In the first of this series on Docker security, I wrote "containers do not contain." In this second article, I'll cover why and what we're doing about it.
Docker, Red Hat, and the open source community are working together to make Docker more secure. When I look at security containers, I am looking to protect the host from the processes within the container, and I'm also looking to protect containers from each other. With Docker we are using the layered security approach, which is "the practice of combining multiple mitigating security controls to protect resources and data."
Basically, we want to put in as many security barriers as possible to prevent a break out. If a privileged process can break out of one containment mechanism, we want to block them with the next. With Docker, we want to take advantage of as many security mechanisms of Linux as possible.
Luckily, with Red Hat Enterprise Linux (RHEL) 7, we get a plethora of security features.
File System Protections
Read-only mount points
Some Linux kernel file systems have to be mounted in a container environment or processes would fail to run. Fortunately, most of these filesystems can be mounted as "read-only". Most apps should never need to write to these file systems.
Docker mounts these file systems into the container as "read-only" mount points.
By mounting these file systems as read-only, privileged container processes cannot write to them. They cannot effect the host system. Of course, we also block the ability of the privileged container processes from remounting the file systems as read/write. We block the ability to mount any file systems at all within the container. I will explain how we block mounts when we get to capabilities.
Copy-on-write file systems
Docker uses copy-on-write file systems. This means containers can use the same file system image as the base for the container. When a container writes content to the image, it gets written to a container specific file system. This prevents one container from seeing the changes of another container even if they wrote to the same file system image. Just as important, one container can not change the image content to effect the processes in another container.
Linux capabilities are explained well on their main page:
Read the entire article: Bringing new security features to Docker - opensource.com.Last updated: April 5, 2018