Add docs to explain why we disabling selinux

Change-Id: Iadbd4a0d77aee692abd9ab4f81ba67ead8949b1e

doc/index.rst

@@ -39,6 +39,7 @@ Kolla Overview
    image-building
    advanced-configuration
    operating-kolla
+   selinux
 
 
 Kolla Services

doc/selinux.rst

@@ -0,0 +1,48 @@
+SELinux in Kolla
+================
+
+Overview
+--------
+The state of SELinux in Kolla is a work in progress. The short answer is you
+must disable it until selinux polices are written for the Docker containers.
+
+The Long Answer
+---------------
+To understand why Kolla needs to set certain selinux policies for services that
+you wouldn't expect to need them (rabbitmq, mariadb, glance, etc.) we must take
+a step back and talk about Docker.
+
+Docker has not had the concept of persistent containerized data until recently.
+This means when a container is run the data it creates is destroyed when the
+container goes away, which is obviously no good in the case of upgrades.
+
+It was suggested data containers could solve this issue by only holding data if
+they were never recreated, leading to a scary state where you could lose access
+to your data if the wrong command was executed. The real answer to this problem
+came in Docker 1.9 with the introduction of named volumes. You could now
+address volumes directly by name removing the need for so called "data
+containers" all together.
+
+Another solution to the persistent data issue is to use a host bind mount which
+involves making, for sake of example, host directory /var/lib/mysql available
+inside the container at /var/lib/mysql. This absolutely solves the problem of
+persistent data, but it introduces another security issue, permissions. With
+this host bind mount solution the data in /var/lib/mysql will be owned by the
+mysql user in the container. Unfortunately, that mysql user in the container
+could have any UID/GID and thats who will own the data outside the container
+introducing a potential security risk. Additionally, this method dirties the
+host and requires host permissions to the directories to bind mount.
+
+The solution Kolla chose is named volumes.
+
+Why does this matter in the case of selinux? Kolla does not run the process it
+is launching as root in most cases. So glance-api is run as the glance user,
+and mariadb is run as the mysql user, and so on. When mounting a named volume
+in the location that the persistent data will be stored it will be owned by the
+root user and group. The mysql user has no permissions to write to this folder
+now. What Kolla does is allow a select few commands to be run with sudo as the
+mysql user. This allows the mysql user to chown a specific, explicit directory
+and store its data in a named volume without the security risk and other
+downsides of host bind mounts. The downside to this is selinux blocks those
+sudo commands and it will do so until we make explicit policies to allow those
+operations.