In this article by Jason Slagle, author of the book Learning Puppet Security, covers using Puppet to manage SELinux and auditd. We learned a lot so far about using Puppet to secure your systems as, well as how to use it to make groups of systems more secure. However, in all of that, we've not yet covered some of the basic OS-level functions that are available to secure a system. In this article, we'll review several of those functions.
(For more resources related to this topic, see here.)
SELinux is a powerful tool in the security arsenal. Most administrators experience with it, is along the lines of "how can I turn that off ?" This is born out of frustration with the poor documentation about the tool, as well as the tedious nature of the configuration.
While Puppet cannot help you with the documentation (which is getting better all the time), it can help you with some of the other challenges that SELinux can bring. That is, ensuring that the proper contexts and policies are in place on the systems being managed.
In this article, we'll cover the following topics related to OS-level security tools:
A brief introduction to SELinux and auditd
The built-in Puppet support for SELinux
Community modules for SELinux
Community modules for auditd
At the end of this article, you should have enough skills so that you no longer need to disable SELinux. However, if you still need to do so, it is certainly possible to do via the modules presented here.
Introducing SELinux and auditd
During the course of this article, we'll explore the SELinux framework for Linux and see how to automate it using Puppet. As part of the process, we'll also review auditd, the logging and auditing framework for Linux. Using Puppet, we can automate the configuration of these often-neglected security tools, and even move the configuration of these tools for various services to the modules that configure those services.
The SELinux framework
SELinux is a security system for Linux originally developed by the United States National Security Agency (NSA). It is an in-kernel protection mechanism designed to provide Mandatory Access Controls (MACs) to the Linux kernel.
SELinux isn't the only MAC framework for Linux. AppArmor is an alternative MAC framework included in the Linux kernel since Version 2.6.30. We choose to implement SELinux; since it is the default framework used under Red Hat Linux, which we're using for our examples.
More information on AppArmor can be found at http://wiki.apparmor.net/index.php/Main_Page.
These access controls work by confining processes to the minimal amount of files and network access that the processes require to run. By doing this, the controls limit the amount of collateral damage that can be done by a process, which becomes compromised.
SELinux was first merged to the Linux mainline kernel for the 2.6.0 release. It was introduced into Red Hat Enterprise Linux with Version 4, and into Ubuntu in Version 8.04. With each successive release of the operating systems, support for SELinux grows, and it becomes easier to use.
SELinux has a couple of core concepts that we need to understand to properly configure it. The first are the concepts of types and contexts. A type in SELinux is a grouping of similar things. Files used by Apache may be httpd_sys_content_t, for instance, which is a type that all content served by HTTP would have. The httpd process itself is of type httpd_t. These types are applied to objects, which represent discrete things, such as files and ports, and become part of the context of that object. The context of an object represents the object's user, role, type, and optionally data on multilevel security. For this discussion, the type is the most important component of the context.
Using a policy, we grant access from the subject, which represents a running process, to various objects that represent files, network ports, memory, and so on. We do that by creating a policy that allows a subject to have access to the types it requires to function.
SELinux has three modes that it can operate in. The first of these modes is disabled. As the name implies, the disabled mode runs without any SELinux enforcement. The second mode is called permissive. In permissive mode, SELinux will log any access violations, but will not act on them. This is a good way to get an idea of where you need to modify your policy, or tune Booleans to get proper system operations. The final mode, enforcing, will deny actions that do not have a policy in place. Under Red Hat Linux variants, this is the default SELinux mode. By default, Red Hat 6 runs SELinux with a targeted policy in enforcing mode. This means, that for the targeted daemons, SELinux will enforce its policy by default.
An example is in order here, to explain this well.
So far, we've been operating with SELinux disabled on our hosts. The first step in experimenting with SELinux is to turn it on. We'll set it to permissive mode at first, while we gather some information. To do this, after starting our master VM, we'll need to modify the SELinux configuration and reboot. While it's possible to change from enforcing mode to either permissive or disabled mode without a reboot, going back requires us to reboot.
