lnd: add strict hardening

Add ProtectSystem=strict, remove PermissionStartOnly.

Extract the section of postStart that needs secrets dir write
access into a separate script with full privileges.

Simplify preStart and fix dataDir quoting.

modules/lnd.nix

@@ -95,41 +95,38 @@ in {
       requires = [ "bitcoind.service" ];
       after = [ "bitcoind.service" ];
       preStart = ''
-        cp ${configFile} ${cfg.dataDir}/lnd.conf
-        chown -R 'lnd:lnd' '${cfg.dataDir}'
-        chmod u=rw,g=r,o= ${cfg.dataDir}/lnd.conf
+        install -m600 ${configFile} '${cfg.dataDir}/lnd.conf'
         echo "bitcoind.rpcpass=$(cat ${secretsDir}/bitcoin-rpcpassword)" >> '${cfg.dataDir}/lnd.conf'
       '';
       serviceConfig = nix-bitcoin-services.defaultHardening // {
-        PermissionsStartOnly = "true";
         ExecStart = "${cfg.package}/bin/lnd --configfile=${cfg.dataDir}/lnd.conf";
         User = "lnd";
         Restart = "on-failure";
         RestartSec = "10s";
-        ProtectSystem = "full"; # ToDo: Make more restrictive
-      } // (if cfg.enforceTor
-          then nix-bitcoin-services.allowTor
-          else nix-bitcoin-services.allowAnyIP
-        ) // nix-bitcoin-services.allowAnyProtocol;  # For ZMQ
-      postStart = let
-        mainnetDir = "${cfg.dataDir}/chain/bitcoin/mainnet";
-      in ''
-        umask 377
-
+        ReadWritePaths = "${cfg.dataDir}";
+        ExecStartPost = [
+          # Run fully privileged for secrets dir write access
+          "+${nix-bitcoin-services.script ''
             attempts=50
             while ! { exec 3>/dev/tcp/127.0.0.1/8080 && exec 3>&-; } &>/dev/null; do
                   ((attempts-- == 0)) && { echo "lnd REST service unreachable"; exit 1; }
                   sleep 0.1
             done
 
-        if [[ ! -f ${secretsDir}/lnd-seed-mnemonic ]]; then
+            mnemonic=${secretsDir}/lnd-seed-mnemonic
+            if [[ ! -f $mnemonic ]]; then
               echo Create lnd seed
 
               ${pkgs.curl}/bin/curl -s \
                 --cacert ${secretsDir}/lnd-cert \
-            -X GET https://127.0.0.1:8080/v1/genseed | ${pkgs.jq}/bin/jq -c '.cipher_seed_mnemonic' > ${secretsDir}/lnd-seed-mnemonic
+                -X GET https://127.0.0.1:8080/v1/genseed | ${pkgs.jq}/bin/jq -c '.cipher_seed_mnemonic' > "$mnemonic"
             fi
-
+            chown lnd: "$mnemonic"
+            chmod 400 "$mnemonic"
+          ''}"
+          "${let
+            mainnetDir = "${cfg.dataDir}/chain/bitcoin/mainnet";
+          in nix-bitcoin-services.script ''
             if [[ ! -f ${mainnetDir}/wallet.db ]]; then
               echo Create lnd wallet
 
@@ -159,7 +156,12 @@ in {
             while ! { exec 3>/dev/tcp/127.0.0.1/${toString cfg.rpcPort}; } &>/dev/null; do
               sleep 0.1
             done
-      '';
+          ''}"
+        ];
+      } // (if cfg.enforceTor
+          then nix-bitcoin-services.allowTor
+          else nix-bitcoin-services.allowAnyIP
+        ) // nix-bitcoin-services.allowAnyProtocol;  # For ZMQ
     };
     users.users.lnd = {
       description = "LND User";

modules/modules.nix

@@ -21,7 +21,7 @@
   options = {
     nix-bitcoin-services = lib.mkOption {
       readOnly = true;
-      default = import ./nix-bitcoin-services.nix lib;
+      default = import ./nix-bitcoin-services.nix lib pkgs;
     };
   };
 

modules/nix-bitcoin-services.nix

@@ -1,7 +1,7 @@
 # See `man systemd.exec` and `man systemd.resource-control` for an explanation
 # of the various systemd options available through this module.
 
-lib:
+lib: pkgs:
 
 with lib;
 {
@@ -42,4 +42,9 @@ with lib;
       to 127.0.0.1;";
     '';
   };
+
+  script = src: pkgs.writers.writeBash "script" ''
+    set -eo pipefail
+    ${src}
+  '';
 }