From e64d3f60fbc77887534838de7208df853275fd01 Mon Sep 17 00:00:00 2001
From: nicoo <nicoo@debian.org>
Date: Sun, 23 Aug 2020 15:12:13 +0200
Subject: [PATCH 1/2] nixos/modules/security/rngd: Disable by default

`rngd` seems to be the root cause for slow boot issues, and its functionality is
redundant since kernel v3.17 (2014), which introduced a `krngd` task (in kernel
space) that takes care of pulling in data from hardware RNGs:

> commit be4000bc4644d027c519b6361f5ae3bbfc52c347
> Author: Torsten Duwe <duwe@lst.de>
> Date:   Sat Jun 14 23:46:03 2014 -0400
>
>     hwrng: create filler thread
>
>     This can be viewed as the in-kernel equivalent of hwrngd;
>     like FUSE it is a good thing to have a mechanism in user land,
>     but for some reasons (simplicity, secrecy, integrity, speed)
>     it may be better to have it in kernel space.
>
>     This patch creates a thread once a hwrng registers, and uses
>     the previously established add_hwgenerator_randomness() to feed
>     its data to the input pool as long as needed. A derating factor
>     is used to bias the entropy estimation and to disable this
>     mechanism entirely when set to zero.

Closes: #96067
---
 nixos/modules/security/rngd.nix | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/nixos/modules/security/rngd.nix b/nixos/modules/security/rngd.nix
index cffa1a5849f..cb885c4762d 100644
--- a/nixos/modules/security/rngd.nix
+++ b/nixos/modules/security/rngd.nix
@@ -10,11 +10,10 @@ in
     security.rngd = {
       enable = mkOption {
         type = types.bool;
-        default = true;
+        default = false;
         description = ''
-          Whether to enable the rng daemon, which adds entropy from
-          hardware sources of randomness to the kernel entropy pool when
-          available.
+          Whether to enable the rng daemon.  Devices that the kernel recognises
+          as entropy sources are handled automatically by krngd.
         '';
       };
       debug = mkOption {
@@ -26,12 +25,6 @@ in
   };
 
   config = mkIf cfg.enable {
-    services.udev.extraRules = ''
-      KERNEL=="random", TAG+="systemd"
-      SUBSYSTEM=="cpu", ENV{MODALIAS}=="cpu:type:x86,*feature:*009E*", TAG+="systemd", ENV{SYSTEMD_WANTS}+="rngd.service"
-      KERNEL=="hw_random", TAG+="systemd", ENV{SYSTEMD_WANTS}+="rngd.service"
-    '';
-
     systemd.services.rngd = {
       bindsTo = [ "dev-random.device" ];
 

From ca674c176918f7b33407608b7894e32e6211dab9 Mon Sep 17 00:00:00 2001
From: worldofpeace <worldofpeace@protonmail.ch>
Date: Wed, 9 Sep 2020 21:50:07 -0400
Subject: [PATCH 2/2] rl-2009: document rngd

---
 nixos/doc/manual/release-notes/rl-2009.xml | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/nixos/doc/manual/release-notes/rl-2009.xml b/nixos/doc/manual/release-notes/rl-2009.xml
index 8bc51c906ae..1b51fbd566d 100644
--- a/nixos/doc/manual/release-notes/rl-2009.xml
+++ b/nixos/doc/manual/release-notes/rl-2009.xml
@@ -815,6 +815,13 @@ CREATE ROLE postgres LOGIN SUPERUSER;
      the value of <option>services.jellyfin.package</option> to <literal>pkgs.jellyfin_10_5</literal>.
     </para>
    </listitem>
+   <listitem>
+    <para>
+     The <literal>security.rngd</literal> service is now disabled by default.
+     This choice was made because there's krngd in the linux kernel space making it (for most usecases)
+     functionally redundent.
+    </para>
+   </listitem>
   </itemizedlist>
  </section>