Add impermanence config

flake.nix

@@ -3,10 +3,14 @@
 
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
     home-manager = {
       url = "github:nix-community/home-manager";
       inputs.nixpkgs.follows = "nixpkgs";
     };
+
+    # doesn't offer much above properly used symlinks but it is convenient
+    impermanence.url = "github:nix-community/impermanence";
   };
 
   outputs = {self, nixpkgs, ...} @ inputs: let

hosts/herugrim/default.nix

@@ -2,7 +2,6 @@
 {
   imports = [
     ./hardware-configuration.nix
-    ./impermanence.nix
   ];
 
   boot.supportedFilesystems = [ "btrfs" ];
@@ -23,11 +22,26 @@
     system = {
       hostname = "herugrim";
       username = "itsdrike";
+
+      impermanence.root = {
+        enable = true;
+        # Some people use /nix/persist/system for this, leaving persistent files in /nix subvolume
+        # I much prefer using a standalone subvolume for this though.
+        persistentMountPoint = "/persist";
+        # Configure automatic root subvolume wiping on boot from initrd
+        autoBtrfsWipe = {
+          devicePath = "/dev/disk/by-label/NIXROOT";
+          subvolumePath = "root";
+          cleanSnapshotPath = "root-blank";
         };
+      };
+    };
+
     device = {
       virtual-machine = false;
       cpu.type = "intel";
     };
+
     home-manager = {
       enable = true;
       stateVersion = "23.11";

hosts/herugrim/impermanence.nix

@@ -1,110 +0,0 @@
-{ config, pkgs, ... }:
-let
-  impermanence = builtins.fetchTarball "https://github.com/nix-community/impermanence/archive/master.tar.gz";
-in
-{
-  imports = [ "${impermanence}/nixos.nix" ];
-
-  users = {
-    # This option makes it that users are not mutable outside our configuration
-    # If you are using impermanence, this will actually be the case regardless of this setting,
-    # however, setting this explicitly is a good idea, because nix will warn us if
-    # our users don't have passwords set
-    mutableUsers = false;
-  
-    # Each existing user needs to have a password file defined here
-    # otherwise, they will not be available to login.
-    # These password files can be generated using the following command:
-    # mkpasswd -m sha-512 > /persist/system/passwords/myuser
-    users = {
-      root = {
-        # password file needs to be in a volume marked `neededForRoot = true`
-        hashedPasswordFile = "/persist/passwords/root";
-      };
-      itsdrike = {
-        hashedPasswordFile = "/persist/passwords/itsdrike";
-      };
-    };
-  };
-
-  # Some people use /nix/persist/system instead, leaving the persistent files in /nix subvolume
-  # I much prefer using a standalone subvolume for this though.
-  environment.persistence."/persist/system" = {
-    hideMounts = true;
-    directories = [
-      "/etc/nixos" # nixos configuration source
-      "/etc/NetworkManager/system-connections" # saved network connections
-      "/var/db/sudo" # keeps track of who got the sudo lecture already
-      "/var/lib/systemd/coredump" # recorded coredumps
-    ];
-    files = [
-      "/etc/machine-id"
-
-      # ssh stuff
-      "/etc/ssh/ssh_host_ed25519_key"
-      "/etc/ssh/ssh_host_ed25519_key.pub"
-      "/etc/ssh/ssh_host_rsa_key"
-      "/etc/ssh/ssh_host_rsa_key.pub"
-    ];
-  };
-
-  # For some reason, NetworkManager needs this instead of the impermanence mode to not get screwed up
-  systemd.tmpfiles.rules = [
-    "L /var/lib/NetworkManager/secret_key - - - - /persist/system/var/lib/NetworkManager/secret_key"
-    "L /var/lib/NetworkManager/seen-bssids - - - - /persist/system/var/lib/NetworkManager/seen-bssids"
-    "L /var/lib/NetworkManager/timestamps - - - - /persist/system/var/lib/NetworkManager/timestamps"
-  ];
-
-  boot.initrd.systemd = {
-    enable = true; # This enables systemd support in stage 1 - required for below setup
-    services.rollback = {
-      description = "Rollback BTRFS root subvolume to a pristine state";
-      wantedBy = [ "initrd.target" ];
-      # make sure it's done after decryption (i.e. LUKS/TPM process)
-      after = [ "systemd-cryptsetup@cryptfs.service" ];
-      # mount the root fs before clearing
-      before = [ "sysroot.mount" ];
-      unitConfig.DefaultDependencies = "no";
-      serviceConfig.Type = "oneshot";
-      script = ''
-        mkdir -p /mnt
-  
-        # We first mount the btrfs root to /mnt
-        # so we can manipulate btrfs subvolumes.
-        mount /dev/mapper/cryptfs /mnt
-
-        # While we're tempted to just delete /root and create
-        # a new snapshot from /root-blank, /root is already
-        # populated at this point with a number of subvolumes,
-        # which makes `btrfs subvolume delete` fail.
-        # So, we remove them first.
-        #
-        # /root contains subvolumes:
-        # - /root/var/lib/portables
-        # - /root/var/lib/machines
-        #
-        # These are probably related to systemd-nspawn, but
-        # since I don't use it, I'm not 100% sure.
-        # Anyhow, deleting these subvolumes hasn't resulted in
-        # any issues so far, except for fairly benign-looking
-        # errors from systemd-tmpfiles.
-        btrfs subvolume list -o /mnt/root |
-          cut -f9 -d' ' |
-          while read subvolume; do
-            echo "deleting /$subvolume subvolume..."
-            btrfs subvolume delete "/mnt/$subvolume"
-          done &&
-          echo "deleting /root subvolume..." &&
-          btrfs subvolume delete /mnt/root
-  
-        echo "restoring blank /root subvolume..."
-        btrfs subvolume snapshot /mnt/root-blank /mnt/root
-  
-        # Once we're done rolling back to a blank snapshot,
-        # we can unmount /mnt and continue on the boot process.
-        umount /mnt
-      '';
-    };
-  };
-}
-