Let's edit the /etc/sysconfig/selinux file and set the SELINUX variable to permissive on our puppetmaster. Remember to start the vagrant machine and SSH in as it is necessary. Once this is done, the file should look as follows:
Once this is complete, we need to reboot. To do so, run the following command:
sudo shutdown -r now
Wait for the system to come back online.
Once the machine is back up and you SSH back into it, run the getenforce command. It should return permissive, which means SELinux is running, but not enforced.
Now, we can make sure our master is running and take a look at its context. If it's not running, you can start the service with the sudo service puppetmaster start command. Now, we'll use the -Z flag on the ps command to examine the SELinux flag. Many commands, such as ps and ls use the -Z flag to view the SELinux data. We'll go ahead and run the following command to view the SELinux data for the running puppetmaster:
ps -efZ|grep puppet
When you do this, you'll see a Linux output, such as follows:
unconfined_u:system_r:initrc_t:s0 puppet 1463 1 1 11:41 ? 00:00:29 /usr/bin/ruby /usr/bin/puppet master
If you take a look at the first part of the output line, you'll see that Puppet is running in the unconfined_u:system_r:initrc_t context. This is actually somewhat of a bug and a result of the Puppet policy on CentOS 6 being out of date. We should actually be running under the system_u:system_r:puppetmaster_t:s0 context, but the policy is for a much older version of Puppet, so it runs unconfined.
Let's take a look at the sshd process to see what it looks like also. To do so, we'll just grep for sshd instead:
ps -efZ|grep sshd
The output is as follows:
system_u:system_r:sshd_t:s0-s0:c0.c1023 root 1206 1 0 11:40 ? 00:00:00 /usr/sbin/sshd
This is a more traditional output one would expect. The sshd process is running under the system_u:system_r:sshd_t context. This actually corresponds to the system user, the system role, and the sshd type.
The user and role are SELinux constructs that help you allow role-based access controls. The users do not map to system users, but allow us to set a policy based on the SELinux user object. This allows role-based access control, based on the SELinux user. Previously the unconfined user was a user that will not be enforced.
Now, we can take a look at some objects. Doing a ls -lZ /etc/ssh command results in the following:
As you can see, each of the files belongs to a context that includes the system user, as well as the object role. They are split among the etc type for configuration files and the sshd_key type for keys.
The SSH policy allows the sshd process to read both of these file types. Other policies, say, for NTP, would potentially allow the ntpd process to read the etc types, but it would not be able to read the sshd_key files.
This very fine-grained control is the power of SELinux. However, with great power comes very complex configuration. Configuration can be confusing to set up, if it doesn't happen correctly. For instance, with Puppet, the wrong type can potentially impact the system if not dealt with.
Fortunately, in permissive mode, we will log data that we can use to assist us with this. This leads us into the second half of the system that we wish to discuss, which is auditd.
In the meantime, there is a bunch of information on SELinux available on its website at http://selinuxproject.org/page/Main_Page. There's also a very funny, but informative, resource available describing SELinux at https://people.redhat.com/duffy/selinux/selinux-coloring-book_A4-Stapled.pdf.
The auditd framework for audit logging
SELinux does a great job at limiting access to system components; however, reporting what enforcement took place was not one of its objectives.
Enter the auditd. The auditd is an auditing framework developed by Red Hat. It is a complete auditing system using rules to indicate what to audit. This can be used to log SELinux events, as well as much more.
Under the hood, auditd has hooks into the kernel to watch system calls and other processes. Using the rules, you can configure logging for any of these events. For instance, you can create a rule that monitors writes to the /etc/passwd file. This would allow you to see if any users were added to the system. We can also add monitoring of files, such as lastlog and wtmp to monitor the login activity. We'll explore this example later when we configure auditd.
