[bug#47013] [PATCH] gnu: Harden filesystem links.

[Leo Famulari <leo@famulari.name>]

These sysctl options are enabled on most GNU/Linux distros, including
Debian, Fedora, NixOS, and OpenSUSE.

I've tested this patch on Guix System for several weeks, and it doesn't
appear to break anything. Plus, we know that Guix works on other distros
that enable these restrictions.

References:

https://sysctl-explorer.net/fs/protected_hardlinks/
https://sysctl-explorer.net/fs/protected_symlinks/

* gnu/services/base.scm (%base-services): Add a default
sysctl-configuration that enables fs.protected_hardlinks and
fs.protected_symlinks.
---
 gnu/services/base.scm | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/gnu/services/base.scm b/gnu/services/base.scm
index f6a490f712..edd2c8e355 100644
--- a/gnu/services/base.scm
+++ b/gnu/services/base.scm
@@ -3,7 +3,7 @@
 ;;; Copyright © 2015, 2016 Alex Kost <alezost@gmail.com>
 ;;; Copyright © 2015, 2016, 2020 Mark H Weaver <mhw@netris.org>
 ;;; Copyright © 2015 Sou Bunnbu <iyzsong@gmail.com>
-;;; Copyright © 2016, 2017 Leo Famulari <leo@famulari.name>
+;;; Copyright © 2016, 2017, 2021 Leo Famulari <leo@famulari.name>
 ;;; Copyright © 2016 David Craven <david@craven.ch>
 ;;; Copyright © 2016 Ricardo Wurmus <rekado@elephly.net>
 ;;; Copyright © 2018 Mathieu Othacehe <m.othacehe@gmail.com>
@@ -35,6 +35,7 @@
   #:use-module (gnu services)
   #:use-module (gnu services admin)
   #:use-module (gnu services shepherd)
+  #:use-module (gnu services sysctl)
   #:use-module (gnu system pam)
   #:use-module (gnu system shadow)                ; 'user-account', etc.
   #:use-module (gnu system uuid)
@@ -2532,6 +2533,12 @@ to handle."
                  (udev-configuration
                    (rules (list lvm2 fuse alsa-utils crda))))
 
+        (service sysctl-service-type
+                 (sysctl-configuration
+                   (settings
+                     '(("fs.protected_hardlinks" . "1")
+                       ("fs.protected_symlinks" . "1")))))
+
         (service special-files-service-type
                  `(("/bin/sh" ,(file-append bash "/bin/sh"))
                    ("/usr/bin/env" ,(file-append coreutils "/bin/env"))))))
-- 
2.30.1