bug#29773: urandom-seed-service should run earlier in the boot process

bug#29773: urandom-seed-service should run earlier in the boot process

[Leo Famulari]

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The urandom-seed-service preserves some of the Linux cryptographic
random number generator's (CRNG) pool across reboots so that freshly
booted systems have access to unpredictable psuedorandom numbers. It
also tries to take advantage of whatever is backing /dev/hwrng.

However, the urandom-seed-service may not be started before certain
applications that assume a good source of randomness.

In some cases, the applications require some random data before any
services are started, during activation. For example, our OpenSSH
service generates its host keys during activation. And even if it
generated host keys during the start of the OpenSSH service, that
service does not depend on urandom-seed-service. [0]

In systemd, there is an abstract sysinit "target" that basically serves
as a checkpoint. All the lower-level system initialization is required
before the sysinit.target is met, and the rest of the services depend on
sysinit. The random seeding is part of sysinit. I've reproduced a graph
of this in [1].

In practice, I'm not sure if it matters. I'd appreciate if GuixSD users
could check /var/log/messages for warnings like this one and report
them:

random: application: uninitialized urandom read (16 bytes read) 

And I'd also appreciate any feedback or ideas for improvement in this
area.

[0] See the attached service graph of a bare-bones system with OpenSSH.
[1] from: <https://www.freedesktop.org/software/systemd/man/bootup.html>

local-fs-pre.target
         |
         v
(various mounts and   (various swap   (various cryptsetup
 fsck services...)     devices...)        devices...)       (various low-level   (various low-level
         |                  |                  |             services: udevd,     API VFS mounts:
         v                  v                  v             tmpfiles, random     mqueue, configfs,
  local-fs.target      swap.target     cryptsetup.target    seed, sysctl, ...)      debugfs, ...)
         |                  |                  |                    |                    |
         \__________________|_________________ | ___________________|____________________/
                                              \|/
                                               v
                                        sysinit.target
                                               |
          ____________________________________/|\________________________________________
         /                  |                  |                    |                    \
         |                  |                  |                    |                    |
         v                  v                  |                    v                    v
     (various           (various               |                (various          rescue.service
    timers...)          paths...)              |               sockets...)               |
         |                  |                  |                    |                    v
         v                  v                  |                    v              rescue.target
   timers.target      paths.target             |             sockets.target
         |                  |                  |                    |
         v                  \_________________ | ___________________/
                                              \|/
                                               v
                                         basic.target
                                               |
          ____________________________________/|                                 emergency.service
         /                  |                  |                                         |
         |                  |                  |                                         v
         v                  v                  v                                 emergency.target
     display-        (various system    (various system
 manager.service         services           services)
         |             required for            |
         |            graphical UIs)           v
         |                  |           multi-user.target
         |                  |                  |
         \_________________ | _________________/
                           \|/
                            v
                  graphical.target

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bug#29773: Service graph

[Leo Famulari]

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I forgot to attach the GuixSD service graph used an example in my last
message. 

Here it is!

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bug#29773: urandom-seed-service should run earlier in the boot process

[Ludovic Courtès]

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Hello,

Leo Famulari <leo@famulari.name> skribis:

> In some cases, the applications require some random data before any
> services are started, during activation. For example, our OpenSSH
> service generates its host keys during activation. And even if it
> generated host keys during the start of the OpenSSH service, that
> service does not depend on urandom-seed-service. [0]
>
> In systemd, there is an abstract sysinit "target" that basically serves
> as a checkpoint. All the lower-level system initialization is required
> before the sysinit.target is met, and the rest of the services depend on
> sysinit. The random seeding is part of sysinit. I've reproduced a graph
> of this in [1].

There’s a ‘user-processes’ service that serves a similar purpose.

With the attached patches ‘urandom-seed’ becomes a dependency of
‘user-processes’, meaning that daemons & co. start after
‘urandom-seed’.

WDYT?

> In practice, I'm not sure if it matters. I'd appreciate if GuixSD users
> could check /var/log/messages for warnings like this one and report
> them:
>
> random: application: uninitialized urandom read (16 bytes read) 

I don’t have any of these.  I guess this is most likely to happen when
running ‘ssh-keygen’ on startup, which isn’t the case on my machine.

Ludo’.


