zaterdag 27 juni 2009

SELinux Lockdown Part Nine: Booleans

Booleans are blocks of policy that can be added or removed on the fly by toggling a boolean. The old NSA Example policy was based on a least privilege model. This means that as little as possible was allowed to successfully achieve a task. Almost each rule that gets added to SELinux policy adds new privileges. To maximize security that SELinux provide the mount of active rules should be kept to a minimum.

In Fedora 11 There is some policy enabled with booleans that your environment may not need. It is recommended that this policy is removed and that it is only added when it is needed.

Some booleans add policy when enabled, others add policy when disabled. A simple description of a booleans functionality can be displayed with the command: sudo semanage boolean -l. These descriptions are usually enough to understand its functionality but the descriptions are short. If you run a AVC denial through the audit2why command than audit2why will display which boolean to set in order to solve the issue if the problem can be solved by toggling a boolean.

Sometimes it is best to look to the contents of booleans to understand what they allow. Booleans are defined in Source Policy. You would have to have access to the Source Policy and you would have to know how to find the blocks of policy that are the content of the boolean.

There are three older tools that help you list, set and toggle SELinux booleans: getsebool, setsebool and togglesebool. In Fedora 11 the functionality that these tools provides has been built into the semanage command: semanage boolean.

The names of booleans should point to the Source Policy Module where the boolean is defined. Unfortunately it often is not that easy to find the location of the boolean definition in Source Policy. In a prefect scenario that name of the boolean has the name of the module where it is defined prepended.

Example: How to find the meaning and content of gpg_agent_env_file boolean:

# semanage boolean -l | grep gpg
gpg_agent_env_file -> off Allow usage of the gpg-agent --write-env-file option. This also allows gpg-agent to manage user files.

As the name of the boolean suggest, the boolean is defined somewhere in the gpg module:

http://oss.tresys.com/projects/refpolicy/browser/trunk/policy/modules/apps/gpg.te
On line 196 to line 203 the actually content of the boolean is defined.

This boolean allows programs that run in the gpg_agent_t SELinux Domain to write files in the user home space with the generic user_home_t type when the boolean is set to on.

I think there is actually a but in the block of policy as the gpg_agent_t may only type transition to user_home_t files and not to directories.

The boolean declaration can be found on line 9 to line 15. The declaration has a short description of the functionality of the boolean.

The reason why i showed you how to find a description and the actual content of a boolean is because i cannot discuss each and every boolean in this article. If it is decided to lock-down booleans one can look up whether it actually adds or removes policy when toggled on and what it actually does. Then the boolean can just be switched on and off to see if you need to policy it provides. If required that AVC denials can be fed to audit2why to see if there is a boolean available to allow the functionality.

There are a few booleans that i would like to highlight in this article:

The unconfined_login boolean:

unconfined_login -> off Allow a user to login as an unconfined domain

This boolean was discussed the Part Eight of this series. If set to on then users can login to the system in the unconfined_t User Domain.
Security can be improved much by setting this to off. The content of this boolean can be found in the unconfineduser.te file in Source Policy.

The ssh_sysadm_login and xdm_sysadm_login booleans:

ssh_sysadm_login -> off Allow ssh logins as sysadm_r:sysadm_t
xdm_sysadm_login -> off Allow xdm logins as sysadm

This boolean is similar to unconfined_login for the sysadm_t User Domain. This boolean currently does NOT work. Therefore it is not recommended to map any SELinux Users to the sysadm_r role. Users with access to this role can log into the system directly in this privileged domain.

The *_allow_exec_content_t booleans:

allow_sysadm_exec_content -> off allow_sysadm_exec_content
allow_xguest_exec_content -> off allow_xguest_exec_content
allow_user_exec_content -> off allow_user_exec_content
allow_staff_exec_content -> off allow_staff_exec_content
allow_guest_exec_content -> off allow_guest_exec_content

The description of this boolean is not very helpfull as you can see. When this boolean is set to on then users that operate in any of the mentioned User Domains can execute user content in their user space. This means files with type user_home_t, user_tmp_t or when nfs or samba home directories are enabled types nfs_t and cifs_t respectively. This boolean is Fedora specific and its contents can be found in the userdomain.if Source Policy file.

The secure_module boolean:

secure_mode -> off Enabling secure mode disallows programs, such as newrole, from transitioning to administrative user domains.

The secure_mode boolean can be enabled to disallow User Domain transitions to privileged User Domains. The content of this boolean can be found in the selinuxutil.te Source policy file: http://oss.tresys.com/projects/refpolicy/browser/trunk/policy/modules/system/selinuxutil.te

The policy for this boolean starts on line 289 and ends on line 295. This boolean currently only works for the newrole command. It does NOT work for the sudo command. Therefore it is encouraged to not depend on this boolean.


The secure_mode_insmod boolean:

secure_mode_insmod -> off Disable transitions to insmod.

When this boolean is set to on then confined users will not be able to transition to the insmod Domain. Restricted user domain will not be able to insert Linux Kernel modules. This boolean is defined in the modutils.te Source Policy file: http://oss.tresys.com/projects/refpolicy/browser/trunk/policy/modules/system/modutils.te

The policy starts on line 120 and end on line 122. The declaration for this boolean can be found on line 4 to line 6.

The secure_mode_policyload boolean:

secure_mode_policyload -> off boolean to determine whether the system permits loading policy, setting enforcing mode, and changing boolean values. Set this to true and you have to reboot to set it back

The description of this boolean is quite good. When enabled there is no policy to permit the loading of policy, setting the enforcing mode and changing of booleans. This policy can be found in the selinux.te Source Policy file: http://oss.tresys.com/projects/refpolicy/browser/trunk/policy/modules/kernel/selinux.te

The content of this boolean starts on line 44 and ends on line 52.

The xserver_object_manager boolean:

xserver_object_manager -> off Support X userspace object manager

This boolean is quite powerful. By enabling this boolean the X Server access control extensions become enable. XACE allows the operator to define how processes can interact with X Server. The default policy still has rough edges. XACE is implemented for the SELinux Multi Level Security Policy Model which enforces confidentiality. By default it is disabled with SELinux Policy Targeted. If you feel brave, enable it and experience the power of Xace.
After you set this boolean to true you are required to restart the X server.

The content of this boolean can be found in the xserver.te Source Policy file: http://oss.tresys.com/projects/refpolicy/browser/trunk/policy/modules/services/xserver.te

The content of this boolean starts on line 749 and ends on line 766.

It is not always easy to find where booleans are defined unfortunately. There is room for improvement with the naming of booleans.

Conclusion:

To achieve greater security minimize the amount of policy.
Sometimes policy is added by turning a boolean on and sometimes policy is added by turning a boolean off.
Content of booleans can be reviewed in Source Policy.

Refer: man getsebool, man setsebool, man togglesebool, man audit2why, man semanage

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