AppArmor & SELinux

Mandatory access control on files and capabilities.

seccomp filters which syscalls a process may make; it says nothing about which files that process may touch. A Linux Security Module — AppArmor on Debian/Ubuntu, SELinux on RHEL/Fedora — adds mandatory access control: a policy the kernel enforces regardless of file permissions or user, confining a container to a declared set of paths, capabilities, and operations. It is the layer that turns “this process can open any file” into “this process can open these files.”

# Docker applies a default AppArmor profile (docker-default) automatically
$ docker run --rm alpine cat /proc/self/attr/current
docker-default (enforce)          # the container is confined by AppArmor
# load and apply a tighter custom profile:
$ sudo apparmor_parser -r -W /etc/apparmor.d/docker-myapp
$ docker run --rm --security-opt apparmor=docker-myapp myapp:1.0

A profile is an allowlist of paths and caps

An AppArmor profile enumerates what the process may do: read these paths, write those, use these capabilities, deny everything else. A tight web-server profile might allow reading its content directory and binding a socket, permit writes only to a cache path, and explicitly deny writing to binary directories or reading /etc/shadow — so even a fully compromised process is boxed into the behavior you declared.

#include <tunables/global>
profile docker-myapp flags=(attach_disconnected,mediate_deleted) {
  #include <abstractions/base>
  network inet tcp,
  /app/** r,                    # read app files
  /var/cache/app/** rw,         # one writable path
  deny /etc/shadow rwkl,        # never touch credentials
  deny /bin/** wl,              # no writing into binary dirs
  deny mount,                   # no mounting (a common escape primitive)
}

SELinux does the same with labels

On RHEL-family hosts the equivalent is SELinux, which labels every process and file and enforces type-based rules — container_t processes may only touch container_file_t content, so a container simply cannot read host files that carry a different label. The concepts map: AppArmor is path-based, SELinux is label-based, both are mandatory access control the kernel enforces beneath the app. Run a new profile in complain/permissive mode first, tune from the denials, then enforce.