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An Introduction to SELinux on CentOS 7 – Part 2: Files and Processes

PostedSeptember 5, 2014 80.7k views Security CentOS

Introduction

In the first part of our SELinux series, we saw how to enable and disable SELinux and how to change some of the policy settings using boolean values. In this second part, we will talk about file and process security contexts.

To refresh your memory from the previous tutorial, a file security context is a type and a process security context is a domain.

Note
The commands, packages, and files shown in this tutorial were tested on CentOS 7. The concepts remain same for other distributions.

In this tutorial, we will be running the commands as the root user unless otherwise stated. If you don't have access to the root account and use another account with sudo privileges, you need to precede the commands with the sudo keyword.

Creating Test User Accounts

First, let's create four user accounts to demonstrate SELinux capabilities as we go along.

  • regularuser
  • switcheduser
  • guestuser
  • restricteduser

You should currently be the root user. Let's run the following command to add the regularuser account:

useradd -c "Regular User" regularuser

Then we run the passwd command to change its password:

passwd regularuser

The output will ask us for new password. Once supplied, the account will be ready for login:

Changing password for user regularuser.
New password:
Retype new password:
passwd: all authentication tokens updated successfully.

Let's create the other accounts too:

useradd -c "Switched User" switcheduser

passwd switcheduser
useradd -c "Guest User" guestuser

passwd guestuser 
useradd -c "Restricted Role User" restricteduser

passwd restricteduser 

SELinux for Processes and Files

The purpose of SELinux is to secure how processes access files in a Linux environment. Without SELinux, a process or application like the Apache daemon will run under the context of the user that started it. So if your system is compromised by a rogue application that's running under the root user, the app can do whatever it wants because root has all-encompassing rights on every file.

SELinux tries to go one step further and eliminate this risk. With SELinux, a process or application will have only the rights it needs to function and NOTHING more. The SELinux policy for the application will determine what types of files it needs access to and what processes it can transition to. SELinux policies are written by app developers and shipped with the Linux distribution that supports it. A policy is basically a set of rules that maps processes and users to their rights.

We begin the discussion of this part of the tutorial by understanding what SELinux contexts and domains mean.

The first part of security puts a label on each entity in the Linux system. A label is like any other file or process attribute (owner, group, date created etc.); it shows the context of the resource. So what's a context? Put simply, a context is a collection of security related information that helps SELinux make access control decisions. Everything in a Linux system can have a security context: a user account, a file, a directory, a daemon, or a port can all have their security contexts. However, security context will mean different things for different types of objects.

SELinux File Contexts

Let's start by understanding SELinux file contexts. Let's look at the output of a regular ls -l command against the /etc directory.

ls -l /etc/*.conf

This will show us a familiar output:

... 
-rw-r--r--. 1 root root    19 Aug 19 21:42 /etc/locale.conf
-rw-r--r--. 1 root root   662 Jul 31  2013 /etc/logrotate.conf
-rw-r--r--. 1 root root  5171 Jun 10 07:35 /etc/man_db.conf
-rw-r--r--. 1 root root   936 Jun 10 05:59 /etc/mke2fs.conf
...

Simple, right? Let's now add the -Z flag:

ls -Z /etc/*.conf

We now have an extra column of information after the user and group ownership:

...
-rw-r--r--. root root system_u:object_r:locale_t:s0    /etc/locale.conf
-rw-r--r--. root root system_u:object_r:etc_t:s0       /etc/logrotate.conf
-rw-r--r--. root root system_u:object_r:etc_t:s0       /etc/man_db.conf
-rw-r--r--. root root system_u:object_r:etc_t:s0       /etc/mke2fs.conf
...

This column shows the security contexts of the files. A file is said to have been labelled with its security context when you have this information available for it. Let's take a closer look at one of the security contexts.

-rw-r--r--. root    root  system_u:object_r:etc_t:s0       /etc/logrotate.conf

The security context is this part:

system_u:object_r:etc_t:s0

There are four parts and each part of the security context is separated by a colon (:). The first part is the SELinux user context for the file. We will discuss SELinux users later, but for now, we can see that it's system_u. Each Linux user account maps to an SELinux user, and in this case, the root user that owns the file is mapped to the system_u SELinux user. This mapping is done by the SELinux policy.