To quickly see how a rule works, we'll manually configure a quick rule that will log the time when the wtmp file was edited. This will add some system logging around users logging in.
To do this, let's edit the /etc/audit/audit.rules file to add a rule to monitor this. Edit the file and add the following lines:
-w /var/log/wtmp -p wa -k logins-w /etc/passwd –p wa –k password
We'll take a look at what the preceding lines do. These lines both start with the –w clauses. These indicate the files that we are monitoring. Second, we have the –p clauses. This lets you set what file operations we monitor. In this case, it is write and append operations. Finally, with the the –k entries, we're setting a keyword that is logged and can be filtered on.
This should go at the end of the file. Once it's done, reload auditd with the following command:
sudo service auditd restart
Once this is complete, go ahead and log another ssh session in. Once you can simply log, back out. Once this is done, take a look at the /var/log/audit/audit.log file. You should see the content like the following:
type=SYSCALL msg=audit(1416795396.816:482): arch=c000003e syscall=2 success=yes exit=8 a0=7fa983c446aa a1=1 a2=2 a3=7fff3f7a6590 items=1 ppid=1206 pid=2202 auid=500 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=(none) ses=51 comm="sshd" exe="/usr/sbin/sshd" subj=system_u:system_r:sshd_t:s0-s0:c0.c1023 key="logins"type=SYSCALL msg=audit(1416795420.057:485): arch=c000003e syscall=2 success=yes exit=7 a0=7fa983c446aa a1=1 a2=2 a3=8 items=1 ppid=1206 pid=2202 auid=500 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=(none) ses=51 comm="sshd" exe="/usr/sbin/sshd" subj=system_u:system_r:sshd_t:s0-s0:c0.c1023 key="logins"
There are tons of fields in this output, including the SELinux context, the userID, and so on. Of interest is the auid, which is the audit user ID. On commands run via the sudo command, this will still contain the user ID of the user who called sudo. This is a great way to log commands performed via sudo.
Auditd also logs SELinux failures. They get logged under the type AVC. These access vector cache logs will be placed in the auditd log file when a SELinux violation occurs.
Much like SELinux, auditd is somewhat complicated. The intricacies of it are beyond the scope of this book. You can get more information at http://people.redhat.com/sgrubb/audit/.
SELinux and Puppet
Puppet has direct support for several features of SELinux. There are two native Puppet types for SELinux: selboolean and selmodule. These types support setting SELinux Booleans and installing SELinux policy modules.
SELinux Booleans are variables that impact on how SELinux behaves. They are set to allow various functions to be permitted. For instance, you set a SELinux Boolean to true to allow the httpd process to access network ports.
SELinux modules are groupings of policies. They allow policies to be loaded in a more granular way. The Puppet selmodule type allows Puppet to load these modules.
The selboolean type
The targeted SELinux policy that most distributions use is based on the SELinux reference policy. One of the features of this policy is the use of Boolean variables that control actions of the policy.
There are over 200 of these Booleans on a Red Hat 6-based machine. We can investigate them by installing the policycoreutils-python package on the operating system. You can do this by executing the following command:
sudo yum install policycoreutils-python
Once installed, we can run the semanage boolean -l command to get a list of the Boolean values, along with their descriptions. The output of this will look as follows:
As you can see, there exists a very large number of settings that can be reconfigured, simply by setting the appropriate Boolean value.
The selboolean Puppet type supports managing these Boolean values. The provider is fairly simple, accepting the following values:
Parameter
Description
name
This contains the name of the Boolean to be set. It defaults to the title.
persistent
This checks whether to write the value to disk for the next boot.
provider
This is the provider for the type. Usually, the default getsetsebool value is accepted.
value
This contains the value of the Boolean, true or false.
Usage of this type is rather simple. We'll show an example that will set the puppetmaster_use_db parameter to true value. If we are using the SELinux Puppet policy, this would allow the master to talk to a database. For our use, it's a simple unused variable that we can use for demonstration purposes.