options/system/default.nix

@@ -2,7 +2,9 @@
   inherit (lib) mkOption;
 in
 {
-  imports = [ ];
+  imports = [
+    ./impermanence.nix
+  ];
 
   options.myOptions.system = {
     hostname = mkOption {

options/system/impermanence.nix

@@ -0,0 +1,81 @@
+{ lib, config, ... }: with lib; let
+  inherit (lib) mkEnableOption mkOption literalExpression;
+
+  cfg = config.myOptions.system.impermanence;
+in
+{
+  options.myOptions.system.impermanence = {
+    root = {
+      enable = mkEnableOption ''
+        the Impermanence module for persisting important state directories.
+      '';
+
+      extraFiles = mkOption {
+        default = [];
+        example = literalExpression ''["/etc/nix/id_rsa"]'';
+        description = ''
+          Additional files in root to link to persistent storage.
+        '';
+      };
+
+      extraDirectories = mkOption {
+        default = [];
+        example = literalExpression ''["/etc/nix/id_rsa"]'';
+        description = ''
+          Additional directories in root to link to persistent storage.
+        '';
+      };
+
+      persistentMountPoint = mkOption {
+        default = "/persist";
+        description = ''
+          Path to a persistent directory (usually a mount point to a
+          standalone partition / subvolume), which will hold the persistent
+          system state files.
+        '';
+      };
+
+      autoBtrfsWipe = {
+        enable = mkOption {
+          default = true;
+          description = ''
+            Enable automatic wiping of the root BTRFS subvolume from initrd.
+
+            Generally, you will want to keep this enabled, as otherwise setting up
+            impermanence is pointless. However in case you're using a non-BTRFS
+            system, or you wish to set up a custom handling for this auto-wiping,
+            which the current handling doesn't support, disable this.
+          '';
+        };
+
+        devicePath = mkOption {
+          default = "/dev/mapper/cryptfs";
+          description = ''
+            Path to the BTRFS block device containing the subvolume to be wiped.
+
+            This device will be mounted from initrd.
+          '';
+        };
+
+        subvolumePath = mkOption {
+          default = "root";
+          description = ''
+            Path to the BTRFS subvolume to be wiped.
+
+            This is a relative path, starting from the BTRFS root.
+          '';
+        };
+
+        cleanSnapshotPath = mkOption {
+          default = "root-blank";
+          description = ''
+            Path to the BTRFS snapshot (subvolume) to be restore 
+            `myOptions.system.impermanence.root.autoWipe.btrfsSubvolume` to.
+
+            This should be a blank snapshot to achieve a complete wipe.
+          '';
+        };
+      };
+    };
+  };
+}

system/default.nix

@@ -4,6 +4,7 @@ _: {
     ./boot
     ./services
     ./nix
+    ./impermanence
     ./programs.nix
     ./system.nix
     ./network.nix

system/impermanence/default.nix

@@ -0,0 +1,7 @@
+{ inputs, ... }:
+{
+  imports = [
+    inputs.impermanence.nixosModules.impermanence
+    ./root.nix
+  ];
+}

