add support for building fully dm-verity protected images with system…

…d-repart (NixOS#343252)

nixos/doc/manual/release-notes/rl-2411.section.md

@@ -46,6 +46,9 @@
   If you experience any issues, please report them.
   The original Perl script can still be used for now by setting `system.switch.enableNg` to `false`.
 
+- Support for mounting filesystems from block devices protected with [dm-verity](https://docs.kernel.org/admin-guide/device-mapper/verity.html)
+  was added through the `boot.initrd.systemd.dmVerity` option.
+
 - The [Xen Hypervisor](https://xenproject.org) is once again available as a virtualisation option under [`virtualisation.xen`](#opt-virtualisation.xen.enable).
   - This release includes Xen [4.17.5](https://wiki.xenproject.org/wiki/Xen_Project_4.17_Release_Notes), [4.18.3](https://wiki.xenproject.org/wiki/Xen_Project_4.18_Release_Notes) and [4.19.0](https://wiki.xenproject.org/wiki/Xen_Project_4.19_Release_Notes), as well as support for booting the hypervisor on EFI systems.
   ::: {.warning}

nixos/modules/image/assert_uki_repart_match.py

@@ -0,0 +1,78 @@
+import json
+import sys
+
+store_verity_type = "@NIX_STORE_VERITY@"  # replaced at import by Nix
+
+
+def extract_uki_cmdline_params(ukify_json: dict) -> dict[str, str]:
+    """
+    Return a dict of the parameters in the .cmdline section of the UKI
+    Exits early if "usrhash" is not included.
+    """
+    cmdline = ukify_json.get(".cmdline", {}).get("text")
+    if cmdline is None:
+        print("Failed to get cmdline from ukify output")
+
+    params = {}
+    for param in cmdline.split():
+        key, val = param.partition("=")[::2]
+        params[key] = val
+
+    if "usrhash" not in params:
+        print(
+            f"UKI cmdline does not contain a usrhash:\n{cmdline}"
+        )
+        exit(1)
+
+    return params
+
+
+def hashes_match(partition: dict[str, str], expected: str) -> bool:
+    """
+    Checks if the value of the "roothash" key in the passed partition object matches `expected`.
+    """
+    if partition.get("roothash") != expected:
+        pretty_part = json.dumps(partition, indent=2)
+        print(
+            f"hash mismatch, expected to find roothash {expected} in:\n{pretty_part}"
+        )
+        return False
+    else:
+        return True
+
+
+def check_partitions(
+    partitions: list[dict], uki_params: dict[str, str]
+) -> bool:
+    """
+    Checks if the usrhash from `uki_params` has a matching roothash
+    for the corresponding partition in `partitions`.
+    """
+    for part in partitions:
+        if part.get("type") == store_verity_type:
+            expected = uki_params["usrhash"]
+            return hashes_match(part, expected)
+
+    return False
+
+
+def main() -> None:
+    ukify_json = json.load(sys.stdin)
+    repart_json_output = sys.argv[1]
+
+    with open(repart_json_output, "r") as r:
+        repart_json = json.load(r)
+
+    uki_params = extract_uki_cmdline_params(ukify_json)
+
+    if check_partitions(repart_json, uki_params):
+        print("UKI and repart verity hashes match")
+    else:
+        print("Compatibility check for UKI and image failed!")
+        print(f"UKI cmdline parameters:\n{uki_params}")
+        print(f"repart config: {repart_json_output}")
+        exit(1)
+
+
+if __name__ == "__main__":
+    main()