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From 5895acdbc345572434d9efae5cf5cdd11e4c1a07 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ludovic=20Court=C3=A8s?= <ludo@gnu.org>
Date: Wed, 20 Dec 2017 11:09:03 +0100
Subject: [PATCH 2/3] services: urandom-seed: Become a dependency of
 'user-processes'.

This ensures that 'urandom-seed' is started before programs that rely on
sources of randomness.

Fixes <https://bugs.gnu.org/29773>.
Reported by Leo Famulari <leo@famulari.name>.

* gnu/services/base.scm (urandom-seed-shepherd-service): Change
'requirement' to (file-systems).
(urandom-seed-service-type): Extend USER-PROCESSES-SERVICE-TYPE.
---
 gnu/services/base.scm | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/gnu/services/base.scm b/gnu/services/base.scm
index 481439d4f..cc59ec573 100644
--- a/gnu/services/base.scm
+++ b/gnu/services/base.scm
@@ -529,7 +529,7 @@ in KNOWN-MOUNT-POINTS when it is stopped."
   (list (shepherd-service
          (documentation "Preserve entropy across reboots for /dev/urandom.")
          (provision '(urandom-seed))
-         (requirement '(user-processes))
+         (requirement '(file-systems))
          (start #~(lambda _
                     ;; On boot, write random seed into /dev/urandom.
                     (when (file-exists? #$%random-seed-file)
@@ -590,7 +590,13 @@ in KNOWN-MOUNT-POINTS when it is stopped."
   (service-type (name 'urandom-seed)
                 (extensions
                  (list (service-extension shepherd-root-service-type
-                                          urandom-seed-shepherd-service)))
+                                          urandom-seed-shepherd-service)
+
+                       ;; Have 'user-processes' depend on 'urandom-seed'.
+                       ;; This ensures that user processes and daemons don't
+                       ;; start until we have seeded the PRNG.
+                       (service-extension user-processes-service-type
+                                          (const '(urandom-seed)))))
                 (description
                  "Seed the @file{/dev/urandom} pseudo-random number
 generator (RNG) with the value recorded when the system was last shut
-- 
2.15.1


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From 8d0714bdb038e525880aed9de29e78af8c021efb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ludovic=20Court=C3=A8s?= <ludo@gnu.org>
Date: Wed, 20 Dec 2017 11:05:11 +0100
Subject: [PATCH 1/3] services: 'user-processes-service-type' can now be
 extended.

* gnu/services/base.scm (user-processes-shepherd-service): New
procedure, taken from former 'user-processes-service-type'.  Add
REQUIREMENTS argument; remove GRACE-DELAY argument.
(user-processes-service-type): Redefine in terms of 'service-type'.
(user-processes-service): Remove.
(file-system-service-type): Extend USER-PROCESSES-SERVICE-TYPE.
* gnu/system.scm (essential-services): Use USER-PROCESSES-SERVICE-TYPE
directly.
---
 gnu/services/base.scm | 236 +++++++++++++++++++++++++++-----------------------
 gnu/system.scm        |   2 +-
 2 files changed, 130 insertions(+), 108 deletions(-)

diff --git a/gnu/services/base.scm b/gnu/services/base.scm
index a3654fd4d..481439d4f 100644
--- a/gnu/services/base.scm
+++ b/gnu/services/base.scm
@@ -57,7 +57,7 @@
             file-system-service-type
             user-unmount-service
             swap-service
-            user-processes-service
+            user-processes-service-type
             host-name-service
             console-keymap-service
             %default-console-font
@@ -162,6 +162,129 @@
 ;;;
 ;;; Code:
 
