Merge pull request #8114 from stephan13360/1.4-maint

add non-root deployment strategy

docs/deployment.rst

@@ -14,3 +14,4 @@ This chapter details deployment strategies for the following scenarios.
    deployment/automated-local
    deployment/image-backup
    deployment/pull-backup
+   deployment/non-root-user

docs/deployment/non-root-user.rst

@@ -0,0 +1,66 @@
+.. include:: ../global.rst.inc
+.. highlight:: none
+.. _non_root_user:
+
+================================
+Backing up using a non-root user
+================================
+
+This section describes how to run borg as a non-root user and still be able to
+backup every file on the system.
+
+Normally borg is run as the root user to bypass all filesystem permissions and
+be able to read all files. But in theory this also allows borg to modify or
+delete files on your system, in case of a bug for example.
+
+To eliminate this possibility, we can run borg as a non-root user and give it read-only
+permissions to all files on the system.
+
+
+Using Linux capabilities inside a systemd service
+=================================================
+
+One way to do so, is to use linux `capabilities
+<https://man7.org/linux/man-pages/man7/capabilities.7.html>`_ within a systemd
+service.
+
+Linux capabilities allow us to give parts of the privileges the root user has to
+a non-root user. This works on a per-thread level and does not give the permission
+to the non-root user as a whole.
+
+For this we need to run our backup script from a systemd service and use the `AmbientCapabilities
+<https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#AmbientCapabilities=>`_
+option added in systemd 229.
+
+A very basic unit file would look like this:
+
+::
+
+    [Unit]
+    Description=Borg Backup
+
+    [Service]
+    Type=oneshot
+    User=borg
+    ExecStart=/usr/local/sbin/backup.sh
+
+    AmbientCapabilities=CAP_DAC_READ_SEARCH
+
+The ``CAP_DAC_READ_SEARCH`` capability gives borg read-only access to all files and directories on the system.
+
+This service can then be started manually using ``systemctl start``, a systemd timer or other methods.
+
+Restore considerations
+======================
+
+When restoring files, the root user should be used. When using the non-root user, borg extract will
+change all files to be owned by the non-root user. Using borg mount will not allow the non-root user
+to access files that it would not have access to on the system itself.
+
+Other than that, the same restore process, that would be used when running the backup as root, can be used.
+
+.. warning::
+
+	When using a local repo and running borg commands as root, make sure to only use commands that do not
+    modify the repo itself, like extract or mount. Modifying the repo using the root user will break
+    the repo for the non-root user, since some files inside the repo will now be owned by root.