Services failed to start after upgrading to CentOS 7.6

Symptoms

Services do not start after upgrade to CentOS 7.6 due to SELinux.

It was disabled before but become enabled after the upgrade:

# getenforce
Enforcing

Cause

Missing /etc/selinux/config file before the upgrade on some preinstalled servers from hosting providers.

Resolution

If an upgrade was done

Connect to the server via SSH and disable SELinux setting the following in /etc/selinux/config:

CONFIG_TEXT: SELINUX=disabled

Reboot the server. 

If an upgrade wasn't done

Before the upgrade on another server, make sure that /etc/selinux/config file exists and contains the following:

# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#     enforcing - SELinux security policy is enforced.
#     permissive - SELinux prints warnings instead of enforcing.
#     disabled - No SELinux policy is loaded.
SELINUX=disabled
# SELINUXTYPE= can take one of three values:
#     targeted - Targeted processes are protected,
#     minimum - Modification of targeted policy. Only selected processes are protected.
#     mls - Multi Level Security protection.
SELINUXTYPE=targeted