hack/dind: update comments around AppArmor

Provide more context to the steps we're doing.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 65cfcc28ab)
Signed-off-by: Cory Snider <csnider@mirantis.com>

hack/dind

@@ -11,8 +11,39 @@ set -e
 # Usage: dind CMD [ARG...]
 
 # apparmor sucks and Docker needs to know that it's in a container (c) @tianon
+#
+# Set the container env-var, so that AppArmor is enabled in the daemon and
+# containerd when running docker-in-docker.
+#
+# see: https://github.com/containerd/containerd/blob/787943dc1027a67f3b52631e084db0d4a6be2ccc/pkg/apparmor/apparmor_linux.go#L29-L45
+# see: https://github.com/moby/moby/commit/de191e86321f7d3136ff42ff75826b8107399497
 export container=docker
 
+# Allow AppArmor to work inside the container;
+#
+#     aa-status
+#     apparmor filesystem is not mounted.
+#     apparmor module is loaded.
+#
+#     mount -t securityfs none /sys/kernel/security
+#
+#     aa-status
+#     apparmor module is loaded.
+#     30 profiles are loaded.
+#     30 profiles are in enforce mode.
+#       /snap/snapd/18357/usr/lib/snapd/snap-confine
+#       ...
+#
+# Note: https://0xn3va.gitbook.io/cheat-sheets/container/escaping/sensitive-mounts#sys-kernel-security
+#
+#     ## /sys/kernel/security
+#
+#     In /sys/kernel/security mounted the securityfs interface, which allows
+#     configuration of Linux Security Modules. This allows configuration of
+#     AppArmor policies, and so access to this may allow a container to disable
+#     its MAC system.
+#
+# Given that we're running privileged already, this should not be an issue.
 if [ -d /sys/kernel/security ] && ! mountpoint -q /sys/kernel/security; then
 	mount -t securityfs none /sys/kernel/security || {
 		echo >&2 'Could not mount /sys/kernel/security.'