nixos/modules/image/repart-verity-store.nix

@@ -0,0 +1,209 @@
+# opinionated module that can be used to build nixos images with
+# a dm-verity protected nix store
+{
+  config,
+  pkgs,
+  lib,
+  ...
+}:
+let
+  cfg = config.image.repart.verityStore;
+
+  verityMatchKey = "store";
+
+  # TODO: make these and other arch mappings available from systemd-lib for example
+  partitionTypes = {
+    usr =
+      {
+        "x86_64" = "usr-x86-64";
+        "arm64" = "usr-arm64";
+      }
+      ."${pkgs.stdenv.hostPlatform.linuxArch}";
+
+    usr-verity =
+      {
+        "x86_64" = "usr-x86-64-verity";
+        "arm64" = "usr-arm64-verity";
+      }
+      ."${pkgs.stdenv.hostPlatform.linuxArch}";
+  };
+
+  verityHashCheck =
+    pkgs.buildPackages.writers.writePython3Bin "assert_uki_repart_match.py"
+      {
+        flakeIgnore = [ "E501" ]; # ignores PEP8's line length limit of 79 (black defaults to 88 characters)
+      }
+      (
+        builtins.replaceStrings [ "@NIX_STORE_VERITY@" ] [
+          partitionTypes.usr-verity
+        ] (builtins.readFile ./assert_uki_repart_match.py)
+      );
+in
+{
+  options.image.repart.verityStore = {
+    enable = lib.mkEnableOption "building images with a dm-verity protected nix store";
+
+    ukiPath = lib.mkOption {
+      type = lib.types.str;
+      default = "/EFI/Linux/${config.system.boot.loader.ukiFile}";
+      defaultText = "/EFI/Linux/\${config.system.boot.loader.ukiFile}";
+      description = ''
+        Specify the location on the ESP where the UKI is placed.
+      '';
+    };
+
+    partitionIds = {
+      esp = lib.mkOption {
+        type = lib.types.str;
+        default = "00-esp";
+        description = ''
+          Specify the attribute name of the ESP.
+        '';
+      };
+      store-verity = lib.mkOption {
+        type = lib.types.str;
+        default = "10-store-verity";
+        description = ''
+          Specify the attribute name of the store's dm-verity hash partition.
+        '';
+      };
+      store = lib.mkOption {
+        type = lib.types.str;
+        default = "20-store";
+        description = ''
+          Specify the attribute name of the store partition.
+        '';
+      };
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    boot.initrd.systemd.dmVerity.enable = true;
+
+    image.repart.partitions = {
+      # dm-verity hash partition
+      ${cfg.partitionIds.store-verity}.repartConfig = {
+        Type = partitionTypes.usr-verity;
+        Verity = "hash";
+        VerityMatchKey = lib.mkDefault verityMatchKey;
+        Label = lib.mkDefault "store-verity";
+      };
+      # dm-verity data partition that contains the nix store
+      ${cfg.partitionIds.store} = {
+        storePaths = [ config.system.build.toplevel ];
+        repartConfig = {
+          Type = partitionTypes.usr;
+          Verity = "data";
+          Format = lib.mkDefault "erofs";
+          VerityMatchKey = lib.mkDefault verityMatchKey;
+          Label = lib.mkDefault "store";
+        };
+      };
+
+    };
+
+    system.build = {
+
+      # intermediate system image without ESP
+      intermediateImage =
+        (config.system.build.image.override {
+          # always disable compression for the intermediate image
+          compression.enable = false;
+        }).overrideAttrs
+          (
+            _: previousAttrs: {
+              # make it easier to identify the intermediate image in build logs
+              pname = "${previousAttrs.pname}-intermediate";
+
+              # do not prepare the ESP, this is done in the final image
+              systemdRepartFlags = previousAttrs.systemdRepartFlags ++ [ "--defer-partitions=esp" ];
+
+              # the image will be self-contained so we can drop references
+              # to the closure that was used to build it
+              unsafeDiscardReferences.out = true;
+            }
+          );
+
+      # UKI with embedded usrhash from intermediateImage
+      uki =
+        let
+          inherit (config.system.boot.loader) ukiFile;
+          cmdline = "init=${config.system.build.toplevel}/init ${toString config.boot.kernelParams}";
+        in
+        # override the default UKI
+        lib.mkOverride 99 (
+          pkgs.runCommand ukiFile
+            {
+              nativeBuildInputs = [
+                pkgs.jq
+                pkgs.systemdUkify
+              ];
+            }
+            ''
+              mkdir -p $out
+
+              # Extract the usrhash from the output of the systemd-repart invocation for the intermediate image.
+              usrhash=$(jq -r \
+                '.[] | select(.type=="${partitionTypes.usr-verity}") | .roothash' \
+                ${config.system.build.intermediateImage}/repart-output.json
+              )
+
+              # Build UKI with the embedded usrhash.
+              ukify build \
+                  --config=${config.boot.uki.configFile} \
+                  --cmdline="${cmdline} usrhash=$usrhash" \
+                  --output="$out/${ukiFile}"
+            ''
+        );
+
+      # final system image that is created from the intermediate image by injecting the UKI from above
+      finalImage =
+        (config.system.build.image.override {
+          # continue building with existing intermediate image
+          createEmpty = false;
+        }).overrideAttrs
+          (
+            finalAttrs: previousAttrs:
+            let
+              copyUki = "CopyFiles=${config.system.build.uki}/${config.system.boot.loader.ukiFile}:${cfg.ukiPath}";
+            in
+            {
+              nativeBuildInputs = previousAttrs.nativeBuildInputs ++ [
+                pkgs.systemdUkify
+                verityHashCheck
+              ];
+
+              postPatch = ''
+                # add entry to inject UKI into ESP
+                echo '${copyUki}' >> $finalRepartDefinitions/${cfg.partitionIds.esp}.conf
+              '';
+
+              preBuild = ''
+                # check that we build the final image with the same intermediate image for
+                # which the injected UKI was built by comparing the UKI cmdline with the repart output
+                # of the intermediate image
+                #
+                # This is necessary to notice incompatible substitutions of
+                # non-reproducible store paths, for example when working with distributed
+                # builds, or when offline-signing the UKI.
+                ukify --json=short inspect ${config.system.build.uki}/${config.system.boot.loader.ukiFile} \
+                  | assert_uki_repart_match.py "${config.system.build.intermediateImage}/repart-output.json"
+
+                # copy the uncompressed intermediate image, so that systemd-repart picks it up
+                cp -v ${config.system.build.intermediateImage}/${config.image.repart.imageFileBasename}.raw .
+                chmod +w ${config.image.repart.imageFileBasename}.raw
+              '';
+
+              # the image will be self-contained so we can drop references
+              # to the closure that was used to build it
+              unsafeDiscardReferences.out = true;
+            }
+          );
+    };
+  };
+
+  meta.maintainers = with lib.maintainers; [
+    nikstur
+    willibutz
+  ];
+}