The second part specifies the SELinux role, which is object_r. To brush up on SELinux roles, look back at the first SELinux article.

What's most important here is the third part, the type of the file that's listed here as etc_t. This is the part that defines what type the file or directory belongs to. We can see that most files belong to the etc_t type in the /etc directory. Hypothetically, you can think of type as a sort of "group" or attribute for the file: it's a way of classifying the file.

We can also see some files may belong to other types, like locale.conf which has a locale_t type. Even when all the files listed here have the same user and group owners, their types could be different.

As another example, let's check the type contexts for user home directories:

ls -Z /home

Home directories will have a different context type: userhomedir_t

drwx------. guestuser      guestuser      unconfined_u:object_r:user_home_dir_t:s0      guestuser
drwx------. root           root           system_u:object_r:lost_found_t:s0 lost+found
drwx------. regularuser    regularuser    unconfined_u:object_r:user_home_dir_t:s0      regularuser
drwx------. restricteduser restricteduser unconfined_u:object_r:user_home_dir_t:s0      restricteduser
drwx------. switcheduser   switcheduser   unconfined_u:object_r:user_home_dir_t:s0      switcheduser
drwx------. sysadmin       sysadmin       unconfined_u:object_r:user_home_dir_t:s0      sysadmin

The fourth part of the security context, s0, has to do with multilevel security or MLS. Basically this is another way of enforcing SELinux security policy, and this part shows the sensitivity of the resource (s0). We will briefly talk about sensitivity and categories later. For most vanilla setups of SELinux, the first three security contexts are more important.

SELinux Process Contexts

Let's now talk about process security contexts.

Start the Apache and SFTP services. We installed these services in the first SELinux tutorial.

service httpd start

service vsftpd start

We can run the ps command with a few flags to show the Apache and SFTP processes running on our server:

ps -efZ | grep 'httpd\|vsftpd'

Once again the -Z flag is used for displaying SELinux contexts. The output shows the user running the process, the process ID, and the parent process ID:

system_u:system_r:httpd_t:s0            root        7126    1       0 16:50 ?        00:00:00 /usr/sbin/httpd -DFOREGROUND
system_u:system_r:httpd_t:s0            apache      7127    7126    0 16:50 ?        00:00:00 /usr/sbin/httpd -DFOREGROUND
system_u:system_r:httpd_t:s0            apache      7128    7126    0 16:50 ?        00:00:00 /usr/sbin/httpd -DFOREGROUND
system_u:system_r:httpd_t:s0            apache      7129    7126    0 16:50 ?        00:00:00 /usr/sbin/httpd -DFOREGROUND
system_u:system_r:httpd_t:s0            apache      7130    7126    0 16:50 ?        00:00:00 /usr/sbin/httpd -DFOREGROUND
system_u:system_r:httpd_t:s0            apache      7131    7126    0 16:50 ?        00:00:00 /usr/sbin/httpd -DFOREGROUND
system_u:system_r:ftpd_t:s0-s0:c0.c1023 root        7209    1       0 16:54 ?        00:00:00 /usr/sbin/vsftpd /etc/vsftpd/vsftpd.conf
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 root 7252 2636  0 16:57 pts/0 00:00:00 grep --color=auto httpd\|vsftpd

The security context is this part:

system_u:system_r:httpd_t:s0

The security context has four parts: user, role, domain, and sensitivity. The user, role, and sensitivity work just like the same contexts for files (explained in the previous section). The domain is unique to processes.

In the example above, we can see that a few processes are running within the httpd_t domain, while one is running within the ftpd_t domain.

So what's the domain doing for processes? It gives the process a context to run within. It's like a bubble around the process that confines it. It tells the process what it can do and what it can't do. This confinement makes sure each process domain can act on only certain types of files and nothing more.

Using this model, even if a process is hijacked by another malicious process or user, the worst it can do is to damage the files it has access to. For example, the vsftp daemon will not have access to files used by say, sendmail or samba. This restriction is implemented from the kernel level: it's enforced as the SELinux policy loads into memory, and thus the access control becomes mandatory.

