services: Prevent following symlinks during activation.

This addresses a potential security issue, where a compromised
service could trick the activation code in changing the permissions,
owner and group of arbitrary files.  However, this patch is
currently only a partial fix, due to a TOCTTOU (time-of-check to
time-of-use) race, which can be fixed once guile has bindings
to openat and friends.

Fixes: <https://lists.gnu.org/archive/html/guix-devel/2021-01/msg00388.html>

* gnu/build/activation.scm: new procedure 'mkdir-p/perms'.
* gnu/services/authentication.scm
  (%nslcd-activation, nslcd-service-type): use new procedure.
* gnu/services/cups.scm (%cups-activation): likewise.
* gnu/services/dbus.scm (dbus-activation): likewise.
* gnu/services/dns.scm (knot-activation): likewise.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/build/activation.scm

@@ -1,6 +1,11 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
-;;; Copyright © 2015 Mark H Weaver <mhw@netris.org>
+;;; Copyright © 2013 Nikita Karetnikov <nikita@karetnikov.org>
+;;; Copyright © 2013 Andreas Enge <andreas@enge.fr>
+;;; Copyright © 2015, 2018 Mark H Weaver <mhw@netris.org>
+;;; Copyright © 2018 Arun Isaac <arunisaac@systemreboot.net>
+;;; Copyright © 2018, 2019 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2021 Maxime Devos <maximedevos@telenet.be>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -37,7 +42,8 @@ (define-module (gnu build activation)
             activate-modprobe
             activate-firmware
             activate-ptrace-attach
-            activate-current-system))
+            activate-current-system
+            mkdir-p/perms))
 
 ;;; Commentary:
 ;;;
@@ -55,6 +61,47 @@ (define %skeleton-directory
 (define (dot-or-dot-dot? file)
   (member file '("." "..")))
 
+;; Based upon mkdir-p from (guix build utils)
+(define (verify-not-symbolic dir)
+  "Verify DIR or its ancestors aren't symbolic links."
+  (define absolute?
+    (string-prefix? "/" dir))
+
+  (define not-slash
+    (char-set-complement (char-set #\/)))
+
+  (define (verify-component file)
+    (unless (eq? 'directory (stat:type (lstat file)))
+      (error "file name component is not a directory" dir)))
+
+  (let loop ((components (string-tokenize dir not-slash))
+             (root       (if absolute?
+                             ""
+                             ".")))
+    (match components
+      ((head tail ...)
+       (let ((file (string-append root "/" head)))
+         (catch 'system-error
+           (lambda ()
+             (verify-component file)
+             (loop tail file))
+           (lambda args
+             (if (= ENOENT (system-error-errno args))
+                 #t
+                 (apply throw args))))))
+      (() #t))))
+
+;; TODO: the TOCTTOU race can be addressed once guile has bindings
+;; for fstatat, openat and friends.
+(define (mkdir-p/perms directory owner bits)
+  "Create the directory DIRECTORY and all its ancestors.
+Verify no component of DIRECTORY is a symbolic link.
+Warning: this is currently suspect to a TOCTTOU race!"
+  (verify-not-symbolic directory)
+  (mkdir-p directory)
+  (chown directory (passwd:uid owner) (passwd:gid owner))
+  (chmod directory bits))
+
 (define* (copy-account-skeletons home
                                  #:key
                                  (directory %skeleton-directory)

gnu/services/authentication.scm

@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2018 Danny Milosavljevic <dannym@scratchpost.org>
 ;;; Copyright © 2018, 2019 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2021 Maxime Devos <maximedevos@telenet.be>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -31,6 +32,7 @@ (define-module (gnu services authentication)
   #:use-module (guix gexp)
   #:use-module (guix records)
   #:use-module (guix packages)
+  #:use-module (guix modules)
   #:use-module (ice-9 match)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-26)
@@ -521,6 +523,16 @@ (module pam-ldap-module))))
 (define (pam-ldap-pam-services config)
   (list (pam-ldap-pam-service config)))
 
+(define %nslcd-activation
+  (with-imported-modules (source-module-closure '((gnu build activation)))
+    #~(begin
+        (use-modules (gnu build activation))
+        (let ((rundir "/var/run/nslcd")
+              (user (getpwnam "nslcd")))
+          (mkdir-p/perms rundir user #o755)
+          (when (file-exists? "/etc/nslcd.conf")
+            (chmod "/etc/nslcd.conf" #o400))))))
+
 (define nslcd-service-type
   (service-type
    (name 'nslcd)
@@ -531,15 +543,7 @@ (define nslcd-service-type
           (service-extension etc-service-type
                              nslcd-etc-service)
           (service-extension activation-service-type
-                             (const #~(begin
+                             (const %nslcd-activation))
-                                        (use-modules (guix build utils))
-                                        (let ((rundir "/var/run/nslcd")
-                                              (user (getpwnam "nslcd")))
-                                          (mkdir-p rundir)
-                                          (chown rundir (passwd:uid user) (passwd:gid user))
-                                          (chmod rundir #o755)
-                                          (when (file-exists? "/etc/nslcd.conf")
-                                            (chmod "/etc/nslcd.conf" #o400))))))
           (service-extension pam-root-service-type
                              pam-ldap-pam-services)
           (service-extension nscd-service-type

