home.

There are two general types of relabeling operations:

  • Deliberately changing the type of a file

  • Restoring files to the default state according to policy

There are also relabeling operations that an administrator performs. These are covered in Section 44.2.2, “Relabeling a File System”.

Tip

The majority of SELinux permission control in the targeted policy is Type Enforcement (TE). Consequently, you can generally ignore the user and role information in a security label and focus on just changing the type. You do not normally need to consider the role and user settings on files.

Note

If relabeling affects the label on a daemon's executable, you should restart the daemon to be sure it is running in the correct domain. For example, if /usr/sbin/mysqld has the wrong security label, and you address this by using a relabeling operation such as restorecon, you must restart mysqld after the relabeling operation. Setting the executable file to have the correct type (mysqld_exec_t) ensures that it transitions to the proper domain when started.

Use the chcon command to change a file to the correct type. You need to know the correct type that you want to apply to use this command. The directories and files in the following example are labeled with the default type defined for file system objects created in /home:

cd ~
ls -Zd public_html/
drwxrwxr-x  auser  auser  user_u:object_r:user_home_t public_html/

ls -Z web_files/
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   1.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   2.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   3.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   4.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   5.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   index.html

If you move these files into the public_html directory, they retain the original type:

mv web_files/* public_html/
ls -Z public_html/
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   1.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   2.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   3.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   4.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   5.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t   index.html

To make these files viewable from a special user public HTML folder, they need to have a type that httpd has permissions to read, presuming the Apache HTTP Server is configured for UserDir and the Boolean value httpd_enable_homedirs is enabled.

chcon -R -t httpd_user_content_t public_html/
ls -Z public_html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t   1.html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t   2.html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t   3.html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t   4.html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t   5.html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t   index.html

ls -Z public_html/ -d
drwxrwxr-x  auser  auser  user_u:object_r:httpd_user_content_t  public_html/

Tip

If the file has no label, such as a file created while SELinux was disabled in the kernel, you need to give it a full label with chcon system_u:object_r:shlib_t foo.so. Otherwise, you will receive an error about applying a partial context to an unlabeled file.

Use the restorecon command to restore files to the default values according to the policy. There are two other methods for performing this operation that work on the entire file system: fixfiles or a policy relabeling operation. Each of these methods requires superuser privileges. Cautions against both of these methods appear in Section 44.2.2, “Relabeling a File System”.

The following example demonstrates restoring the default user home directory context to a set of files that have different types. The first two sets of files have different types, and are being moved into a directory for archiving. Their contexts are different from each other, and are incorrect for a standard user's home directory:

ls -Z /tmp/
-rw-rw-r--  auser  auser  user_u:object_r:tmp_t            /tmp/file1
-rw-rw-r--  auser  auser  user_u:object_r:tmp_t            /tmp/file2
-rw-rw-r--  auser  auser  user_u:object_r:tmp_t            /tmp/file3

mv /tmp/{1,2,3} archives/
mv public_html/* archives/
ls -Z archives/
-rw-rw-r--  auser  auser  user_u:object_r:tmp_t            file1
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t    file1.html
-rw-rw-r--  auser  auser  user_u:object_r:tmp_t            file2
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t    file2.html
-rw-rw-r--  auser  auser  user_u:object_r:tmp_t            file3
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t    file3.html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t    file4.html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t    file5.html
-rw-rw-r--  auser  auser  user_u:object_r:httpd_user_content_t  index.html

The archives/ directory already has the default type because it was created in the user's home directory:

ls -Zd archives/
drwxrwxr-x  auser  auser  user_u:object_r:user_home_t  archives/

Using the restorecon command to relabel the files uses the default file contexts set by the policy, so these files are labeled with the default label for their current directory.

/sbin/restorecon -R archives/
ls -Z archives/
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    file1
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    file1.html
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    file2
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    file2.html
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    file3
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    file3.html
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    file4.html
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    file5.html
-rw-rw-r--  auser  auser  system_u:object_r:user_home_t    index.html

44.1.4. Creating Archives That Retain Security Contexts

You can use either the tar or star utilities to create archives that retain SELinux security contexts. The following example uses star to demonstrate how to create such an archive. You need to use the appropriate -xattr and -H=exustar options to ensure that the extra attributes are captured and that the header for the *.star file is of a type that fully supports xattrs. Refer to the man page for more information about these and other options.

The following example illustrates the creation and extraction of a set of html files and directories. Note that the two directories have different labels. Unimportant parts of the file context have been omitted for printing purposes (indicated by ellipses '...'):

ls -Z public_html/ web_files/

public_html/:
-rw-rw-r--  auser  auser  ...httpd_user_content_t 1.html
-rw-rw-r--  auser  auser  ...httpd_user_content_t 2.html
-rw-rw-r--  auser  auser  ...httpd_user_content_t 3.html
-rw-rw-r--  auser  auser  ...httpd_user_content_t 4.html
-rw-rw-r--  auser  auser  ...httpd_user_content_t 5.html
-rw-rw-r--  auser  auser  ...httpd_user_content_t index.html
web_files/:
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  1.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  2.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  3.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  4.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  5.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  index.html

The following command creates the archive, retaining all of the SELinux security contexts:

star -xattr -H=exustar -c -f all_web.star public_html/ web_files/
star: 11 blocks + 0 bytes (total of 112640 bytes = 110.00k).

Use the ls command with the -Z option to validate the security context:

ls -Z all_web.star
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t \  all_web.star

You can now copy the archive to a different directory. In this example, the archive is copied to /tmp. If there is no specific policy to make a derivative temporary type, the default behavior is to acquire the tmp_t type.

cp all_web.star /tmp/ cd /tmp/

ls -Z all_web.star
-rw-rw-r--  auser  auser  user_u:object_r:tmp_t  all_web.star

Now you can expand the archives using star and it restores the extended attributes:

star -xattr -x -f all_web.star
star: 11 blocks + 0 bytes (total of 112640 bytes = 110.00k).

ls -Z /tmp/public_html/ /tmp/web_files/
/tmp/public_html/:
-rw-rw-r--  auser  auser  ...httpd_sys_content_t 1.html
-rw-rw-r--  auser  auser  ...httpd_sys_content_t 2.html
-rw-rw-r--  auser  auser  ...httpd_sys_content_t 3.html
-rw-rw-r--  auser  auser  ...httpd_sys_content_t 4.html
-rw-rw-r--  auser  auser  ...httpd_sys_content_t 5.html
-rw-rw-r--  auser  auser  ...httpd_sys_content_t index.html
/tmp/web_files/:
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  1.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  2.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  3.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  4.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  5.html
-rw-rw-r--  auser  auser  user_u:object_r:user_home_t  \ index.html

Caution

If you use an absolute path when you create an archive using star, the archive expands on that same path. For example, an archive made with this command restores the files to /var/log/httpd/:

star -xattr -H=exustar -c -f httpd_logs.star /var/log/httpd/

If you attempt to expand this archive, star issues a warning if the files in the path are newer than the ones in the archive.


 
 
  Published under the terms of the GNU General Public License Design by Interspire