crush depth

Having run FreeBSD for nearly two decades, I eventually got frustrated with it for various reasons and moved my workloads from "a lot of virtual machines on a FreeBSD bhyve host" to "a lot of OCI containers on a Linux host".

I try to run containers in as restricted and isolated a fashion as possible. Ultimately, I'm trying to limit the possible damage caused by a compromise of a single service. I've tried to take a layered approach starting from the kernel upwards. We don't have secure systems and probably never will, so all of my choices are based around making systems harder to exploit, and around limiting damage if and when something is compromised.

Distribution

I'm running containers on RHEL derivatives. There are multiple reasons for this:

• The distributions have a very up-to-date targeted SELinux policy that assigns each container a unique label. This severely limits what compromised container processes can actually do: Even if a container is completely compromised, the SELinux label effectively stops the container's processes from interacting with anything on the host system, and with any other containers.

• The distribution userland is compiled with a lot of exploit mitigation (stack protection, bounds-checked libc function replacements, hardened allocators, read-only executable sections, control flow integrity, etc). This makes it harder for any vulnerable processes to be exploited successfully. There are distributions that enable even more hardening, but these often require building from source (Hardened Gentoo, etc) and nobody has time for that.

• The distribution kernel typically has most of the various kernel hardening features enabled. I'm told that Fedora is an early adopter of most of these features, and they percolate down into RHEL derivatives when it's determined that they aren't terrible. This makes it harder for kernel exploits to actually work.

• The distribution kernel tends to exclude a lot of old drivers that don't have any place on modern "enterprise" hardware. Old drivers can be sources of vulnerabilities.

• The distributions have an incredibly boring and slow release schedule with no incompatible changes between major releases. This keeps things stable. It also means that it's safe to run an automatic package updater (such as dnf-automatic) to keep the host kernel up-to-date with the latest security patches.

I tried running CoreOS for a few years, but I found it too cumbersome. A one-line configuration file change requires a complete re-deploy of the entire virtual machine, and this can take 5-10 minutes depending on the VPS host. I understand what they're trying to do, but I just found it too annoying to work with in practice. Maybe if it was a lot faster...

I like Debian, but the lack of kernel hardening and the lack of a good container SELinux policy means that I've rejected it as a container host.

My requirement is that I want a userland and kernel that's hard to exploit. Containers don't have their own virtualized kernels in the manner that bhyve and other hypervisor guests do, so it's important to pick a kernel that's going to be less subject to any local kernel exploits that might be launched inside containers. RHEL derivatives seem to be about the best option that doesn't involve paying someone to build custom kernels as their full-time job.

Podman

I'm running containers with podman. The set of different ways one can run a podman container could fill a book. I've landed on the following set of configuration practices:

Rootful Containers

Containers can either be rootful (meaning the host's root user is the user that starts the container) or rootless (meaning a non-root user on the host starts the container).

I ran entirely rootless containers for over a year, with each container being run by different user on the host. This sounds like it should be optimal from a security perspective; if a container never had root privileges, then it presumably can't be the victim of privilege escalation.

Unfortunately, this turned out to be bad. Rootless containers are convenient for allowing users to run containers locally for development work and so on, but they're horrible for production use:

• Linux container networking is already obscenely complicated, error-prone, and fragile. Couple this with the requirement to be able to set up container networking without root privileges, and it eventually becomes necessary to more or less implement an entire fake virtual network stack. They are, as far as I'm aware, already onto their third "userland network stack". It's exactly as complex and error-prone as it looks, and when it fails, it does so silently. With rootful containers, you can simply use macvlan networking, assign each container an IP address, and sidestep most of the horrible complexity.

• Convenient commands like podman stats and podman ps only show the containers from the current user (by design). If all of the containers are being executed by different users, then the utility of these commands is basically lost. With rootful containers, you can run these commands as root and see all of the containers.

• Rootless containers actually end up somehow being less secure in many cases, because there are sandboxing features that require some privileges to be present in the first place in order to enable the features. In other words, a process needs to be slightly more privileged in order to be able to use sandboxing features to drop itself to a less privileged position than an ordinary unprivileged process. I'll cover this in a bit.

• The actual storage for container images ends up being spread over all of the home directories of all of the "users" that run the containers. With rootful containers, image data ends up in /var/lib/containers, and that's it.

