docs: add documentation for privileged and host devices

This commit adds documentation for privileged containers and the mounting of host devices
when privileged is used. It has instructions for disabling this functionality when using
Containerd and CRI.

Fixes kata-containers#529

Signed-off-by: Alex Price <[email protected]>

Limitations.md

@@ -231,6 +231,8 @@ This is also true with using `securityContext privileged=true` with Kubernetes.
 The container may also be granted full access to a subset of host devices
 (https://github.com/kata-containers/runtime/issues/1568).
 
+See [Privileged Kata Containers](how-to/privileged.md) for how to configure some of this behaviour. 
+
 # Miscellaneous
 
 This section lists limitations where the possible solutions are uncertain.

how-to/README.md

@@ -9,3 +9,4 @@
 - [Setting Sysctls with Kata](how-to-use-sysctls-with-kata.md)
 - [What Is VMCache and How To Enable It](what-is-vm-cache-and-how-do-I-use-it.md)
 - [What Is VM Templating and How To Enable It](what-is-vm-templating-and-how-do-I-use-it.md)
+- [Privileged Kata Containers](privileged.md)

how-to/privileged.md

@@ -0,0 +1,46 @@
+# Privileged Kata Containers
+
+Kata Containers supports creation of containers that are "privileged" - i.e. have additional capabilities and access
+that isn't normally granted.
+
+* [Caveats](#caveats)
+    * [Host Devices](#host-devices)
+        * [Containerd and CRI](#containerd-and-cri)
+
+## Warnings
+
+**Warning:** Whilst this functionality is supported, it can decrease the security of Kata Containers if not configured 
+correctly.
+
+### Host Devices
+
+By default, when privileged is enabled for a container, all of the `/dev/*` block devices from the host are mounted
+into the guest. This will allow the privileged container inside the Kata guest to gain access to mount any block device 
+from the host, a potentially undesirable side-effect that decreases the security of Kata.
+
+The following sections document how to configure this behaviour in different container runtimes.
+
+#### Containerd and CRI
+
+The Containerd CRI allows configuring the privileged host devices behaviour for each runtime in the CRI config. This is 
+done with the `privileged_without_host_devices` option. Setting this to `true` will disable hot plugging of the host 
+devices into the guest, even when privileged is enabled.
+
+See below example config:
+
+```toml
+[plugins]
+  [plugins.cri]
+    [plugins.cri.containerd]
+       [plugins.cri.containerd.runtimes.runc]
+         runtime_type = "io.containerd.runc.v1"
+         privileged_without_host_devices = false
+       [plugins.cri.containerd.runtimes.kata]
+         runtime_type = "io.containerd.kata.v2"
+         privileged_without_host_devices = true
+         [plugins.cri.containerd.runtimes.kata.options]
+           ConfigPath = "/opt/kata/share/defaults/kata-containers/configuration.toml"
+```
+
+ - [Kata Containers with Containerd and CRI documentation](how-to-use-k8s-with-cri-containerd-and-kata.md)
+ - [Containerd CRI config documentation](https://github.com/containerd/cri/blob/master/docs/config.md)