From 36659d1efa4f745787026b5892f3f045293573ff Mon Sep 17 00:00:00 2001
From: Martin Weinelt <hexa@darmstadt.ccc.de>
Date: Wed, 16 Jun 2021 21:31:24 +0200
Subject: [PATCH] nixos/home-assistant: update hardening

This makes access to serial devices contingent on using certain
components and restricts the default setup even further.
---
 .../modules/services/misc/home-assistant.nix  | 56 +++++++++++++++++--
 nixos/tests/home-assistant.nix                |  2 +
 2 files changed, 54 insertions(+), 4 deletions(-)

diff --git a/nixos/modules/services/misc/home-assistant.nix b/nixos/modules/services/misc/home-assistant.nix
index c8ffead7b1ac..d68d7b05c173 100644
--- a/nixos/modules/services/misc/home-assistant.nix
+++ b/nixos/modules/services/misc/home-assistant.nix
@@ -268,6 +268,52 @@ in {
           "CAP_NET_BIND_SERVICE"
           "CAP_NET_RAW"
         ]));
+        componentsUsingBluetooth = [
+          # Components that require the AF_BLUETOOTH address family
+          "bluetooth_tracker"
+          "bluetooth_le_tracker"
+        ];
+        componentsUsingSerialDevices = [
+          # Components that require access to serial devices (/dev/tty*)
+          # List generated from home-assistant documentation:
+          #   git clone https://github.com/home-assistant/home-assistant.io/
+          #   cd source/_integrations
+          #   rg "/dev/tty" -l | cut -d'/' -f3 | cut -d'.' -f1 | sort
+          # And then extended by references found in the source code, these
+          # mostly the ones using config flows already.
+          "acer_projector"
+          "alarmdecoder"
+          "arduino"
+          "blackbird"
+          "dsmr"
+          "edl21"
+          "elkm1"
+          "elv"
+          "enocean"
+          "firmata"
+          "flexit"
+          "gpsd"
+          "insteon"
+          "kwb"
+          "lacrosse"
+          "mhz19"
+          "modbus"
+          "modem_callerid"
+          "mysensors"
+          "nad"
+          "numato"
+          "rflink"
+          "rfxtrx"
+          "scsgate"
+          "serial"
+          "serial_pm"
+          "sms"
+          "upb"
+          "velbus"
+          "w800rf32"
+          "xbee"
+          "zha"
+        ];
       in {
         ExecStart = "${package}/bin/hass --runner --config '${cfg.configDir}'";
         ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";
@@ -281,11 +327,11 @@ in {
         # Hardening
         AmbientCapabilities = capabilities;
         CapabilityBoundingSet = capabilities;
-        DeviceAllow = [
+        DeviceAllow = (optionals (any useComponent componentsUsingSerialDevices) [
           "char-ttyACM rw"
           "char-ttyAMA rw"
           "char-ttyUSB rw"
-        ];
+        ]);
         DevicePolicy = "closed";
         LockPersonality = true;
         MemoryDenyWriteExecute = true;
@@ -314,13 +360,15 @@ in {
           "AF_INET6"
           "AF_NETLINK"
           "AF_UNIX"
-        ] ++ optionals (useComponent "bluetooth_tracker" || useComponent "bluetooth_le_tracker") [
+        ] ++ optionals (any useComponent componentsUsingBluetooth) [
           "AF_BLUETOOTH"
         ];
         RestrictNamespaces = true;
         RestrictRealtime = true;
         RestrictSUIDSGID = true;
-        SupplementaryGroups = [ "dialout" ];
+        SupplementaryGroups = optionals (any useComponent componentsUsingSerialDevices) [
+          "dialout"
+        ];
         SystemCallArchitectures = "native";
         SystemCallFilter = [
           "@system-service"
diff --git a/nixos/tests/home-assistant.nix b/nixos/tests/home-assistant.nix
index c75dd248ecb3..f8f8e9fd183f 100644
--- a/nixos/tests/home-assistant.nix
+++ b/nixos/tests/home-assistant.nix
@@ -45,6 +45,7 @@ in {
           payload_on = "let_there_be_light";
           payload_off = "off";
         }];
+        # tests component-based capability assignment (CAP_NET_BIND_SERVICE)
         emulated_hue = {
           host_ip = "127.0.0.1";
           listen_port = 80;
@@ -100,6 +101,7 @@ in {
         assert "let_there_be_light" in output_log
 
     with subtest("Check systemd unit hardening"):
+        hass.log(hass.succeed("systemctl show home-assistant.service"))
         hass.log(hass.succeed("systemd-analyze security home-assistant.service"))
   '';
 })