Drop capabilities in containers

We run containers as a non-root user (no effective capabilities).

Still, if a setuid binary is available in a container image, it could
potentially be used to give the user the default capabilities that the
container was started with. For Docker, the default set currently is:
- "CAP_CHOWN"
- "CAP_DAC_OVERRIDE"
- "CAP_FSETID"
- "CAP_FOWNER"
- "CAP_MKNOD"
- "CAP_NET_RAW"
- "CAP_SETGID"
- "CAP_SETUID"
- "CAP_SETFCAP"
- "CAP_SETPCAP"
- "CAP_NET_BIND_SERVICE"
- "CAP_SYS_CHROOT"
- "CAP_KILL"
- "CAP_AUDIT_WRITE"

We'd rather prevent such a potential escalation by dropping ALL
capabilities.

The problem is nicely explained here: https://github.com/projectatomic/atomic-site/issues/203

roles/matrix-corporal/templates/systemd/matrix-corporal.service.j2

@@ -12,6 +12,7 @@ ExecStartPre=-/usr/bin/docker rm matrix-corporal
 ExecStart=/usr/bin/docker run --rm --name matrix-corporal \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
+			--cap-drop=ALL \
 			--network={{ matrix_docker_network }} \
 			{% if matrix_corporal_container_expose_ports %}
 			-p 127.0.0.1:41080:41080 \

roles/matrix-coturn/templates/systemd/matrix-coturn.service.j2

@@ -12,6 +12,7 @@ ExecStartPre=-/usr/bin/docker rm matrix-coturn
 ExecStart=/usr/bin/docker run --rm --name matrix-coturn \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
+			--cap-drop=ALL \
 			-p 3478:3478 \
 			-p 3478:3478/udp \
 			-p {{ matrix_coturn_turn_udp_min_port }}-{{ matrix_coturn_turn_udp_max_port }}:{{ matrix_coturn_turn_udp_min_port }}-{{ matrix_coturn_turn_udp_max_port }}/udp \

roles/matrix-mailer/templates/systemd/matrix-mailer.service.j2

@@ -10,6 +10,7 @@ ExecStartPre=-/usr/bin/docker rm matrix-mailer
 ExecStart=/usr/bin/docker run --rm --name matrix-mailer \
 			--log-driver=none \
 			--user={{ matrix_mailer_container_user_uid }}:{{ matrix_mailer_container_user_gid }} \
+			--cap-drop=ALL \
 			--network={{ matrix_docker_network }} \
 			--env-file={{ matrix_mailer_base_path }}/env-mailer \
 			--hostname={{ hostname_matrix }} \

roles/matrix-mxisd/templates/systemd/matrix-mxisd.service.j2

@@ -15,6 +15,7 @@ ExecStartPre=-/usr/bin/docker rm matrix-mxisd
 ExecStart=/usr/bin/docker run --rm --name matrix-mxisd \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
+			--cap-drop=ALL \
 			--network={{ matrix_docker_network }} \
 			{% if matrix_mxisd_container_expose_port %}
 			-p 127.0.0.1:8090:8090 \

roles/matrix-nginx-proxy/templates/systemd/matrix-nginx-proxy.service.j2

@@ -15,6 +15,7 @@ ExecStartPre=-/usr/bin/docker rm matrix-nginx-proxy
 ExecStart=/usr/bin/docker run --rm --name matrix-nginx-proxy \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
+			--cap-drop=ALL \
 			--network={{ matrix_docker_network }} \
 			-p 80:8080 \
 			-p 443:8443 \

roles/matrix-postgres/templates/systemd/matrix-postgres.service.j2

@@ -10,6 +10,7 @@ ExecStartPre=-/usr/bin/docker rm matrix-postgres
 ExecStart=/usr/bin/docker run --rm --name matrix-postgres \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
+			--cap-drop=ALL \
 			--network={{ matrix_docker_network }} \
 			--env-file={{ matrix_postgres_base_path }}/env-postgres-server \
 			-v {{ matrix_postgres_data_path }}:/var/lib/postgresql/data \

roles/matrix-riot-web/templates/systemd/matrix-riot-web.service.j2

@@ -12,6 +12,7 @@ ExecStartPre=-/usr/bin/docker rm matrix-riot-web
 ExecStart=/usr/bin/docker run --rm --name matrix-riot-web \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
+			--cap-drop=ALL \
 			-v {{ matrix_riot_web_data_path }}/nginx.conf:/etc/nginx/nginx.conf:ro \
 			-v /dev/null:/etc/nginx/conf.d/default.conf:ro \
 			-v {{ matrix_riot_web_data_path }}/config.json:/etc/riot-web/config.json:ro \

roles/matrix-synapse/templates/synapse/systemd/matrix-synapse.service.j2

@@ -21,6 +21,7 @@ ExecStartPre=/bin/sleep 5
 ExecStart=/usr/bin/docker run --rm --name matrix-synapse \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
+			--cap-drop=ALL \
 			--entrypoint=python \
 			--network={{ matrix_docker_network }} \
 			-e SYNAPSE_CACHE_FACTOR={{ matrix_synapse_cache_factor }} \