From: Maxime Devos <maximedevos@telenet.be>
To: 47584@debbugs.gnu.org
Subject: bug#47584: Race condition in ‘copy-account-skeletons’: possible privilege escalation.
Date: Fri, 21 Oct 2022 11:31:14 +0200 [thread overview]
Message-ID: <5c6c936c-7558-a6a1-5a36-ba8bb38db530@telenet.be> (raw)
In-Reply-To: <1a6ed722dfdd96dc8d53f939aa8e440ca7c29213.camel@telenet.be>
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Now openat etc is in Guile, I've looked into adjusting mkdir-p/perms
appropriately. TODO: change the Guile used for activation to some
commit that has openat etc, adjust patch according to test failures.
(Not tested yet)
Greetings,
Maxime.
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diff --git a/gnu/build/activation.scm b/gnu/build/activation.scm
index 10c9045740..ee52bb1979 100644
--- a/gnu/build/activation.scm
+++ b/gnu/build/activation.scm
@@ -5,7 +5,7 @@
;;; 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>
+;;; Copyright © 2021, 2022 Maxime Devos <maximedevos@telenet.be>
;;; Copyright © 2020 Christine Lemmer-Webber <cwebber@dustycloud.org>
;;; Copyright © 2021 Brice Waegeneire <brice@waegenei.re>
;;;
@@ -65,45 +65,61 @@ (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 (mkdir-p/perms directory owner bits)
+ "Create directory DIRECTORY and all its ancestors.
+
+Additionally, verify no component of DIRECTORY is a symbolic link,
+without TOCTTOU races. However, if OWNER differs from the the current
+(process) uid/gid, there is a small window in which DIRECTORY is set to the
+current (process) uid/gid instead of OWNER. This is not expected to be
+a problem in practice.
+
+The permission bits and owner of DIRECTORY are set to BITS and OWNER.
+Anything above DIRECTORY that already exists keeps
+its old owner and bits. For components that do not exist yet, the owner
+and bits are set according to the default behaviour of 'mkdir'."
(define absolute?
- (string-prefix? "/" dir))
+ (string-prefix? "/" directory))
(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?
- ""
- ".")))
+ ;; By combining O_NOFOLLOW and O_DIRECTORY, this procedure automatically
+ ;; verifies that no components are symlinks.
+ (define open-flags (logior O_CLOEXEC ; don't pass the port on to subprocesses
+ O_NOFOLLOW ; don't follow symlinks
+ O_DIRECTORY ; reject anything not a directory
+ O_PATH)) ; TODO: Does Hurd have O_PATH?
+
+ (let loop ((components (string-tokenize directory not-slash))
+ (root (open (if absolute? "/" ".") open-flags)))
(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))
+ (let retry ()
+ ;; In the usual case, we expect HEAD to already exist.
+ (match (catch 'system-error
+ (lambda ()
+ (openat root head open-flags))
+ (lambda args
+ (if (= ENOENT (system-error-errno args))
+ #false
+ (apply throw args))))
+ ((? port? new-root)
+ (close root)
+ (loop tail new-root))
+ (#false
+ ;; If not, create it.
+ (catch 'system-error
+ (lambda _
+ (mkdirat root head))
+ (lambda args
+ ;; Someone else created the directory. Unexpected but fine.
+ (unless (= EEXIST (system-error-errno args))
+ (apply throw args))))
+ (retry)))))
+ (()
+ (chown directory (passwd:uid owner) (passwd:gid owner))
+ (chmod directory bits)))))
(define* (copy-account-skeletons home
#:key
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next prev parent reply other threads:[~2022-10-21 9:34 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-03 16:09 bug#47584: Race condition in ‘copy-account-skeletons’: possible privilege escalation Maxime Devos
2021-04-03 16:22 ` Maxime Devos
2021-04-03 16:32 ` Maxime Devos
2021-04-03 20:15 ` Ludovic Courtès
2021-04-03 16:26 ` Maxime Devos
2021-04-03 20:45 ` Ludovic Courtès
2021-04-03 20:49 ` Ludovic Courtès
2021-04-04 13:29 ` Maxime Devos
2021-04-03 20:27 ` Ludovic Courtès
2021-04-03 20:33 ` Ludovic Courtès
2021-04-04 7:36 ` Maxime Devos
2021-04-05 19:54 ` Ludovic Courtès
2021-04-06 9:56 ` Maxime Devos
2021-04-06 11:57 ` Ludovic Courtès
2021-04-07 18:28 ` Maxime Devos
2022-10-21 9:31 ` Maxime Devos [this message]
2022-10-28 16:03 ` bug#47584: [DRAFT PATCH v2 0/4] Fix race condition in mkdir-p/perms Maxime Devos
2022-10-28 16:04 ` bug#47584: [PATCH 1/3] guile-next: Update to 3.0.8-793fb46 Maxime Devos
2022-10-28 16:04 ` bug#47584: [PATCH 2/3] WIP gnu: Change the Guile used for activation to one that has 'openat' Maxime Devos
2022-10-28 16:04 ` bug#47584: [PATCH 3/3] activation: Fix TOCTTOU in mkdir-p/perms Maxime Devos
2022-10-28 16:05 ` bug#47584: [PATCH 1/3] guile-next: Update to 3.0.8-793fb46 Maxime Devos
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