I'm now exclusively running rootful containers and using a set of configuration options that result in containers that are, as far as I'm aware, as isolated from each other as possible, and as isolated from the host as possible.

read-only

This is the first blunt instrument in the box. When a container is run with --read-only, it's not possible for processes inside the container to write to anything in the filesystem except for in explicitly read/write mounted volumes. This has the following benefits:

• Containers become stateless (other than any explicit state in volumes that the admin provides).

• Compromised processes can't overwrite system binaries inside the container. A compromised process attempting to replace /usr/sbin/sshd will find that it can't.

• It breaks any poorly designed images that run massive piles of setup scripts on startup that write all over the filesystem. This means you learn quite quickly that an image is incompetently packaged, and you can replace it with a better one.

I couple this with exclusively using --rm so that every time a container is started, it's guaranteed to be starting from a pristine "has never run before" state.

cap-drop all

On Linux, the root user is simply an ordinary user that happens to have more capabilities by default. When a podman container is run by the root user, podman does automatically drop some capabilities that it knows containers should never have.

When the --cap-drop all command-line option is used, all of the possible capabilities are dropped, turning all of the processes inside the container into utterly unprivileged processes. A user that appears to be root inside the container won't be able to do any of the things the root user can usually do, such as chown files it doesn't own, and etc.

security-opt=no-new-privileges

The Linux kernel has a no new privileges flag that can be set for processes. This simply prevents any process from gaining any new privileges via an execve() call. This is essentially paranoia that I use in combination with --cap-drop all.

userns auto

On Linux, a user with ID 0 inside a container is equivalent to a user with ID 0 outside of the container. This means that, should a process running under user ID 0 somehow get access to parts of the host filesystem, then it can read and write files that are owned by user ID 0 on the host. It can also send signals to other processes that have user ID 0.

The --userns auto option eliminates this problem. Instead of the container's processes running under the same ID of the user that started the container (root), podman consults /etc/subuid and /etc/subgid and then picks a random region of 65536 UID and GID values that are guaranteed not to overlap with any on the host system, and guaranteed not to overlap with any in any other container. /etc/subuid and /etc/subgid tend to look like this:

# cat /etc/subuid
containers:2147483647:2147483647

# cat /etc/subgid
containers:2147483647:2147483647

The first number is the starting UID/GID, and the second number is the number of ID values that are permitted to be used. This essentially means that all container processes will be assigned UID/GID values in the range [2147483647, 4294967294].

For example, inside a container, nginx believes it is running as UID 0. However, on the host system, we can see that it isn't:

# ps -eo uid:20,pid,cmd | grep nginx | grep master
  2147479553  319065 nginx: master process nginx -p /nginx/ -c /nginx/etc/nginx.conf -e /nginx/var/error.log

It is actually running as UID 2147479553. The processes are also placed into an isolated user namespace.

When I mentioned earlier that there are sandboxing features that require privileges in order to get to a less privileged position, this is what I was referring to.

Volume Z flag

As mentioned, each container on a RHEL derivative gets a unique label. The SELinux policy is written such that volumes mounted in the container need to be assigned the same label or the container processes won't be able to read or write anything on the volume. The Z flag, when used on a volume, assigns a label that's unique to the specific container being executed, whilst the z flag assigns a label that can be used by all containers (if containers need to share data via the filesystem). For example:

# podman run --volume /data/containers/example01/data:/data:rw,Z ...

If another container was compromised and somehow got access to /data/containers/example01/data, it still wouldn't be able to actually read or write anything there because the SELinux label wouldn't match.

If a volume doesn't have a Z or z flag, the containers simply won't be able to read or write at all. It follows that, as the host system files and processes won't have these labels, compromised containers have no ability to read/write to locations on the host filesystem.

idmap

If containers need to share data via the filesystem, volumes can use the idmap flag. This performs a bit of magic that allows files to have the same UID/GID between different containers - something the --userns auto option would normally specifically prevent. The idmap flag requires a compatible filesystem (ZFS and XFS are good candidates) and also requires rootful containers, and so is another instance of having to start out with extra privileges in order to end up with fewer privileges.

seccomp

I don't apply any extra seccomp policies because it's too much of a maintenance burden. I do want to point out that podman applies its own restrictive seccomp policy to all containers by default.

For example, containers are forbidden from calling any of the system calls that are used to load kernel modules. This means that, even if you didn't apply any of the other security mechanisms that are listed here, a process running as root inside a container still wouldn't be quite as capable as a process running as root on the host.

It also means that, if a particular system call has an exploitable bug in it, a process inside the container might not necessarily be able to make the call and exploit the bug.