Naming Conventions

Before we go any further, here is a note about SELinux naming convention. SELinux Users are suffixed by "u", roles are suffixed by "r" and types (for files) or domains (for processes) are suffixed by "_t".

How Processes Access Resources

So far we have seen that files and processes can have different contexts, and that they are restricted to their own types or domains. So how does a process run? To run, a process needs to access its files and perform some actions on them (open, read, modify, or execute). We have also learned that each process can have access to only certain types of resources (files, directories, ports, etc.).

SELinux stipulates these access rules in a policy. The access rules follow a standard allow statement structure:

allow <domain> <type>:<class> { <permissions> };

We have already talked about domains and types. Class defines what the resource actually represents (file, directory, symbolic link, device, ports, cursor etc.)

Here's what this generic allow statement means:

  • If a process is of certain domain
  • And the resource object it's trying to access is of certain class and type
  • Then allow the access
  • Else deny access

To see how this works, let's consider the security contexts of the httpd daemon running on our CentOS 7 system:

system_u:system_r:httpd_t:s0     7126 ?        00:00:00 httpd
system_u:system_r:httpd_t:s0     7127 ?        00:00:00 httpd
system_u:system_r:httpd_t:s0     7128 ?        00:00:00 httpd
system_u:system_r:httpd_t:s0     7129 ?        00:00:00 httpd
system_u:system_r:httpd_t:s0     7130 ?        00:00:00 httpd
system_u:system_r:httpd_t:s0     7131 ?        00:00:00 httpd

The default home directory for the web server is /var/www/html. Let's create a file within that directory and check its context:

touch /var/www/html/index.html

ls -Z /var/www/html/*

The file context for our web content will be httpdsyscontent_t:

-rw-r--r--. root root unconfined_u:object_r:httpd_sys_content_t:s0 /var/www/html/index.html

We can use the sesearch command to check the type of access allowed for the httpd daemon:

sesearch --allow --source httpd_t --target httpd_sys_content_t --class file

The flags used with the command are fairly self-explanatory: the source domain is httpd_t, the same domain Apache is running in. We are interested about target resources that are files and have a type context of httpdsyscontent_t. Your output should look like this:

Found 4 semantic av rules:
   allow httpd_t httpd_sys_content_t : file { ioctl read getattr lock open } ;
   allow httpd_t httpd_content_type : file { ioctl read getattr lock open } ;
   allow httpd_t httpd_content_type : file { ioctl read getattr lock open } ;
   allow httpd_t httpdcontent : file { ioctl read write create getattr setattr lock append unlink link rename execute open } ;

Notice the first line:

allow httpd_t httpd_sys_content_t : file { ioctl read getattr lock open } ;

This says that the httpd daemon (the Apache web server) has I/O control, read, get attribute, lock, and open access to files of the httpdsyscontent type. In this case our index.html file has the same type.

Going one step further, let's first modify the web page (/var/www/html/index.html). Edit the file to contain this content:

<html>
    <title>
        This is a test web page
    </title>
    <body>
        <h1>This is a test web page</h1>
    </body>
</html>

Next, we will change the permission of the /var/www/ folder and its contents, followed by a restart of the httpd daemon:

chmod -R 755 /var/www
service httpd restart

We will then try to access it from a browser:

Accessing Web Page with Correct SELiunux Setting

Note
Depending on how your server is set up, you may have to enable port 80 in the IPTables firewall for allowing incoming HTTP traffic from outside the server. We won't go into the details of enabling ports in IPTables here. There are some excellent DigitalOcean articles on the topic which you can use.

So far so good. The httpd daemon is authorized to access a particular type of file and we can see it when accessing via the browser. Next, let's make things a little different by changing the context of the file. We will use the chcon command for it. The --type flag for the command allows us to specify a new type for the target resource. Here, we are changing the file type to var_t.

chcon --type var_t /var/www/html/index.html

We can confirm the type change:

ls -Z /var/www/html/
-rwxr-xr-x. root root unconfined_u:object_r:var_t:s0   index.html

Next, when we try to access the web page (i.e. the httpd daemon tries to read the file), you may get a Forbidden error, or you may see the generic CentOS "Testing 123" page:

Accessing Web Page with Incorrect SELinux Setting

So what's happening here? Obviously some access is now being denied, but whose access is it? As far as SELinux is concerned, the web server is authorized to access only certain types of files and vart is not one of those contexts. Since we changed the context of the index.html file to vart, Apache can no longer read it and we get an error.