+
+\f
+;;;
+;;; User processes.
+;;;
+
+(define %do-not-kill-file
+  ;; Name of the file listing PIDs of processes that must survive when halting
+  ;; the system.  Typical example is user-space file systems.
+  "/etc/shepherd/do-not-kill")
+
+(define (user-processes-shepherd-service requirements)
+  "Return the 'user-processes' Shepherd service with dependencies on
+REQUIREMENTS (a list of service names).
+
+This is a synchronization point used to make sure user processes and daemons
+get started only after crucial initial services have been started---file
+system mounts, etc.  This is similar to 'target' in systemd."
+  (define grace-delay
+    ;; Delay after sending SIGTERM and before sending SIGKILL.
+    4)
+
+  (list (shepherd-service
+         (documentation "When stopped, terminate all user processes.")
+         (provision '(user-processes))
+         (requirement requirements)
+         (start #~(const #t))
+         (stop #~(lambda _
+                   (define (kill-except omit signal)
+                     ;; Kill all the processes with SIGNAL except those listed
+                     ;; in OMIT and the current process.
+                     (let ((omit (cons (getpid) omit)))
+                       (for-each (lambda (pid)
+                                   (unless (memv pid omit)
+                                     (false-if-exception
+                                      (kill pid signal))))
+                                 (processes))))
+
+                   (define omitted-pids
+                     ;; List of PIDs that must not be killed.
+                     (if (file-exists? #$%do-not-kill-file)
+                         (map string->number
+                              (call-with-input-file #$%do-not-kill-file
+                                (compose string-tokenize
+                                         (@ (ice-9 rdelim) read-string))))
+                         '()))
+
+                   (define (now)
+                     (car (gettimeofday)))
+
+                   (define (sleep* n)
+                     ;; Really sleep N seconds.
+                     ;; Work around <http://bugs.gnu.org/19581>.
+                     (define start (now))
+                     (let loop ((elapsed 0))
+                       (when (> n elapsed)
+                         (sleep (- n elapsed))
+                         (loop (- (now) start)))))
+
+                   (define lset= (@ (srfi srfi-1) lset=))
+
+                   (display "sending all processes the TERM signal\n")
+
+                   (if (null? omitted-pids)
+                       (begin
+                         ;; Easy: terminate all of them.
+                         (kill -1 SIGTERM)
+                         (sleep* #$grace-delay)
+                         (kill -1 SIGKILL))
+                       (begin
+                         ;; Kill them all except OMITTED-PIDS.  XXX: We would
+                         ;; like to (kill -1 SIGSTOP) to get a fixed list of
+                         ;; processes, like 'killall5' does, but that seems
+                         ;; unreliable.
+                         (kill-except omitted-pids SIGTERM)
+                         (sleep* #$grace-delay)
+                         (kill-except omitted-pids SIGKILL)
+                         (delete-file #$%do-not-kill-file)))
+
+                   (let wait ()
+                     ;; Reap children, if any, so that we don't end up with
+                     ;; zombies and enter an infinite loop.
+                     (let reap-children ()
+                       (define result
+                         (false-if-exception
+                          (waitpid WAIT_ANY (if (null? omitted-pids)
+                                                0
+                                                WNOHANG))))
+
+                       (when (and (pair? result)
+                                  (not (zero? (car result))))
+                         (reap-children)))
+
+                     (let ((pids (processes)))
+                       (unless (lset= = pids (cons 1 omitted-pids))
+                         (format #t "waiting for process termination\
+ (processes left: ~s)~%"
+                                 pids)
+                         (sleep* 2)
+                         (wait))))
+
+                   (display "all processes have been terminated\n")
+                   #f))
+         (respawn? #f))))
+
+(define user-processes-service-type
+  (service-type
+   (name 'user-processes)
+   (extensions (list (service-extension shepherd-root-service-type
+                                        user-processes-shepherd-service)))
+   (compose concatenate)
+   (extend append)
+
+   ;; The value is the list of Shepherd services 'user-processes' depends on.
+   ;; Extensions can add new services to this list.
+   (default-value '())
+
+   (description "The @code{user-processes} service is responsible for
+terminating all the processes so that the root file system can be re-mounted
+read-only, just before rebooting/halting.  Processes still running after a few
+seconds after @code{SIGTERM} has been sent are terminated with
+@code{SIGKILL}.")))
+
 \f
 ;;;
 ;;; File systems.
@@ -349,7 +472,11 @@ FILE-SYSTEM."
                  (list (service-extension shepherd-root-service-type
                                           file-system-shepherd-services)
                        (service-extension fstab-service-type
-                                          identity)))
+                                          identity)
+
+                       ;; Have 'user-processes' depend on 'file-systems'.
+                       (service-extension user-processes-service-type
+                                          (const '(file-systems)))))
                 (compose concatenate)
                 (extend append)
                 (description
@@ -389,111 +516,6 @@ file systems, as well as corresponding @file{/etc/fstab} entries.")))
 in KNOWN-MOUNT-POINTS when it is stopped."
   (service user-unmount-service-type known-mount-points))
 
