Properly handle fwupd update capsules

Co-Authored-By: Janne Heß <janne@hess.ooo>

README.md

@@ -89,6 +89,12 @@ the initrd into the signed UKI.
 
 The stub lives in `rust/stub`.
 
+### Fwupd
+
+When both Lanzaboote and `services.fwupd` are enabled, for
+`fwupd.service` a `preStart` will be added that ensures a signed fwupd
+binary is placed in `/run` that fwupd will use.
+
 ## State of Upstreaming to Nixpkgs
 
 Secure Boot is available as an Nixpkgs out-of-tree feature using the

nix/modules/lanzaboote.nix

@@ -74,5 +74,20 @@ in
           /nix/var/nix/profiles/system-*-link
       '';
     };
+
+    systemd.services.fwupd = lib.mkIf config.services.fwupd.enable {
+      # Tell fwupd to load its efi files from /run
+      environment.FWUPD_EFIAPPDIR = "/run/fwupd-efi";
+      # Place the fwupd efi files in /run and sign them
+      preStart = ''
+        mkdir -p /run/fwupd-efi
+        cp ${config.services.fwupd.package.fwupd-efi}/libexec/fwupd/efi/fwupd*.efi /run/fwupd-efi/
+        ${pkgs.sbsigntool}/bin/sbsign --key '${cfg.privateKeyFile}' --cert '${cfg.publicKeyFile}' /run/fwupd-efi/fwupd*.efi
+      '';
+    };
+
+    services.fwupd.uefiCapsuleSettings = lib.mkIf config.services.fwupd.enable {
+      DisableShimForSecureBoot = true;
+    };
   };
 }