Subject: [bug#48923] [PATCH] build: utils: Add ‘call-with-outp

From: Xinglu Chen <[email-masked]>
Date: Tue, 08 Jun 2021 17:40:52 +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/utils.scm (call-with-output-file*): New procedure.
* doc/guix.texi (Build Utilities): Document it.
---
 doc/guix.texi        | 19 +++++++++++++++++++
 guix/build/utils.scm | 10 ++++++++++
 2 files changed, 29 insertions(+)

diff --git a/doc/guix.texi b/doc/guix.texi
index 59b4ac11b4..7e15cd9e92 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -8612,6 +8612,25 @@ Be careful about using @code{$} to match the end of a line; by itself it
 won't match the terminating newline of a line.
 @end deffn
 
+@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
+
 @subsection File Search
 
 @cindex file, searching
diff --git a/guix/build/utils.scm b/guix/build/utils.scm
index 419c10195b..df960eee84 100644
--- a/guix/build/utils.scm
+++ b/guix/build/utils.scm
@@ -5,6 +5,7 @@
 ;;; Copyright © 2015, 2018 Mark H Weaver <[email-masked]>
 ;;; Copyright © 2018 Arun Isaac <[email-masked]>
 ;;; Copyright © 2018, 2019 Ricardo Wurmus <[email-masked]>
+;;; Copyright © 2021 Xinglu Chen <[email-masked]>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -66,6 +67,7 @@
             file-name-predicate
             find-files
             false-if-file-not-found
+            call-with-output-file*
 
             search-path-as-list
             set-path-environment-variable
@@ -448,6 +450,14 @@ also be included.  If FAIL-ON-ERROR? is true, raise an exception upon error."
           #f
           (apply throw args)))))
 
+;; 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)))
+
 
 ;;;
 ;;; Search paths.

base-commit: 503c2039a280dd52a751a6852b4157fccd1b4195
-- 
2.32.0