Subject: [PATCH] services: nix: Mount Nix store read only.

From: Oleg Pykhalov <[email-masked]>
Date: Sun, 19 May 2024 22:26:15 +0300

* gnu/services/nix.scm (nix-shepherd-service): Mount Nix store read only.
(%nix-store-directory, %immutable-nix-store): New variables.
(%nix-store-prefix): New parameter.
(nix-activation): Move /nix/store provision to 'nix-shepherd-service'.

Change-Id: I18a5d58c92c1f2b5b6dcecc3d5b439cc15bf4e49
---
 gnu/services/nix.scm | 47 +++++++++++++++++++++++++++++++++++++-------
 1 file changed, 40 insertions(+), 7 deletions(-)

diff --git a/gnu/services/nix.scm b/gnu/services/nix.scm
index 82853253f6..343b42c13a 100644
--- a/gnu/services/nix.scm
+++ b/gnu/services/nix.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2019, 2020, 2021 Oleg Pykhalov <[email-masked]>
+;;; Copyright © 2019, 2020, 2021, 2024 Oleg Pykhalov <[email-masked]>
 ;;; Copyright © 2020 Peng Mei Yu <[email-masked]>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -97,12 +97,9 @@ (define (nix-activation _)
   #~(begin
       (use-modules (guix build utils)
                    (srfi srfi-26))
-      (for-each (cut mkdir-p <>) '("/nix/store" "/nix/var/log"
+      (for-each (cut mkdir-p <>) '("/nix/var/log"
                                    "/nix/var/nix/gcroots/per-user"
                                    "/nix/var/nix/profiles/per-user"))
-      (chown "/nix/store"
-             (passwd:uid (getpw "root")) (group:gid (getpw "nixbld01")))
-      (chmod "/nix/store" #o775)
       (for-each (cut chmod <> #o777) '("/nix/var/nix/profiles"
                                        "/nix/var/nix/profiles/per-user"))))
 
@@ -129,6 +126,24 @@ (define nix-service-etc
                                     '#$build-sandbox-items))
                     (for-each (cut display <>) '#$extra-config)))))))))))
 
+(define %nix-store-directory
+  "/nix/store")
+
+(define %nix-store-prefix
+  ;; Absolute path to the Nix store.
+  (make-parameter %nix-store-directory))
+
+(define %immutable-nix-store
+  ;; Read-only store to avoid users or daemons accidentally modifying it.
+  ;; 'nix-daemon' has provisions to remount it read-write in its own name
+  ;; space.
+  #~(file-system
+      (device #$(%nix-store-prefix))
+      (mount-point #$(%nix-store-prefix))
+      (type "none")
+      (check? #f)
+      (flags '(read-only bind-mount))))
+
 (define nix-shepherd-service
   ;; Return a <shepherd-service> for Nix.
   (match-lambda
@@ -139,8 +154,26 @@ (define nix-shepherd-service
        (documentation "Run nix-daemon.")
        (requirement '())
        (start #~(make-forkexec-constructor
-                 (list (string-append #$package "/bin/nix-daemon")
-                       #$@extra-options)
+                 (list
+                  #$(program-file
+                     "nix-daemon-wrapper"
+                     (with-imported-modules (source-module-closure '((gnu build file-systems)
+                                                                     (gnu system file-systems)))
+                       #~(begin
+                           (use-modules (gnu build file-systems)
+                                        (gnu system file-systems)
+                                        (guix build syscalls)
+                                        (guix build utils))
+                           (unless (member #$(%nix-store-prefix) (mount-points))
+                             (mkdir-p "/nix/store")
+                             (chown "/nix/store"
+                                    (passwd:uid (getpw "root"))
+                                    (group:gid (getpw "nixbld01")))
+                             (chmod "/nix/store" #o775)
+                             (mount-file-system #$%immutable-nix-store
+                                                #:root "/"))
+                           (execl #$(file-append package "/bin/nix-daemon")
+                                  "nix-daemon" #$@extra-options)))))
                  #:environment-variables
                  (list (string-append "TMPDIR=" #$build-directory)
                        "PATH=/run/current-system/profile/bin")))

base-commit: dd03be186adb64bdb77265dfd0ad53fe50ec016e
-- 
2.41.0