To make things work again, let's change the file type with the restorecon command. The -v switch shows the change of context labels:

restorecon -v /var/www/html/index.html
restorecon reset /var/www/html/index.html context unconfined_u:object_r:var_t:s0->unconfined_u:object_r:httpd_sys_content_t:s0

If we try to access the page now, it will show our "This is a test web page" text again.

This is an important concept to understand: making sure files and directories have the correct context is pivotal to making sure SELinux is behaving as it should. We will see a practical use case at the end of this section, but before that, let's talk about a few more things.

Context Inheritance for Files and Directories

SELinux enforces something we can term as "context inheritance". What this means is that unless specified by the policy, processes and files are created with the contexts of their parents.

So if we have a process called "proca" spawning anoher process called "procb", the spawned process will run in the same domain as "proc_a" unless specified otherwise by the SELinux policy.

Similarly, if we have a directory with a type of "somecontextt", any file or directory created under it will have the same type context unless the policy says otherwise.

To illustrate this, let's check the contexts of the /var/www/ directory:

ls -Z /var/www

The html directory within /var/www/ has the httpdsyscontent_t type context. As we saw before, the index.html file within it has the same context (i.e., the context of the parent):

drwxr-xr-x. root root system_u:object_r:httpd_sys_script_exec_t:s0 cgi-bin
drwxr-xr-x. root root system_u:object_r:httpd_sys_content_t:s0 html

This inheritance is not preserved when files are copied to another location. In a copy operation, the copied file or directory will assume the type context of the target location. In the code snippet below, we are copying the index.html file (with "httpdsyscontent_t" type context) to the /var/ directory:

cp /var/www/html/index.html /var/

If we check the copied file's context, we will see it has changed to var_t, the context of its current parent directory:

ls -Z /var/index.html
-rwxr-xr-x. root root unconfined_u:object_r:var_t:s0   /var/index.html

This change of context can be overridden by the --preserver=context clause in the cp command.

When files or directories are moved, original contexts are preserved. In the following command, we are moving the /var/index.html to the /etc/ directory:

mv  /var/index.html  /etc/

When we check the moved file's context, we see that the var_t context has been preserved under the /etc/ directory:

ls -Z /etc/index.html
-rwxr-xr-x. root root unconfined_u:object_r:var_t:s0   /etc/index.html

So why are we so concerned with file contexts? Why is this copy and move concept important? Think about it: maybe you decided to copy all your web server's HTML files to a separate directory under the root folder. You have done this to simplify you backup process and also to tighten security: you don't want any hacker to easily guess where your website's files are. You have updated the directory's access control, changed the web config file to point to the new location, restarted the service, but it still doesn't work. Perhaps you can then look at the contexts of the directory and its files as the next troubleshooting step. Let's run it as a practical example.

SELinux in Action: Testing a File Context Error

First, let's create a directory named www under the root. We will also create a folder called html under www.

mkdir -p /www/html

If we run the ls -Z command, we will see these directories have been created with the default_t context:

ls -Z /www/
drwxr-xr-x. root root unconfined_u:object_r:default_t:s0 html

Next we copy the contents of the /var/www/html directory to /www/html:

cp /var/www/html/index.html /www/html/

The copied file will have a context of default_t. That's the context of the parent directory.

We now edit the httpd.conf file to point to this new directory as the web site's root folder. We will also have to relax the access rights for this directory.

vi /etc/httpd/conf/httpd.conf

First we comment out the existing location for document root and add a new DocumentRoot directive to /www/html:

# DocumentRoot "/var/www/html"

DocumentRoot "/www/html"

We also comment out the access rights section for the existing document root and add a new section:

#<Directory "/var/www">
#    AllowOverride None
    # Allow open access:
#    Require all granted
#</Directory>

<Directory "/www">
    AllowOverride None
    # Allow open access:
    Require all granted
</Directory>

We leave the location of the cgi-bin directory as it is. We are not getting into detailed Apache configuration here; we just want our site to work for SELinux purposes.