Resource Limits

I run each container in its own systemd slice which, essentially, means that each container gets its own cgroup.

I only use CPU and memory accounting, and I tend to limit container resources on a somewhat trial-and-error basis. I use ZFS filesystem quotas, in combination with giving each container its own filesystem, to prevent runaway container processes from filling up disks.

I tend to set memory limits such that containers run at roughly 80% of their prescribed limits. For example:

ID            NAME        CPU %       MEM USAGE / LIMIT  MEM %
97c719b8ea55  objects02   0.56%       406MB / 512MB      79.30%

Containers that exceed limits are simply killed and restarted (and I have monitoring to indicate OOM conditions, and to notify me if a container is repeatedly restarting).

Frustratingly, there seems to be some kind of bug in podman, at the time of writing, when running rootful containers. Sometimes resource limits are not applied. For safety, I specify the limits redundantly in the systemd unit file for each container:

[Service]
Slice=services-objects02.slice
Restart=on-failure
RestartSec=10s
TimeoutStopSec=70
TimeoutStartSec=300

CPUAccounting=true
CPUQuota=400%
MemoryAccounting=true
MemoryHigh=512000000
MemoryMax=512000000

[Container]
ContainerName=objects02
PodmanArgs=--cpus=4
PodmanArgs=--memory=512000000b

This ensures that, if podman fails to apply resource limits, the systemd service will still have those limits applied to the slice in which the container is placed, and so it will still be subject to those limits.

Networking

I'm using macvlan. This seems to be the "simplest" option in terms of moving parts; there's no NAT involved in the host kernel. You simply assign each container its own IP address, and the container appears as if it was a (real or virtual) machine on the LAN.

All Linux container networking schemes seem to have one or more fatal flaws that make them unusable for one workload or another, but the macvlan scheme seems to have the fewest of those, and is only available to rootful containers.

All of the schemes that are available to rootless containers have nightmarish lists of caveats and horrible failure modes that can result in excruciating amounts of debugging just to work out why packets aren't getting from one place to another. In my opinion, life is too short to bother with them.

I run extremely restrictive default-deny firewall rules on all hosts, but I won't go into detail on that here because it's not specific to containers.

Summary

• I run containers that start out life as rootful containers, but drop all privileges to the point that they're less privileged than rootless containers.
• I run containers as random unique UID and GID values in isolated user namespaces.
• I run containers with all capabilities dropped, and with process flags that prevent any acquisitions of new capabilities.
• I run containers on read-only filesystems.
• I set strict resource limits to quickly kill runaway (maliciously, or accidentally) processes.
• I prevent containers from having access to the host or to each other with SELinux labels.
• I run containers on reasonably hardened kernels.

At the time of writing, I believe this is about as "secure" as is practically possible. The biggest weakness of the system is kernel-level exploits; if someone in a container successfully exploits the kernel locally, then none of the security features matter. This is, however, also true of virtual machines except that the compromised virtual machine has to take one step further and compromise the host kernel.

[Interface]
Address    = 10.10.0.3/32, 70.0.0.1
ListenPort = 51820
MTU        = 1410
PrivateKey = ...

[Peer]
# Router01
AllowedIPs   = 10.10.100.1/32
PreSharedKey = ...
PublicKey    = ...

nat on $nic_wg1 from $nic_dmz:network to any -> ($nic_wg1)

[Interface]
PrivateKey = ...

[Peer]
AllowedIPs          = 10.10.0.0/16
Endpoint            = router02.io7m.com:51820
PreSharedKey        = ...
PublicKey           = ...
PersistentKeepalive = 1

# sysctl net.ipv4.ip_forward=1

table ip filter {
	chain FORWARD {
		type filter hook forward priority filter; policy accept;
		iifname "wg0" oifname "eth1" accept
		iifname "eth1" oifname "wg0" accept
	}
}
table ip nat {
	chain POSTROUTING {
		type nat hook postrouting priority srcnat; policy accept;
		ip saddr 10.10.100.1 oifname "eth1" snat to 10.10.0.3
	}
}

# ifconfig tun1
tun1: flags=1008051<UP,POINTOPOINT,RUNNING,MULTICAST,LOWER_UP> metric 0 mtu 1492

# ifconfig wg0 mtu 1410

# ip link set mtu 1410 up dev wg0

240516_80103601JK3300_xxxxxxxxxxxxx
                      ^^^^^^^^^^^^^

240511_8010F013826077_xxxxxxxxxxxxx
                      ^^^^^^^^^^^^^