Restricting even root access to a folder


Sven Vermeulen Sat 11 July 2015

In a comment Robert asked how to use SELinux to prevent even root access to a directory. The trivial solution would be not to assign an administrative role to the root account (which is definitely possible, but you want some way to gain administrative access otherwise ;-)

Restricting root is one of the commonly referred features of a MAC (Mandatory Access Control) system. With a well designed user management and sudo environment, it is fairly trivial - but if you need to start from the premise that a user has direct root access, it requires some thought to implement it correctly. The main "issue" is not that it is difficult to implement policy-wise, but that most users will start from a pre-existing policy (such as the reference policy) and build on top of that.

The use of a pre-existing policy means that some roles are already identified and privileges are already granted to users - often these higher privileged roles are assigned to the Linux root user as not to confuse users. But that does mean that restricting root access to a folder means that some additional countermeasures need to be implemented.

The policy

But first things first. Let's look at a simple policy for restricting access to /etc/private:

policy_module(myprivate, 1.0)

type etc_private_t;

This simple policy introduces a type (etc_private_t) which is allowed to be used for files (it associates with a file system). Do not use the files_type() interface as this would assign a set of attributes that many user roles get read access on.

Now, it is not sufficient to have the type available. If we want to assign it to a type, someone or something needs to have the privileges to change the security context of a file and directory to this type. If we would just load this policy and try to do this from a privileged account, it would fail:

~# chcon -t etc_private_t /etc/private
chcon: failed to change context of '/etc/private' to 'system_u:object_r:etc_private_t:s0': Permission denied

With the following rule, the sysadm_t domain (which I use for system administration) is allowed to change the context to etc_private_t:

allow sysadm_t etc_private_t:{dir file} relabelto;

With this in place, the administrator can label resources as etc_private_t without having read access to these resources afterwards. Also, as long as there are no relabelfrom privileges assigned, the administrator cannot revert the context back to a type that he has read access to.

The countermeasures

But this policy is not sufficient. One way that administrators can easily access the resources is to disable SELinux controls (as in, put the system in permissive mode):

~# cat /etc/private/README
cat: /etc/private/README: Permission denied
~# setenforce 0
~# cat /etc/private/README
Hello World!

To prevent this, enable the secure_mode_policyload SELinux boolean:

~# setsebool secure_mode_policyload on

This will prevent any policy and SELinux state manipulation... including permissive mode, but also including loading additional SELinux policies or changing booleans. Definitely experiment with this setting without persisting (i.e. do not use -P in the above command yet) to make sure it is manageable for you.

Still, this isn't sufficient. Don't forget that the administrator is otherwise a full administrator - if he cannot access the /etc/private location directly, then he might be able to access it indirectly:

  • If the resource is on a non-critical file system, he can unmount the file system and remount it with a context= mount option. This will override the file-level contexts. Bind-mounting does not seem to allow overriding the context.
  • If the resource is on a file system that cannot be unmounted, the administrator can still reboot the system in a mode where he can access the file system regardless of SELinux controls (either through editing /etc/selinux/config or by booting with enforcing=0, etc.
  • The administrator can still access the block device files on which the resources are directly. Specialized tools can allow for extracting files and directories without actually (re)mounting the device.

A more extensive list of methods to potentially gain access to such resources is iterated in Limiting file access with SELinux alone.

This set of methods for gaining access is due to the administrative role already assigned by the existing policy. To further mitigate these risks with SELinux (although SELinux will never completely mitigate all risks) the roles assigned to the users need to be carefully revisited. If you grant people administrative access, but you don't want them to be able to reboot the system, (re)mount file systems, access block devices, etc. then create a user role that does not have these privileges at all.

Creating such user roles does not require leaving behind the policy that is already active. Additional user domains can be created and granted to Linux accounts (including root). But in my experience, when you need to allow a user to log on as the "root" account directly, you probably need him to have true administrative privileges. Otherwise you'd work with personal accounts and a well-designed /etc/sudoers file.