-(define %do-not-kill-file
-  ;; Name of the file listing PIDs of processes that must survive when halting
-  ;; the system.  Typical example is user-space file systems.
-  "/etc/shepherd/do-not-kill")
-
-(define user-processes-service-type
-  (shepherd-service-type
-   'user-processes
-   (lambda (grace-delay)
-     (shepherd-service
-      (documentation "When stopped, terminate all user processes.")
-      (provision '(user-processes))
-      (requirement '(file-systems))
-      (start #~(const #t))
-      (stop #~(lambda _
-                (define (kill-except omit signal)
-                  ;; Kill all the processes with SIGNAL except those listed
-                  ;; in OMIT and the current process.
-                  (let ((omit (cons (getpid) omit)))
-                    (for-each (lambda (pid)
-                                (unless (memv pid omit)
-                                  (false-if-exception
-                                   (kill pid signal))))
-                              (processes))))
-
-                (define omitted-pids
-                  ;; List of PIDs that must not be killed.
-                  (if (file-exists? #$%do-not-kill-file)
-                      (map string->number
-                           (call-with-input-file #$%do-not-kill-file
-                             (compose string-tokenize
-                                      (@ (ice-9 rdelim) read-string))))
-                      '()))
-
-                (define (now)
-                  (car (gettimeofday)))
-
-                (define (sleep* n)
-                  ;; Really sleep N seconds.
-                  ;; Work around <http://bugs.gnu.org/19581>.
-                  (define start (now))
-                  (let loop ((elapsed 0))
-                    (when (> n elapsed)
-                      (sleep (- n elapsed))
-                      (loop (- (now) start)))))
-
-                (define lset= (@ (srfi srfi-1) lset=))
-
-                (display "sending all processes the TERM signal\n")
-
-                (if (null? omitted-pids)
-                    (begin
-                      ;; Easy: terminate all of them.
-                      (kill -1 SIGTERM)
-                      (sleep* #$grace-delay)
-                      (kill -1 SIGKILL))
-                    (begin
-                      ;; Kill them all except OMITTED-PIDS.  XXX: We would
-                      ;; like to (kill -1 SIGSTOP) to get a fixed list of
-                      ;; processes, like 'killall5' does, but that seems
-                      ;; unreliable.
-                      (kill-except omitted-pids SIGTERM)
-                      (sleep* #$grace-delay)
-                      (kill-except omitted-pids SIGKILL)
-                      (delete-file #$%do-not-kill-file)))
-
-                (let wait ()
-                  ;; Reap children, if any, so that we don't end up with
-                  ;; zombies and enter an infinite loop.
-                  (let reap-children ()
-                    (define result
-                      (false-if-exception
-                       (waitpid WAIT_ANY (if (null? omitted-pids)
-                                             0
-                                             WNOHANG))))
-
-                    (when (and (pair? result)
-                               (not (zero? (car result))))
-                      (reap-children)))
-
-                  (let ((pids (processes)))
-                    (unless (lset= = pids (cons 1 omitted-pids))
-                      (format #t "waiting for process termination\
- (processes left: ~s)~%"
-                              pids)
-                      (sleep* 2)
-                      (wait))))
-
-                (display "all processes have been terminated\n")
-                #f))
-      (respawn? #f)))))
-
-(define* (user-processes-service #:key (grace-delay 4))
-  "Return the service that is responsible for terminating all the processes so
-that the root file system can be re-mounted read-only, just before
-rebooting/halting.  Processes still running GRACE-DELAY seconds after SIGTERM
-has been sent are terminated with SIGKILL.
-
-The returned service will depend on 'file-systems', meaning that it is
-considered started after all the auto-mount file systems have been mounted.
-
-All the services that spawn processes must depend on this one so that they are
-stopped before 'kill' is called."
-  (service user-processes-service-type grace-delay))
-
 \f
 ;;;
 ;;; Preserve entropy to seed /dev/urandom on boot.
diff --git a/gnu/system.scm b/gnu/system.scm
index 7466ed780..df89ca06d 100644
--- a/gnu/system.scm
+++ b/gnu/system.scm
@@ -449,7 +449,7 @@ a container or that of a \"bare metal\" system."
          (other-fs  (non-boot-file-system-service os))
          (unmount   (user-unmount-service known-fs))
          (swaps     (swap-services os))
-         (procs     (user-processes-service))
+         (procs     (service user-processes-service-type))
          (host-name (host-name-service (operating-system-host-name os)))
          (entries   (operating-system-directory-base-entries
                      os #:container? container?)))
-- 
2.15.1

bug#29773: urandom-seed-service should run earlier in the boot process

[Leo Famulari]