nixos/modules/image/repart.nix

@@ -69,6 +69,10 @@ let
   }) opts;
 in
 {
+  imports = [
+    ./repart-verity-store.nix
+  ];
+
   options.image.repart = {
 
     name = lib.mkOption {

nixos/modules/module-list.nix

@@ -1625,6 +1625,7 @@
   ./system/boot/stage-2.nix
   ./system/boot/systemd.nix
   ./system/boot/systemd/coredump.nix
+  ./system/boot/systemd/dm-verity.nix
   ./system/boot/systemd/initrd-secrets.nix
   ./system/boot/systemd/initrd.nix
   ./system/boot/systemd/journald.nix

nixos/modules/system/boot/systemd/dm-verity.nix

@@ -0,0 +1,61 @@
+{ config, lib, ... }:
+
+let
+  cfg = config.boot.initrd.systemd.dmVerity;
+in
+{
+  options = {
+    boot.initrd.systemd.dmVerity = {
+      enable = lib.mkEnableOption "dm-verity" // {
+        description = ''
+          Mount verity-protected block devices in the initrd.
+
+          Enabling this option allows to use `systemd-veritysetup` and
+          `systemd-veritysetup-generator` in the initrd.
+        '';
+      };
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    assertions = [
+      {
+        assertion = config.boot.initrd.systemd.enable;
+        message = ''
+          'boot.initrd.systemd.dmVerity.enable' requires 'boot.initrd.systemd.enable' to be enabled.
+        '';
+      }
+    ];
+
+    boot.initrd = {
+      availableKernelModules = [
+        "dm_mod"
+        "dm_verity"
+      ];
+
+      # dm-verity needs additional udev rules from LVM to work.
+      services.lvm.enable = true;
+
+      # The additional targets and store paths allow users to integrate verity-protected devices
+      # through the systemd tooling.
+      systemd = {
+        additionalUpstreamUnits = [
+          "veritysetup-pre.target"
+          "veritysetup.target"
+          "remote-veritysetup.target"
+        ];
+
+        storePaths = [
+          "${config.boot.initrd.systemd.package}/lib/systemd/systemd-veritysetup"
+          "${config.boot.initrd.systemd.package}/lib/systemd/system-generators/systemd-veritysetup-generator"
+        ];
+      };
+    };
+  };
+
+  meta.maintainers = with lib.maintainers; [
+    msanft
+    nikstur
+    willibutz
+  ];
+}

nixos/tests/all-tests.nix

@@ -128,6 +128,7 @@ in {
   apcupsd = handleTest ./apcupsd.nix {};
   apfs = runTest ./apfs.nix;
   appliance-repart-image = runTest ./appliance-repart-image.nix;
+  appliance-repart-image-verity-store = runTest ./appliance-repart-image-verity-store.nix;
   apparmor = handleTest ./apparmor.nix {};
   archi = handleTest ./archi.nix {};
   aria2 = handleTest ./aria2.nix {};