Daniel J Walsh wrote:
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Murray McAllister wrote:
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
confined domain, and subjects that are not targeted run in an unconfined
domain, For example logged in users by default log in as unconfined_t
while system processes started by init run in initrc_t. Both of these
domains are unconfined.
NOTE:
Even unconfined domains are subject to executable/writable memory
checks. execmem, execstack, execheap. By default processes run as an
unconfined domain can not allocate writeable memory and execute it.
This is a common attack vector call buffer overflow attacks. Some
applications require this type of access (java, wine, mono and a few
others).
Does this mean applications running in a Java Virtual Machine, and in Wine?
I'll change my response below based on the answer to this.
These applications need to be labeled correctly to allow the
access. There are booleans that can turn off this protection for the
unconfined user unconfined_t. allow_execmem, allow_execstack,
allow_execheap.
You can turn the booleans on using setsebool
setsebool -P allow_execmem 1
I will use these examples later on.
How about:
Targeted policy is the default SELinux policy used in Fedora 10. When
using targeted policy, subjects that are targeted run in a confined
domain, and subjects that are not targeted run in an unconfined domain.
For example, by default, logged in users run in the unconfined_t domain,
and system processes started by init run in the initrc_t domain - both
of these domains are unconfined.
Unconfined domains (as well as confined domains) are subject to
executable and writeable memory checks. By default, subjects running in
an unconfined domain can not allocate writeable memory and execute it.
[I think I changed the meaning. Is it still correct? ]
This reduces vulnerability to buffer overflow attacks. Some subjects
require this access, including but not limited to Java™, Wine and Mono.
To allow this access, these subjects must be labeled correctly, and
memory checks need to be disabled for the users running in the
unconfined_t domain. These memory checks are disable by setting
booleans, which allow the SELinux policy to be modified during runtime.
Configuring booleans is discussed later.
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
large number of subjects are protected, and are therefore confined by
Greater then 200, subjects are not protected they are confined. IE A
subject could still be compromized but other processes on the system are
protected from the confined process.
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).
This is an old RHEL4 list. Almost every domain that listens to the
internet is now confined. And most processes that run as root and do
stuff for users are confined.
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.
How about:
Almost every subject that has network access is confined in Fedora 10.
Most subjects that run as the Linux root user and perform tasks for
users, such as the passwd application, are confined. When a subject is
confined, it runs in its own domain, such as the httpd subject running
in the httpd_t domain. When a confined subject is compromised by an
attacker, depending on SELinux policy configuration, the attacker's
access is to resources and the possible damage they can do is 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
Make sure the machine is in enforcing mode, setenforce 1.
I am adding more steps and example output.
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? ]
Not used on objects so it is just the default.
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]
Yes it will update /etc/selinux/targeted/contexts/files/file_contexts
-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
setroubleshoot requires audit subsystem to be running.
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
Unconfined subjects run in the unconfined domain. 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.
I think this would be better stated:
If an unconfined subject is compromized, SELinux will not prevent the
attacker from gaining access to system resources and data, the system
will still have DAC protection.
How about:
When an unconfined subject is comprised, SELinux does not prevent the
attacker from gaining access to system resources and data, and DAC rules
are used.
(I should probably change these to "If an...")
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?
See above, Also there are booleans available to allow the transition to
a confined domain from the unconfined_t. The one I would like to
encourage is.
allow_unconfined_nsplugin_transition
Which will transition firefox plugins to a confined domain.
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Thanks for your help.
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