bug#23605: /dev/urandom not seeded across reboots

[Leo Famulari <leo@famulari.name>]

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Okay, I finally have some code that works! (attached)

I tested it by using `inotifywait -m /dev/urandom` [0] to see that
/dev/urandom was indeed being open and closed for writing when running
`herd {start, stop} urandom-seed`. If you can suggest a way to see
exactly what is being written to /dev/urandom, I would like to verify it
more fully.

If you test it, don't be surprised that the random-seed is empty the
first time you start the service. It is `touch`-ed at boot but filled
during shutdown.

The big caveat is that the only way I could make it work was to use the
old service API. I just couldn't figure out how to use the extensible
service API for a service that takes to arguments.

If this is a problem, then your help is wanted!

I also had to change the location of %random-seed-file from
/var/run/random-seed to /var/lib/random-seed, because /var/run is
cleared upon reboot. This is specified in the FHS standard [1]. Debian
puts it at /var/lib/urandom/random-seed.

[0] From the inotify-tools package

[1]
http://www.pathname.com/fhs/pub/fhs-2.3.html#VARRUNRUNTIMEVARIABLEDATA

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From 8d41d37e2d0f8a0edf7ab9d659d1a2c9315965c1 Mon Sep 17 00:00:00 2001
From: Leo Famulari <leo@famulari.name>
Date: Sun, 8 May 2016 03:08:46 -0400
Subject: [PATCH] services: Add urandom-seed-service.

* gnu/services/base.scm (urandom-seed-service): New procedure.
(%random-seed-file, urandom-seed-service-type): New variables.
(%urandom-seed-shepherd-service): New procedure.
* doc/guix.texi (Base Services): Document it.
---
 doc/guix.texi         | 10 ++++++++++
 gnu/services/base.scm | 52 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 61 insertions(+), 1 deletion(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index bb75425..34a51a8 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -7355,6 +7355,16 @@ Return a service that runs the Guix build daemon according to
 Run @var{udev}, which populates the @file{/dev} directory dynamically.
 @end deffn
 
+@deffn {Scheme Procedure} urandom-seed-service @var{#f}
+Save some entropy in @var{%random-seed-file} to seed @file{/dev/urandom}
+when rebooting.
+@end deffn
+
+@deftp {Data Type} %random-seed-file
+This is where some random bytes are saved by @var{urandom-seed-service}
+to seed @file{/dev/urandom} when rebooting.
+@end deftp
+
 @deffn {Scheme Procedure} console-keymap-service @var{files} ...
 @cindex keyboard layout
 Return a service to load console keymaps from @var{files} using
diff --git a/gnu/services/base.scm b/gnu/services/base.scm
index 96bf8da..329a989 100644
--- a/gnu/services/base.scm
+++ b/gnu/services/base.scm
@@ -3,6 +3,7 @@
 ;;; Copyright © 2015, 2016 Alex Kost <alezost@gmail.com>
 ;;; Copyright © 2015 Mark H Weaver <mhw@netris.org>
 ;;; Copyright © 2015 Sou Bunnbu <iyzsong@gmail.com>
+;;; Copyright © 2016 Leo Famulari <leo@famulari.name>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -93,6 +94,8 @@
             gpm-service-type
             gpm-service
 
+            urandom-seed-service
+
             %base-services))
 
 ;;; Commentary:
@@ -422,6 +425,53 @@ stopped before 'kill' is called."
 
 \f
 ;;;
+;;; Preserve entropy to seed /dev/urandom on boot.
+;;;
+
+(define %random-seed-file
+  "/var/lib/random-seed")
+
+(define %urandom-seed-shepherd-service
+  (shepherd-service
+    (documentation "Preserve entropy across reboots for /dev/urandom.")
+    (provision '(urandom-seed))
+    (requirement '(user-processes))
+    (start #~(lambda _
+               ;; "Activation"
+               (mkdir-p (dirname #$%random-seed-file))
+               (close-port (open-file #$%random-seed-file "a0b"))
+               (chmod #$%random-seed-file #o600)
+               ;; On boot, write random seed into /dev/urandom
+               (when (file-exists? #$%random-seed-file)
+                 (call-with-input-file #$%random-seed-file
+                   (lambda (seed)
+                     (call-with-output-file "/dev/urandom"
+                       (lambda (urandom)
+                         (dump-port seed urandom))))))
+               #t))
+    (stop #~(lambda _
+              ;; During shutdown, write from /dev/urandom into random seed
+              (let ((buf (make-bytevector 512)))
+                (call-with-input-file "/dev/urandom"
+                  (lambda (urandom)
+                    (get-bytevector-n! urandom buf 0 512)
+                    (call-with-output-file #$%random-seed-file
+                      (lambda (seed)
+                        (put-bytevector seed buf)))
+                    #t)))))
+    (modules `((rnrs bytevectors)
+               (rnrs io ports)
+               ,@%default-modules))))
+
+(define urandom-seed-service-type
+  (shepherd-service-type 'urandom-seed
+                         (const %urandom-seed-shepherd-service)))
+
+(define (urandom-seed-service)
+  (service urandom-seed-service-type #f))
+
+\f
+;;;
 ;;; System-wide environment variables.
 ;;;
 
@@ -1200,7 +1250,6 @@ extra rules from the packages listed in @var{rules}."
   "Return a service that uses @var{device} as a swap device."
   (service swap-service-type device))
 
-
 (define-record-type* <gpm-configuration>
   gpm-configuration make-gpm-configuration gpm-configuration?
   (gpm      gpm-configuration-gpm)                ;package
@@ -1283,6 +1332,7 @@ This is the GNU operating system, welcome!\n\n")))
           (syslog-service)
           (guix-service)
           (nscd-service)
+          (urandom-seed-service)
 
           ;; The LVM2 rules are needed as soon as LVM2 or the device-mapper is
           ;; used, so enable them by default.  The FUSE and ALSA rules are
-- 
2.8.3