services: urandom-seed: Try using a HWRNG to seed the Linux CRNG at boot.

* gnu/services/base.scm (urandom-seed-shepherd-service): Try to read from
'/dev/hwrng' at boot, as a supplement to any saved random seed.
* doc/guix.texi (Base Services): Document the new feature.

doc/guix.texi

@@ -10025,7 +10025,9 @@ well as in the @var{groups} field of the @var{operating-system} record.
 
 @deffn {Scheme Procedure} urandom-seed-service
 Save some entropy in @var{%random-seed-file} to seed @file{/dev/urandom}
-when rebooting.
+when rebooting.  It also tries to seed @file{/dev/urandom} from
+@file{/dev/hwrng} while booting, if @file{/dev/hwrng} exists and is
+readable.
 @end deffn
 
 @defvr {Scheme Variable} %random-seed-file

gnu/services/base.scm

@@ -516,6 +516,24 @@ stopped before 'kill' is called."
                           (call-with-output-file "/dev/urandom"
                             (lambda (urandom)
                               (dump-port seed urandom))))))
+
+                    ;; Try writing from /dev/hwrng into /dev/urandom.
+                    ;; It seems that the file /dev/hwrng always exists, even
+                    ;; when there is no hardware random number generator
+                    ;; available. So, we handle a failed read or any other error
+                    ;; reported by the operating system.
+                    (let ((buf (catch 'system-error
+                                 (lambda ()
+                                   (call-with-input-file "/dev/hwrng"
+                                     (lambda (hwrng)
+                                       (get-bytevector-n hwrng 512))))
+                                 ;; Silence is golden...
+                                 (const #f))))
+                      (when buf
+                        (call-with-output-file "/dev/urandom"
+                          (lambda (urandom)
+                            (put-bytevector urandom buf)))))
+
                     ;; Immediately refresh the seed in case the system doesn't
                     ;; shut down cleanly.
                     (call-with-input-file "/dev/urandom"