As a reminder, the SElinux policy for Puppet on CentOS 6 is outdated, so setting the Boolean does not impact the version of Puppet we're running. It does, however, serve to show how a Boolean is set.
To do this, we'll create a sample role and profile for our puppetmaster. This is something that would likely exist in a production environment to manage the configuration of the master. In this example, we'll simply build a small profile and role for the master.
Let's start with the profile. Copy over the profiles module we've slowly been building up, and let's add a puppetmaster.pp profile. To do so, edit the profiles/manifests/puppetmaster.pp file and make it look as follows:
class profiles::puppetmaster {selboolean { 'puppetmaster_use_db': value => on, persistent => true,}}
Then, we'll move on to the role. Copy the roles, and edit the roles/manifests/puppetmaster.pp file there and make it look as follows:
class roles::puppetmaster {include profiles::puppetmaster}
Once this is done, we can apply it to our host. Edit the /etc/puppet/manifests/site.pp file. We'll apply the puppetmaster role to the puppetmaster machine, as follows:
node 'puppet.book.local' {include roles::puppetmaster}
Now, we'll run Puppet and get the output as follows:
As you can see, it set the value to on when run. Using this method, we can set any of the SELinux Boolean values we need for our system to operate properly.
More information on SELinux Booleans with information on how to obtain a list of them can be found at https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Security-Enhanced_Linux/sect-Security-Enhanced_Linux-Working_with_SELinux-Booleans.html.
The selmodule type
The other native type inside Puppet is a type to manage the SELinux modules. Modules are compiled collections of the SELinux policy. They're loaded into the kernel using the selmodule command. This Puppet type provides support for this mechanism.
The available parameters are as follows:
Parameter
Description
name
This contains the name of the module— it defaults to the title
ensure
This is the desired state—present or absent
provider
This specifies the provider for the type—it should be selmodule
selmoduledir
This is the directory that contains the module to be installed
selmodulepath
This provides the complete path to the module to be installed if not present in selmoduledir
syncversion
This checks whether to resync the module if a new version is found, such as ensure => latest
Using the module, we can take our compiled module and serve it onto the system with Puppet. We can then use the module to ensure that it gets installed on the system. This lets us centrally manage the module with Puppet.
We'll see an example where this module compiles a policy and then installs it, so we won't show a specific example here. Instead, we'll move on to talk about the last SELinux-related component in Puppet.
File parameters for SELinux
The final internal support for SELinux types comes in the form of the file type.
The file type parameters are as follows:
Parameter
Description
selinux_ignore_defaults
By default, Puppet will use the matchpathcon function to set the context of a file. This overrides that behavior if set to true value.
Selrange
This sets the SELinux range component. We've not really covered this. It's not used in most mainstream distributions at the time this book was written.
Selrole
This sets the SELinux role on the file.
seltype
This sets the SELinux type on the file.
seluser
This sets the SELinux role on the file.
Usually, if you place files in the correct location (the expected location for a service) on the filesystem, Puppet will get the SELinux properties correct via its use of the matchpathcon function. This function (which also has a matching utility) applies a default context based on the policy settings. Setting the context manually is used in cases where you're storing data outside the normal location. For instance, you might be storing web data under the /opt file.
The preceding types and providers provide the basics that allow you to manage SELinux on a system. We'll now take a look at a couple of community modules that build on these types and create a more in-depth solution.
Summary
This article looked at what SELinux and auditd were, and gave a brief example of how they can be used. We looked at what they can do, and how they can be used to secure your systems.
After this, we looked at the specific support for SELinux in Puppet. We looked at the two built-in types to support it, as well as the parameters on the file type.
Then, we took a look at one of the several community modules for managing SELinux. Using this module, we can store the policies as text instead of compiled blobs.
Resources for Article:
Further resources on this subject:
The anatomy of a report processor [Article]
Module, Facts, Types and Reporting tools in Puppet [Article]
Designing Puppet Architectures [Article]
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