Finally, restart the httpd daemon:

service httpd restart

Once the server has been restarted, accessing the web page will give us the same "403 Forbidden" error (or default "Testing 123" page) we saw before.

The error is happening because the index.html file's context changed during the copy operation. It needs to be changed back to its original context (httpdsyscontent_t).

But how do we do that?

Changing and Restoring SELinux File Contexts

In a previous code sample we saw two commands for changing file contents: chcon and restorecon. Running chcon is a temporary measure. You can use it to temporarily change file or directory contexts for troubleshooting access denial errors. However, this method is only temporary: a file system relabel or running the restorecon command will revert the file back to its original context.

Also, running chcon requires you to know the correct context for the file; the --type flag specifies the context for the target. restorecon doesn't need this specified. If you run restorecon, the file will have the correct context re-applied and the changes will be made permanent.

But if you don't know the file's correct context, how does the system know which context to apply when it runs restorecon?

Conveniently, SELinux "remembers" the context of every file or directory in the server. In CentOS 7, contexts of files already existing in the system are listed in the /etc/selinux/targeted/contexts/files/file_contexts file. It's a large file and it lists every file type associated with every application supported by the Linux distribution. Contexts of new directories and files are recorded in the /etc/selinux/targeted/contexts/files/file_contexts.local file. So when we run the restorecon command, SELinux will look up the correct context from one of these two files and apply it to the target.

The code snippet below shows an extract from one of the files:

cat /etc/selinux/targeted/contexts/files/file_contexts
...
/usr/(.*/)?lib(/.*)?    system_u:object_r:lib_t:s0
/opt/(.*/)?man(/.*)?    system_u:object_r:man_t:s0
/dev/(misc/)?agpgart    -c      system_u:object_r:agp_device_t:s0
/usr/(.*/)?sbin(/.*)?   system_u:object_r:bin_t:s0
/opt/(.*/)?sbin(/.*)?   system_u:object_r:bin_t:s0
/etc/(open)?afs(/.*)?   system_u:object_r:afs_config_t:s0
...

To permanently change the context of our index.html file under /www/html, we have to follow a two-step process.

  • First we run the semanage fcontext command. This will write the new context to the /etc/selinux/targeted/contexts/files/file_contexts.local file. But it won't relabel the file itself. We'll do this for both directories.
semanage fcontext --add --type httpd_sys_content_t "/www(/.*)?"
semanage fcontext --add --type httpd_sys_content_t "/www/html(/.*)?"

To make sure, we can check the file context database (note that we are using the file_contexts.local file):

cat /etc/selinux/targeted/contexts/files/file_contexts.local

You should see the updated contexts:

# This file is auto-generated by libsemanage
# Do not edit directly.

/www(/.*)?    system_u:object_r:httpd_sys_content_t:s0
/www/html(/.*)?    system_u:object_r:httpd_sys_content_t:s0

Next, we will run the restorecon command. This will relabel the file or directory with what's been recorded in the previous step:

restorecon -Rv /www

This should reset the context in three levels: the top level /www directory, the /www/html directory under it and the index.html file under /www/html:

restorecon reset /www context unconfined_u:object_r:default_t:s0->unconfined_u:object_r:httpd_sys_content_t:s0
restorecon reset /www/html context unconfined_u:object_r:default_t:s0->unconfined_u:object_r:httpd_sys_content_t:s0
restorecon reset /www/html/index.html context unconfined_u:object_r:default_t:s0->unconfined_u:object_r:httpd_sys_content_t:s0

If we now try to access the web page, it should work.

There is a nifty tool called matchpathcon that can help troubleshoot context-related problems. This command will look at the current context of a resource and compare it with what's listed under the SELinux context database. If different, it will suggest the change required. Let's test this with the /www/html/index.html file. We will use the -V flag that verifies the context:

matchpathcon -V /www/html/index.html

The matchpathcon output should show that the context is verified.

/www/html/index.html verified.

For an incorrectly labelled file, the message will say what the context should be:

/www/html/index.html has context unconfined_u:object_r:default_t:s0, should be system_u:object_r:httpd_sys_content_t:s0 

Domain Transition

So far we have seen how processes access file system resources. We will now see how processes access other processes.

