Subject: [PATCH v2] activation: Add ‘call-with-output-file*’ procedure.

From: Xinglu Chen <[email-masked]>
Date: Tue, 08 Jun 2021 20:30:13 +0200

Using ‘call-with-output-file*’ instead of ‘call-with-output-file’ and ‘chmod’
will prevent secrets from being leaked.  See
<https://issues.guix.gnu.org/48872>.

* guix/build/activation.scm (call-with-output-file*): New procedure.
* doc/guix.texi (Activation): New section; document ‘call-with-output-file*’.
---
Changes since v1:

* Moved ‘call-with-output-file*’ from (gnu build utils) to (gnu build
  activation).

* Added a “Activation” section in the manual to document the new
  procedure.

 doc/guix.texi            | 31 +++++++++++++++++++++++++++++++
 gnu/build/activation.scm | 13 ++++++++++++-
 2 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index 59b4ac11b4..643c7ff126 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -321,6 +321,7 @@ System Configuration
 * Invoking guix deploy::        Deploying a system configuration to a remote host.
 * Running Guix in a VM::        How to run Guix System in a virtual machine.
 * Defining Services::           Adding new service definitions.
+* Activation::                  Setting up system-wide files and directories.
 
 Services
 
@@ -13386,6 +13387,7 @@ instance to support new system services.
 * Invoking guix deploy::        Deploying a system configuration to a remote host.
 * Running Guix in a VM::        How to run Guix System in a virtual machine.
 * Defining Services::           Adding new service definitions.
+* Activation::                  Setting up system-wide files and directories.
 @end menu
 
 @node Using the Configuration System
@@ -34633,6 +34635,35 @@ system:
 This service represents PID@tie{}1.
 @end defvr
 
+@node Activation
+@section Activation
+
+@dfn{Activation} is the process that sets up system-wide files and
+directories so that an @code{operating-system} (@pxref{operating-system
+Reference}) configuration becomes active.  This will happen when
+invoking commands like @command{guix system reconfigure} or
+@command{guix system switch-generation}, but not when invoking
+@command{guix system build} (@pxref{Invoking guix system}).
+
+@deffn {Scheme Procedure} call-with-output-file* @var{file} @var{proc} @
+  [#:perms #o666]
+Open FILE for output, set the file permission bits to @var{perms}, and
+call @code{(PROC port)} with the resulting port.
+
+The advantage of using this procedure compared to something like this
+
+@lisp
+(call-with-output-file "FILE"
+  (lambda (port)
+    (display "top secret" port)))
+(chmod "FILE" #o400)
+@end lisp
+
+is that, with the latter, an unpriviliged user could open @var{file}
+before the permission was changed to @code{#o400}, thus making it
+possible to leak sensitive information.
+@end deffn
+
 
 @node Documentation
 @chapter Documentation
diff --git a/gnu/build/activation.scm b/gnu/build/activation.scm
index 2af1d44b5f..0054079cb6 100644
--- a/gnu/build/activation.scm
+++ b/gnu/build/activation.scm
@@ -6,6 +6,7 @@
 ;;; Copyright © 2018 Arun Isaac <[email-masked]>
 ;;; Copyright © 2018, 2019 Ricardo Wurmus <[email-masked]>
 ;;; Copyright © 2021 Maxime Devos <[email-masked]>
+;;; Copyright © 2021 Xinglu Chen <[email-masked]>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -34,6 +35,7 @@
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-11)
   #:use-module (srfi srfi-26)
+  #:use-module (srfi srfi-60)
   #:export (activate-users+groups
             activate-user-home
             activate-etc
@@ -43,7 +45,8 @@
             activate-firmware
             activate-ptrace-attach
             activate-current-system
-            mkdir-p/perms))
+            mkdir-p/perms
+            call-with-output-file*))
 
 ;;; Commentary:
 ;;;
@@ -102,6 +105,14 @@ Warning: this is currently suspect to a TOCTTOU race!"
   (chown directory (passwd:uid owner) (passwd:gid owner))
   (chmod directory bits))
 
+;; Prevent secrets from leaking, see <https://issues.guix.gnu.org/48872>
+(define* (call-with-output-file* file proc #:key (perms #o666))
+  "FILE should be string containg the path to a file, PROC should be a procedure
+that accepts the port as an argument, and PERMS should be the permission bits
+of the file, the default is 666."
+  (let ((port (open file (bitwise-ior O_WRONLY O_CREAT) perms)))
+    (call-with-port port proc)))
+
 (define* (copy-account-skeletons home
                                  #:key
                                  (directory %skeleton-directory)

base-commit: 503c2039a280dd52a751a6852b4157fccd1b4195
-- 
2.32.0