system/impermanence/root.nix

@@ -0,0 +1,110 @@
+{ config }: let
+  cfgSystem = config.myOptions.system;
+  cfg = config.myOptions.system.impermanence.root;
+in
+{
+  config = mkIf cfg.enable {
+    users = {
+      # This option makes it that users are not mutable outside of our configuration.
+      # If you're using root impermanence, this will actually be the case regardless
+      # of this setting, however, setting this explicitly is a good idea, because nix
+      # will warn us if our users don't have passwords set, preventing lock outs.
+      mutableUsers = false;
+
+      # Each existing user needs to have a password file defined here, otherwise
+      # they will not be available to login. These password files can be generated with:
+      # mkpasswd -m sha-512 > /persist/passwords/myuser
+      users = {
+        root = {
+          hashedPasswordFile = "${cfg.persistentMountPoint}/passwords/root";
+        };
+        ${cfgSystem.username} = {
+          hashedPasswordFile = "${cfg.persistentMountPoint}/passwords/${cfgSystem.username}";
+        };
+      };
+    };
+
+    environment.persistence."${cfg.persistentMountPoint}/system" = {
+      hideMounts = true;
+      directories = [
+        "/etc/nixos" # NixOS configuration source
+        "/etc/NetworkManager/system-connections" # saved network connections
+        "/var/db/sudo" # keeps track of who got the sudo lecture already
+        "/var/lib/systemd/coredump" # captured coredumps 
+      ] ++ cfg.extraDirectories;
+
+      files = [
+        "/etc/machine-id"
+      ] ++ cfg.extraFiles;
+    };
+
+    # For some reason, NetworkManager needs this instead of the impermanence mode
+    # to not get screwed up
+    systemd.tmpfiles.rules = [
+      "L /var/lib/NetworkManager/secret_key - - - - ${cfg.persistentMountPoint}/system/var/lib/NetworkManager/secret_key"
+      "L /var/lib/NetworkManager/seen-bssids - - - - ${cfg.persistentMountPoint}/system/var/lib/NetworkManager/seen-bssids"
+      "L /var/lib/NetworkManager/timestamps - - - - ${cfg.persistentMountPoint}/system/var/lib/NetworkManager/timestamps"
+    ];
+
+    # Define host key paths in the persistent mount point instead of using impermanence for these.
+    # This works better, because these keys also get auto-created if they don't already exist.
+    services.openssh.hostKeys = mkForce [
+      {
+        bits = 4096;
+        path = "${cfg.persistentMountPoint}/system/etc/ssh/ssh_host_rsa_key";
+        type = "rsa";
+      }
+      {
+        bits = 4096;
+        path = "${cfg.persistentMountPoint}/system/etc/ssh/ssh_host_ed25519_key";
+        type = "ed25519";
+      }
+    ];
+
+    boot.initrd.systemd = let
+      cfgWipe = cfg.autoBtrfsWipe;
+    in {
+      enable = true; # This enables systemd support in stage 1 - required for below setup
+      services.rollback-root = {
+        description = "Rollback BTRFS root subvolume to a pristine state";
+        enable = cfgWipe.enable;
+        wantedby = [ "initrd.target" ];
+        # Make sure it's done after decryption (i.e. LUKS/TPM process)
+        after = [ "systemd-cryptsetup@cryptfs.service" ];
+        # mount the root fs before clearing
+        before = [ "sysroot.mount" ];
+        unitConfig.DefaultDependencies = "no";
+        serviceConfig.Type = "oneshot";
+        script = ''
+          # Mount the BTRFS root to /mnt, so we can manipulate the subvolumes
+          mount --mkdir ${cfgWipe.devicePath} /mnt
+          
+          # To restore the root subvolume, we will first delete it, and then create
+          # a new snapshot from the blank snapshot, which will become our new root subvolume
+
+          # However, at this point, root subvol is already populated and contains a number
+          # of subvolumes, which would make `btrfs subvolume delete` fail.
+          #
+          # These existing subvolumes get created automatically, and we can safely remove
+          # them. They are: /srv, /var/lib/portables, /var/lib/machines, /var/tmp
+          sudo btrfs subvolume list -o "/mnt/${cfgWipe.subvolumePath}" | cut -f9 -d' ' |
+            while read subvolme; do
+              echo "deleting $subvolume subvolume..." &&
+              btrfs subvolume delete "/mnt/$subvolume"
+            done
+
+          # Now we can remove the root subvolume, and restore it from a snapshot
+          echo "deleting ${cfgWipe.subvolumePath} (root) subvolume..."
+          btrfs subvolume delete "/mnt/${cfg.subvolumePath}"
+          echo "restoring ${cfgWipe.subvolumePath} (root) subvolume..."
+          btrfs subvolume snapshot "/mnt/${cfgWipe.cleanSnapshotPath}"
+          "/mnt/${cfgWipe.subvolumePath}"
+
+          # Once we're done rolling back to a blank snapshot,
+          # we can unmount /mnt and continue on the boot process
+          umount /mnt
+        '';
+      };
+    };
+  };
+}