Hi,
The following is a draft of the "Targeted Policy" sections for the
SELinux User Guide. Any comments and corrections are appreciated.
Thanks.
Targeted Policy
Targeted policy is the default SELinux policy used in Fedora 10. When
using targeted policy, subjects that are targeted run in their own
domain type, and subjects that are not targeted run in the unconfined_t
domain type. When a subject runs in the unconfined_t domain type,
SELinux rules do not apply, and only DAC rules are used.
Confined Subjects
A large number of subjects are protected, and are therefore confined by
the SELinux targeted policy, including the Apache HTTP Server (httpd),
Samba (samba), FTP (vsftpd), Kerberos (krb5-server), ISC BIND (bind and
bind-chroot), NFS (nfs-utils), and NIS (ypserv). When a subject is
confined, it runs in its own domain type, such as the httpd subject
running in the httpd_t domain type. When a confined subject is
compromised by an attacker, the damage an attacker can do and the data
they can access is greatly limited.
The following example demonstrates how SELinux prevents the Apache HTTP
Server (httpd) from reading files that are not correctly labeled, such
as files intended for use by another subject. This is an example, and
should not be used in production. It assumes that the httpd and wget
packages are installed, that the SELinux targeted policy is used, and
that SELinux is running in enforcing mode:
1. As the Linux root user, run the touch /var/www/html/testfile command.
2. Run the ls -Z /var/www/html/testfile command to view the SELinux
context:
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0
/var/www/html/testfile
By default, Linux users run unconfined on Fedora 10, which is why the
testfile file object is labeled with the SELinux unconfined_u user. The
object_r role is a standard role, and does not affect access control.
The httpd_sys_content_t file type allows the httpd subject to access
this object.
[ What is object_r really for? ]
3. As the Linux root user, start the Apache HTTP Server: /sbin/service
httpd start. When the server has started, change into a directory where
your Linux user has write access to, and run the wget
http://localhost/testfile command. Unless there are any changes to the
default configuration, this command succeeds.
4. The /usr/bin/chcon command relabels files; however, such label
changes do not survive when the file system is relabeled. For permanent
changes that survive a file system relabel, use the /usr/sbin/semanage
command, which is discussed later. As the Linux root user, run the
/usr/bin/chcon -t samba_share_t /var/www/html/testfile command to change
the file type, to a file type that is used by Samba. Run the ls -Z
/var/www/html/testfile command to verify the changes:
[ If a file has an entry in file_contexts, and is relabeled with
semanage fcontext, does that update
/etc/selinux/targeted/contexts/files/file_contexts with the change? I
was going to try, but forgot how to change the file type with semanage]
-rw-r--r-- root root unconfined_u:object_r:samba_share_t:s0
/var/www/html/testfile
5. Note: the current DAC permissions allow the httpd subject access to
this file. Change into a directory where your Linux user has write
access to, and run the wget http://localhost/testfile command. Unless
there are any changes to the default configuration, this command fails:
HTTP request sent, awaiting response... 403 Forbidden
2008-08-22 03:48:40 ERROR 403: Forbidden.
This example demonstrates the additional security added by SELinux.
Although the httpd subject had access to the object in step 5, because
the object was labeled with a file type that httpd subject does not have
access to, SELinux denied access. After step 5, an error such as the
following is logged to /var/log/messages:
Aug 22 03:48:40 localhost setroubleshoot: SELinux is preventing httpd
(httpd_t) "getattr"
to /var/www/html/testfile (samba_share_t). For complete SELinux messages.
run sealert -l c05911d3-e680-4e42-8e36-fe2ab9f8e654
Also, if the audit package is installed and the auditd subject is
running, a more detailed denial is logged to /var/log/audit/audit.log.
These denials are discussed later.
Unconfined Subjects
Unconfined subjects run in the unconfined_t domain type. This means that
SELinux policy rules do not apply, and only DAC permissions are used.
When an unconfined subject is comprised, an attacker may gain access to
a large number of system resources and data.
The following example demonstrates how the Apache HTTP Server (httpd)
can access data intended for use by another subject, when running
unconfined. Note: on Fedora 10, the httpd subject runs in the confined
httpd_t domain type by default. This is an example, and should not be
used in production. It assumes that the httpd and wget packages are
installed, that the SELinux targeted policy is used, and that SELinux is
running in enforcing mode:
1. As the Linux root user, run the touch /var/www/html/test2file command.
2. Run the ls -Z /var/www/html/test2file command to view the SELinux
context:
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0
/var/www/html/test2file
By default, Linux users run unconfined on Fedora 10, which is why the
test2file file object is labeled with the SELinux unconfined_u user. The
object_r role is a standard role, and does not affect access control.
The httpd_sys_content_t file type allows the httpd subject to access
this object.
3. The /usr/bin/chcon command relabels files; however, such label
changes do not survive when the file system is relabeled. For permanent
changes that survive a file system relabel, use the /usr/sbin/semanage
command, which is discussed later. As the Linux root user, run the
/usr/bin/chcon -t samba_share_t /var/www/html/test2file command to
change the file type, to a file type that is used by Samba. Run the ls
-Z /var/www/html/test2file command to verify the changes:
-rw-r--r-- root root unconfined_u:object_r:samba_share_t:s0
/var/www/html/test2file
4. To simulate the httpd subject running unconfined, run the
/usr/sbin/setenforce 0 command as the Linux root user to temporarily
disable SELinux. Confirm SELinux is disabled by running the
/usr/sbin/getenforce command. When SELinux is disabled,
/usr/sbin/getenforce returns Permissive:
$ getenforce
Permissive
5. As the Linux root user, start the Apache HTTP Server: /sbin/service
httpd start. Change into a directory where your Linux user has write
access to, and run the wget http://localhost/test2file command. Unless
there are any changes to the default configuration, this command succeeds.
6. Enable SELinux by running /usr/sbin/setenforce 1 command. When
SELinux is enabled, /usr/sbin/getenforce returns Enforcing:
$ getenforce
Enforcing
The examples in these sections demonstrate how data can be protected
from a compromised confined-subject (protected by SELinux), as well as
how data is more accessible to an attacker from a compromised
unconfined-subject (not protected by SELinux).
Confined and Unconfined User Domains
In progress. Introduction to restrictions on certain domains (user_t,
guest_t etc).
Are there any SELinux restrictions on what users can do when they run
unconfined?
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