From: Maxime Devos <maximedevos@telenet.be>
To: 47584@debbugs.gnu.org
Cc: Maxime Devos <maximedevos@telenet.be>
Subject: bug#47584: [PATCH 3/3] activation: Fix TOCTTOU in mkdir-p/perms.
Date: Fri, 28 Oct 2022 18:04:09 +0200 [thread overview]
Message-ID: <20221028160409.31887-3-maximedevos@telenet.be> (raw)
In-Reply-To: <20221028160409.31887-1-maximedevos@telenet.be>
I removed the 'Based upon mkdir-p from (guix build utils)'
comment because it's quite a bit different now.
* gnu/build/activation.scm (verify-not-symbolic): Delete.
(mkdir-p/perms): Rewrite in terms of 'openat'.
---
gnu/build/activation.scm | 90 +++++++++++++++++++++++++---------------
1 file changed, 57 insertions(+), 33 deletions(-)
diff --git a/gnu/build/activation.scm b/gnu/build/activation.scm
index 10c9045740..29c6f2ce4c 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>
;;;
@@ -64,46 +64,70 @@ (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 (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)))
+ ;; 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
- (let loop ((components (string-tokenize dir not-slash))
- (root (if absolute?
- ""
- ".")))
+ (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
+ (begin
+ (close-port root)
+ (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))
+ (close-port root)
+ (apply throw args))))
+ (retry)))))
+ (()
+ (catch 'system-error
+ (lambda ()
+ (chown root (passwd:uid owner) (passwd:gid owner))
+ (chmod root bits))
+ (lambda args
+ (close-port root)
+ (apply throw args)))
+ (close-port root)
+ (values)))))
(define* (copy-account-skeletons home
#:key
--
2.38.0
next prev parent reply other threads:[~2022-10-28 16:06 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
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 ` Maxime Devos [this message]
2022-10-28 16:05 ` bug#47584: [PATCH 1/3] guile-next: Update to 3.0.8-793fb46 Maxime Devos
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