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On Wed, Dec 20, 2017 at 11:19:36AM +0100, Ludovic Courtès wrote:
> There’s a ‘user-processes’ service that serves a similar purpose.
> 
> With the attached patches ‘urandom-seed’ becomes a dependency of
> ‘user-processes’, meaning that daemons & co. start after
> ‘urandom-seed’.
> 
> WDYT?

In general, I think it's a good approach.

Currently, the urandom-seed-service seems to non-deterministically but
typically start after the udev-service, so that /dev/hwrng is always set
up by udev before the urandom-seed-service tries to use it.

With these patches, that's not the case. This breaks the hwrng seeding
feature added in 9a56cf2b5b (services: urandom-seed: Try using a HWRNG
to seed the Linux CRNG at boot).

I'll try rearranging the service dependency graph.

> > Leo Famulari <leo@famulari.name> skribis:
> > In practice, I'm not sure if it matters. I'd appreciate if GuixSD users
> > could check /var/log/messages for warnings like this one and report
> > them:
> >
> > random: application: uninitialized urandom read (16 bytes read) 
> 
> I don’t have any of these.  I guess this is most likely to happen when
> running ‘ssh-keygen’ on startup, which isn’t the case on my machine.

Watching a fresh system boot repeatedly, I noticed that the host keys
always seem to be generated immediately after Linux reports "random:
crng init done".

To me, this suggests that OpenSSH is using the getrandom() syscall. If
so, any GuixSD host keys created with glibc >= 2.25 and OpenSSH >= 7.2
should be unpredictable. But I'm not sure if that's what's happening or
not.

> +(define (user-processes-shepherd-service requirements)
> +  "Return the 'user-processes' Shepherd service with dependencies on
> +REQUIREMENTS (a list of service names).
> +
> +This is a synchronization point used to make sure user processes and daemons
> +get started only after crucial initial services have been started---file
> +system mounts, etc.  This is similar to 'target' in systemd."

To clarify, user-processes may be similar to the sysinit target in
systemd. Systemd targets are sort of like run-levels, and there are
several of them, such as the multi-user target, the graphical target,
etc.

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bug#29773: urandom-seed-service should run earlier in the boot process

[Ludovic Courtès]

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Leo Famulari <leo@famulari.name> skribis:

> On Wed, Dec 20, 2017 at 11:19:36AM +0100, Ludovic Courtès wrote:
>> There’s a ‘user-processes’ service that serves a similar purpose.
>> 
>> With the attached patches ‘urandom-seed’ becomes a dependency of
>> ‘user-processes’, meaning that daemons & co. start after
>> ‘urandom-seed’.
>> 
>> WDYT?
>
> In general, I think it's a good approach.
>
> Currently, the urandom-seed-service seems to non-deterministically but
> typically start after the udev-service, so that /dev/hwrng is always set
> up by udev before the urandom-seed-service tries to use it.
>
> With these patches, that's not the case. This breaks the hwrng seeding
> feature added in 9a56cf2b5b (services: urandom-seed: Try using a HWRNG
> to seed the Linux CRNG at boot).
>
> I'll try rearranging the service dependency graph.

The attached patch does the trick, AFAICS:


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diff --git a/gnu/services/base.scm b/gnu/services/base.scm
index acc5c33f5..7fc8f6aa7 100644
--- a/gnu/services/base.scm
+++ b/gnu/services/base.scm
@@ -529,7 +529,10 @@ in KNOWN-MOUNT-POINTS when it is stopped."
   (list (shepherd-service
          (documentation "Preserve entropy across reboots for /dev/urandom.")
          (provision '(urandom-seed))
-         (requirement '(file-systems))
+
+         ;; Depend on udev so that /dev/hwrng is available.
+         (requirement '(file-systems udev))
+
          (start #~(lambda _
                     ;; On boot, write random seed into /dev/urandom.
                     (when (file-exists? #$%random-seed-file)

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> Watching a fresh system boot repeatedly, I noticed that the host keys
> always seem to be generated immediately after Linux reports "random:
> crng init done".
>
> To me, this suggests that OpenSSH is using the getrandom() syscall. If
> so, any GuixSD host keys created with glibc >= 2.25 and OpenSSH >= 7.2
> should be unpredictable. But I'm not sure if that's what's happening or
> not.

