From 1ac86e14c746140a7fa9a6e81a3471739655366b Mon Sep 17 00:00:00 2001
From: Daniel Frank <git@danielfrank.net>
Date: Fri, 18 Oct 2019 00:53:48 +0200
Subject: [PATCH 1/2] swap: depend on rngd if enabled and randomEncryption is
 configured to avoid entropy starvation during boot

---
 nixos/modules/config/swap.nix | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/nixos/modules/config/swap.nix b/nixos/modules/config/swap.nix
index d0fc0d4a3ea4..adb4e2294213 100644
--- a/nixos/modules/config/swap.nix
+++ b/nixos/modules/config/swap.nix
@@ -185,6 +185,8 @@ in
           { description = "Initialisation of swap device ${sw.device}";
             wantedBy = [ "${realDevice'}.swap" ];
             before = [ "${realDevice'}.swap" ];
+            # If swap is encrypted, depending on rngd resolves a possible entropy starvation during boot
+            after = mkIf (config.security.rngd.enable && sw.randomEncryption.enable) [ "rngd.service" ];
             path = [ pkgs.utillinux ] ++ optional sw.randomEncryption.enable pkgs.cryptsetup;
 
             script =

From d14ba1e1add6cca568515f87508ad5d00180bf16 Mon Sep 17 00:00:00 2001
From: Daniel Frank <git@danielfrank.net>
Date: Fri, 18 Oct 2019 00:30:11 +0200
Subject: [PATCH 2/2] security.rngd: start rngd during early boot to reduce
 entropy starvation due to encrypted swap and remove PrivateTmp to avoid a
 circular dependency

---
 nixos/modules/security/rngd.nix | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/nixos/modules/security/rngd.nix b/nixos/modules/security/rngd.nix
index d9d6d9c9f253..5566c53897dc 100644
--- a/nixos/modules/security/rngd.nix
+++ b/nixos/modules/security/rngd.nix
@@ -39,12 +39,15 @@ in
 
       description = "Hardware RNG Entropy Gatherer Daemon";
 
+      # rngd may have to start early to avoid entropy starvation during boot with encrypted swap
+      unitConfig.DefaultDependencies = false;
       serviceConfig = {
         ExecStart = "${pkgs.rng-tools}/sbin/rngd -f"
           + optionalString cfg.debug " -d";
+        # PrivateTmp would introduce a circular dependency if /tmp is on tmpfs and swap is encrypted,
+        # thus depending on rngd before swap, while swap depends on rngd to avoid entropy starvation.
         NoNewPrivileges = true;
         PrivateNetwork = true;
-        PrivateTmp = true;
         ProtectSystem = "full";
         ProtectHome = true;
       };