Add vault_harden_file_perms to set chmod 0550 on config/plugins path

The [Production
Hardening](https://learn.hashicorp.com/tutorials/vault/production-hardening)
have a bullet point "Allow minimal write privileges". It states: "its
executable binary or any Vault configuration files".

Prior to this change, the config and plugins path had chmod 0750, so
Vault could actually write config files and change plugins.

This commit adds a new parameter named vault_harden_file_perms (turned
off by default). When enabled, it changes the chmod of config and
plugins path to 0550 to effectively disallow Vault from writing into
these dirs.

Signed-off-by: Albin Kerouanton <albinker@gmail.com>

README.md

@@ -171,6 +171,12 @@ The role defines variables in `defaults/main.yml`:
 - PID file location
 - Default value: `/var/run/vault`
 
+### `vault_harden_file_perms`
+
+- Whether this role should disallow Vault from writing into config and plugin
+  path. This should be enabled to follow [Production Hardening](https://learn.hashicorp.com/tutorials/vault/production-hardening).
+- Default value: false
+
 ### `vault_manage_user`
 
 - Should this role manage the vault user?

defaults/main.yml

@@ -38,6 +38,7 @@ vault_data_path: /var/vault
 vault_log_path: /var/log/vault
 vault_run_path: /var/run/vault
 vault_home: "/home/{{ vault_user }}"
+vault_harden_file_perms: true
 
 # System user and group
 vault_manage_user: true

tasks/main.yml

@@ -104,17 +104,22 @@
 - name: Create directories
   become: true
   file:
-    dest: "{{ item }}"
+    dest: "{{ item.path }}"
     state: directory
     owner: "{{ vault_user }}"
     group: "{{ vault_group }}"
-    mode: 0750
+    mode: "{{ item.mode }}"
   with_items:
-    - "{{ vault_config_path }}"
-    - "{{ vault_plugin_path }}"
-    - "{{ vault_data_path }}"
-    - "{{ vault_log_path }}"
-    - "{{ vault_run_path }}"
+    - path: "{{ vault_config_path }}"
+      mode: "{{ vault_harden_file_perms | ternary('0550', '0750') }}"
+    - path: "{{ vault_plugin_path }}"
+      mode: "{{ vault_harden_file_perms | ternary('0550', '0750') }}"
+    - path: "{{ vault_data_path }}"
+      mode: "0750"
+    - path: "{{ vault_log_path }}"
+      mode: "0750"
+    - path: "{{ vault_run_path }}"
+      mode: "0750"
 
 - name: Enable logrotate for vault
   become: true