Nice.

The problem though is that ‘ssh-keygen -A’ runs from the activation
snippet, which itself runs before shepherd is started.

To work around that, we should either introduce a separate ‘ssh-keygen’
service that ‘ssh-daemon’ would depend on, or invoke ‘ssh-keygen’ from
the ‘start’ method of the ‘ssh-daemon’ service.

>> +(define (user-processes-shepherd-service requirements)
>> +  "Return the 'user-processes' Shepherd service with dependencies on
>> +REQUIREMENTS (a list of service names).
>> +
>> +This is a synchronization point used to make sure user processes and daemons
>> +get started only after crucial initial services have been started---file
>> +system mounts, etc.  This is similar to 'target' in systemd."
>
> To clarify, user-processes may be similar to the sysinit target in
> systemd. Systemd targets are sort of like run-levels, and there are
> several of them, such as the multi-user target, the graphical target,
> etc.

Indeed, I’ve fixed it locally.

If that’s OK I’ll push these patches later today.

Thank you,
Ludo’.

bug#29773: urandom-seed-service should run earlier in the boot process

[Leo Famulari]

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On Thu, Dec 21, 2017 at 10:10:29AM +0100, Ludovic Courtès wrote:
> 
> The attached patch does the trick, AFAICS:
> 

> diff --git a/gnu/services/base.scm b/gnu/services/base.scm
> index acc5c33f5..7fc8f6aa7 100644
> --- a/gnu/services/base.scm
> +++ b/gnu/services/base.scm
> @@ -529,7 +529,10 @@ in KNOWN-MOUNT-POINTS when it is stopped."
>    (list (shepherd-service
>           (documentation "Preserve entropy across reboots for /dev/urandom.")
>           (provision '(urandom-seed))
> -         (requirement '(file-systems))
> +
> +         ;; Depend on udev so that /dev/hwrng is available.
> +         (requirement '(file-systems udev))
> +
>           (start #~(lambda _
>                      ;; On boot, write random seed into /dev/urandom.
>                      (when (file-exists? #$%random-seed-file)

Yes, it seems to work for me.

I'm unsure if the stop action of urandom-seed-service is being executed
on shutdown.

I added some print statements and sleep delays to the stop action but
the system halts faster than I expected and I don't see any writes to
/var/lib/random-seed.

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bug#29773: urandom-seed-service should run earlier in the boot process

[Ludovic Courtès]

Hello,

Leo Famulari <leo@famulari.name> skribis:

> On Thu, Dec 21, 2017 at 10:10:29AM +0100, Ludovic Courtès wrote:
>> 
>> The attached patch does the trick, AFAICS:
>> 
>
>> diff --git a/gnu/services/base.scm b/gnu/services/base.scm
>> index acc5c33f5..7fc8f6aa7 100644
>> --- a/gnu/services/base.scm
>> +++ b/gnu/services/base.scm
>> @@ -529,7 +529,10 @@ in KNOWN-MOUNT-POINTS when it is stopped."
>>    (list (shepherd-service
>>           (documentation "Preserve entropy across reboots for /dev/urandom.")
>>           (provision '(urandom-seed))
>> -         (requirement '(file-systems))
>> +
>> +         ;; Depend on udev so that /dev/hwrng is available.
>> +         (requirement '(file-systems udev))
>> +
>>           (start #~(lambda _
>>                      ;; On boot, write random seed into /dev/urandom.
>>                      (when (file-exists? #$%random-seed-file)
>
> Yes, it seems to work for me.

Great, I’ve pushed the whole series.

> I'm unsure if the stop action of urandom-seed-service is being executed
> on shutdown.
>
> I added some print statements and sleep delays to the stop action but
> the system halts faster than I expected and I don't see any writes to
> /var/lib/random-seed.

/var/lib/random-seed is definitely being updated on shutdown on my
system, as can be seen from its mtime.

Thanks,
Ludo’.