Domain transition is the method where a process changes its context from one domain to another. To understand it, let's say you have a process called proca running within a context of contextat. With domain transition, proca can run an application (a program or an executable script) called appx that would spawn another process. This new process could be called procb and it could be running within the contextbt domain. So effectively, contextat is transitioning to contextbt through appx. The appx executable is working as an entrypoint to contextb_t. The flow can be illustrated below:

SELinux Domain Transition

The case of domain transition is fairly common in SELinux. Let's consider the vsftpd process running on our server. If it is not running, we can run the service vsftpd start command to start the daemon.

Next we consider the systemd process. This is the ancestor of all processes. This is the replacement of the System V init process and runs within a context of init_t. :

ps -eZ  | grep init
system_u:system_r:init_t:s0         1 ?        00:00:02 systemd
system_u:system_r:mdadm_t:s0      773 ?        00:00:00 iprinit

The process running within the init_t domain is a short-lived one: it will invoke the binary executable /usr/sbin/vsftpd, which has a type context of ftpdexect. When the binary executable starts, it becomes the vsftpd daemon itself and runs within the ftpd_t domain.

We can check the domain contexts of the files and processes:

ls -Z /usr/sbin/vsftpd

Shows us:

-rwxr-xr-x. root root system_u:object_r:ftpd_exec_t:s0 /usr/sbin/vsftpd

Checking the process:

ps -eZ | grep vsftpd

Shows us:

system_u:system_r:ftpd_t:s0-s0:c0.c1023 7708 ? 00:00:00 vsftpd

So here the process running in the init_t domain is executing a binary file with the ftpdexect type. That file starts a daemon within the ftpd_t domain.

This transition is not something the application or the user can control. This has been stipulated in the SELinux policy that loads into memory as the system boots. In a non-SELinux server a user can start a process by switching to a more powerful account (provided she or he has the right to do so). In SELinux, such access is controlled by pre-written policies. And that's another reason SELinux is said to implement Mandatory Access Control.

Domain transition is subject to three strict rules:

  • The parent process of the source domain must have the execute permission for the application sitting between both the domains (this is the entrypoint).

  • The file context for the application must be identified as an entrypoint for the target domain.

  • The original domain must be allowed to transition to the target domain.

Taking the vsftpd daemon example above, let's run the sesearch command with different switches to see if the daemon conforms to these three rules.

First, the source domain initt needs to have execute permission on the entrypoint application with the ftpdexec_t context. So if we run the following command:

sesearch -s init_t -t ftpd_exec_t -c file -p execute -Ad

The result shows that processes within initt domain can read, get attribute, execute, and open files of ftpdexec_t context:

Found 1 semantic av rules:
   allow init_t ftpd_exec_t : file { read getattr execute open } ;

Next, we check if the binary file is the entrypoint for the target domain ftpd_t:

sesearch -s ftpd_t -t ftpd_exec_t -c file -p entrypoint -Ad

And indeed it is so:

Found 1 semantic av rules:
   allow ftpd_t ftpd_exec_t : file { ioctl read getattr lock execute execute_no_trans entrypoint open } ;

And finally, the source domain initt needs to have permission to transition to the target domain ftpdt:

sesearch -s init_t -t ftpd_t -c process -p transition -Ad

As we can see below, the source domain has that permission:

Found 1 semantic av rules:
   allow init_t ftpd_t : process transition ;

Unconfined Domains

When we introduced the concept of domains, we compared it to a hypothetical bubble around the process: something that stipulates what the process can and can't do. This is what confines the process.

SELinux also has processes that run within unconfined domains. As you can imagine, unconfined processes would have all types of access in the system. Even then, this full access is not arbitrary: full access is also specified in the SELinux policy.

Example of an unconfined process domain would be unconfined_t. This is the same domain logged in users run their processes by default. We will talk about users and their accesses to process domains in subsequent sections.

Conclusion

We have covered some very important SELinux concepts here today. Managing file and process context is at the heart of a successful SELinux implementation. As we will see in the next and final part of this series, there's another piece of the puzzle remaining: the SELinux user.

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