gnu/services/cups.scm

@@ -4,6 +4,7 @@
 ;;; Copyright © 2018 Ricardo Wurmus <rekado@elephly.net>
 ;;; Copyright © 2019 Alex Griffin <a@ajgrf.com>
 ;;; Copyright © 2019 Tobias Geerinckx-Rice <me@tobias.gr>
+;;; Copyright © 2021 Maxime Devos <maximedevos@telenet.be>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -31,6 +32,7 @@ (define-module (gnu services cups)
   #:use-module (guix packages)
   #:use-module (guix records)
   #:use-module (guix gexp)
+  #:use-module (guix modules)
   #:use-module (ice-9 match)
   #:use-module ((srfi srfi-1) #:select (append-map find))
   #:export (cups-service-type
@@ -871,13 +873,11 @@ (define-configuration opaque-cups-configuration
 
 (define %cups-activation
   ;; Activation gexp.
-  (with-imported-modules '((guix build utils))
+  (with-imported-modules (source-module-closure '((gnu build activation)
+                                                  (guix build utils)))
     #~(begin
-        (use-modules (guix build utils))
+        (use-modules (gnu build activation)
-        (define (mkdir-p/perms directory owner perms)
+                     (guix build utils))
-          (mkdir-p directory)
-          (chown directory (passwd:uid owner) (passwd:gid owner))
-          (chmod directory perms))
         (define (build-subject parameters)
           (string-concatenate
            (map (lambda (pair)

gnu/services/dbus.scm

@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2015 Sou Bunnbu <iyzsong@gmail.com>
+;;; Copyright © 2021 Maxime Devos <maximedevos@telenet.be>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -28,6 +29,7 @@ (define-module (gnu services dbus)
   #:use-module (guix gexp)
   #:use-module ((guix packages) #:select (package-name))
   #:use-module (guix records)
+  #:use-module (guix modules)
   #:use-module (srfi srfi-1)
   #:use-module (ice-9 match)
   #:export (dbus-configuration
@@ -161,24 +163,23 @@ (define dbus-setuid-programs
 
 (define (dbus-activation config)
   "Return an activation gexp for D-Bus using @var{config}."
+  (with-imported-modules (source-module-closure
+                          '((gnu build activation)
+                            (guix build utils)))
     #~(begin
-      (use-modules (guix build utils))
+        (use-modules (gnu build activation)
-
+                     (guix build utils))
-      (mkdir-p "/var/run/dbus")
 
         (let ((user (getpwnam "messagebus")))
-        (chown "/var/run/dbus"
-               (passwd:uid user) (passwd:gid user))
-
           ;; This directory contains the daemon's socket so it must be
           ;; world-readable.
-        (chmod "/var/run/dbus" #o755))
+          (mkdir-p/perms "/var/run/dbus" user #o755))
 
         (unless (file-exists? "/etc/machine-id")
           (format #t "creating /etc/machine-id...~%")
           (invoke (string-append #$(dbus-configuration-dbus config)
                                  "/bin/dbus-uuidgen")
-                "--ensure=/etc/machine-id"))))
+                  "--ensure=/etc/machine-id")))))
 
 (define dbus-shepherd-service
   (match-lambda

gnu/services/dns.scm

@@ -2,6 +2,7 @@
 ;;; Copyright © 2017 Julien Lepiller <julien@lepiller.eu>
 ;;; Copyright © 2018 Oleg Pykhalov <go.wigust@gmail.com>
 ;;; Copyright © 2020 Pierre Langlois <pierre.langlois@gmx.com>
+;;; Copyright © 2021 Maxime Devos <maximedevos@telenet.be>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -28,6 +29,7 @@ (define-module (gnu services dns)
   #:use-module (guix packages)
   #:use-module (guix records)
   #:use-module (guix gexp)
+  #:use-module (guix modules)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-34)
@@ -607,17 +609,14 @@ (define %knot-accounts
           (shell (file-append shadow "/sbin/nologin")))))
 
 (define (knot-activation config)
+  (with-imported-modules (source-module-closure '((gnu build activation)))
     #~(begin
-      (use-modules (guix build utils))
+        (use-modules (gnu build activation))
-      (define (mkdir-p/perms directory owner perms)
-        (mkdir-p directory)
-        (chown directory (passwd:uid owner) (passwd:gid owner))
-        (chmod directory perms))
         (mkdir-p/perms #$(knot-configuration-run-directory config)
                        (getpwnam "knot") #o755)
         (mkdir-p/perms "/var/lib/knot" (getpwnam "knot") #o755)
         (mkdir-p/perms "/var/lib/knot/keys" (getpwnam "knot") #o755)
-      (mkdir-p/perms "/var/lib/knot/keys/keys" (getpwnam "knot") #o755)))
+        (mkdir-p/perms "/var/lib/knot/keys/keys" (getpwnam "knot") #o755))))
 
 (define (knot-shepherd-service config)
   (let* ((config-file (knot-config-file config))