crontab: split into crontab and setuid helper crontab-access.

If a user did somehow manage to install this crontab as functioning
setuid-root in its current state (despite linux ignoring the setuid bit when
executing scripts), it would be a very bad thing for them. It currently has
several glaring security holes. In approximate order from most to least
severe:

1. It blindly calls system() with the user-supplied value of VISUAL or
   EDITOR, without dropping privileges. I can't fathom what the author was
   thinking, considering (mcron scripts crontab) is littered with comments and
   evidence that this is supposed to be a setuid-root program. An attacker
   could simply run

   EDITOR='sh #' crontab -e

   and get a root shell. If you try this, you may find that it coincidentally
   doesn't work because bash in particular always drops privileges on startup
   if it detects differing real and effective ids. I don't know whether other
   shells do this, but it actually doesn't matter as long as you're using
   glibc, because its system() consults PATH looking for sh. One false entry
   in there and an attacker is running arbitrary code as root. And crontab
   doesn't do any sanitizing of *any* environment variables.
2. No attempt is made to sanitize any environment variables. Also, depending
   on Guile's startup behavior, trying to sanitize them in guile may be too
   late. A wrapper is needed, which would be needed anyway in order to use a
   setuid script.
3. No attempt is made to ensure that the temporary file being edited is
   newly-created, so an attacker could guess or deduce the filename to be
   used, create it in advance, keep it open while crontab opens it, and
   overwrite it right before it is copied, allowing them to execute arbitrary
   code as any user that dared edit their crontab, including root.
4. Its replace mode accepts a filename. It does no validation whatsoever on
   this, opens it, and copies it to the user's crontab as long as it's valid
   vixie cron syntax. So for example,
   crontab /var/cron/tabs/root && crontab --list
   will let you freely read root's (and in a similar manner any other user's)
   crontab. Vixie cron includes comments in its valid syntax, so any file that
   consists entirely of comments can also be dumped. Also, any file for which
   opening it and reading from it has side-effects can have those side-effects
   triggered even if it isn't valid vixie cron syntax.
5. Crontabs created in /tmp for editing, as well as crontabs created in
   /var/cron/tabs, are world-readable with typical inherited umask.

(1) and (4) are resolved by splitting crontab into two programs: crontab,
which is no longer setuid, and crontab-access, which is. The setuid program no
longer opens any files except for the user's crontab and the allow/deny files,
and it runs no external programs whatsoever. Crontab is run as the invoking
user, so the usual kernel-level permissions checks regarding which files can
be opened for reading apply. The editor is run from crontab, as the invoking
user, so sanitizing of the environment in the setuid helper has no effect on
the editor's environment.

(2) to be resolved shortly with a wrapper program.

(3) is resolved by using mkstemp. The inability to control the mode it is
created with, along with (5), are resolved by setting the umask properly.

* src/mcron/scripts/crontab-access.scm: new module.
* src/mcron/scripts/crontab.scm: move list, delete, and replace
  implementation to crontab-access.
* src/crontab-access.in: new file to invoke main of crontab-access.
* Makefile.am: inform of crontab-access.in and crontab-access.scm.

1 files changed, 121 insertions, 0 deletions

diff --git a/src/mcron/scripts/crontab-access.scm b/src/mcron/scripts/crontab-access.scm
new file mode 100644
index 0000000..d97fc62
--- /dev/null
+++ b/src/mcron/scripts/crontab-access.scm
@@ -0,0 +1,121 @@
+;;;; crontab -- edit user's cron tabs
+;;; Copyright © 2003, 2004 Dale Mellor <>
+;;; Copyright © 2016 Mathieu Lirzin <mthl@gnu.org>
+;;;
+;;; This file is part of GNU Mcron.
+;;;
+;;; GNU Mcron is free software: you can redistribute it and/or modify
+;;; it under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation, either version 3 of the License, or
+;;; (at your option) any later version.
+;;;
+;;; GNU Mcron is distributed in the hope that it will be useful,
+;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Mcron.  If not, see <http://www.gnu.org/licenses/>.
+(define-module (mcron scripts crontab-access)
+  #:use-module (ice-9 rdelim)
+  #:use-module (mcron config)
+  #:use-module (mcron utils)
+  #:use-module (mcron vixie-specification)
+  #:export (main))
+
+(define (hit-server user-name)
+  "Tell the running cron daemon that the user corresponding to
+USER-NAME has modified his crontab.  USER-NAME is written to the
+'/var/cron/socket' UNIX socket."
+  (catch #t
+    (lambda ()
+      (let ((socket (socket AF_UNIX SOCK_STREAM 0)))
+        (connect socket AF_UNIX config-socket-file)
+        (display user-name socket)
+        (close socket)))
+    (lambda (key . args)
+      (display "Warning: a cron daemon is not running.\n"))))
+
+(define (in-access-file? file name)
+  "Scan FILE which should contain one user name per line (such as
+'/var/cron/allow' and '/var/cron/deny').  Return #t if NAME is in there, and
+#f otherwise.  If FILE cannot be opened, a value that is neither #t nor #f
+is returned."
+  (catch #t
+    (lambda ()
+      (with-input-from-file file
+        (lambda ()
+          (let loop ((input (read-line)))
+            (cond ((eof-object? input)
+                   #f)
+                  ((string=? input name)
+                   #t)
+                  (else
+                   (loop (read-line))))))))
+    (const '())))
+
+(define (main --user --replace --list --remove)
+  (when config-debug  (debug-enable 'backtrace))
+  (let ((crontab-real-user
+         ;; This program should have been installed SUID root. Here we get
+         ;; the passwd entry for the real user who is running this program.
+         (passwd:name (getpw (getuid)))))
+
+    ;; If the real user is not allowed to use crontab due to the
+    ;; /var/cron/allow and/or /var/cron/deny files, bomb out now.
+    (if (or (eq? (in-access-file? config-allow-file crontab-real-user) #f)
+            (eq? (in-access-file? config-deny-file crontab-real-user) #t))
+        (mcron-error 6 "Access denied by system operator."))
+
+    ;; Check that no more than one of the mutually exclusive options are
+    ;; being used.
+    (when (<  1  (+ (if --list 1 0) (if --remove 1 0) (if --replace 1 0)))
+      (mcron-error 7 "Only one of options -l, -r or -R can be used."))
+
+    ;; Check that a non-root user is trying to read someone else's files.
+    (when (and (not (zero? (getuid))) --user)
+      (mcron-error 8 "Only root can use the -u option."))
+
+    ;; Crontabs being written should not have global or group access.
+    (umask #o077)
+
+    (letrec* ( ;; Iff the --user option is given, the crontab-user may be
+              ;; different from the real user.
+              (crontab-user (or --user crontab-real-user))
+              ;; So now we know which crontab file we will be manipulating.
+              (crontab-file
+               (string-append config-spool-dir "/" crontab-user)))
+      ;; There are three possible actions: list, remove, and replace (via
+      ;; stdin).
+      (cond
+       ;; In the remove personality we simply make an effort to delete the
+       ;; crontab and wake the daemon. No worries if this fails.
+       (--remove (catch #t (λ ()  (delete-file crontab-file)
+                              (hit-server crontab-user))
+                   noop))
+
+       ;; Read crontab from stdin, verify it, replace it, wake daemon.
+       (--replace
+        (let ((input-string (read-string)))
+          (catch-mcron-error
+           (read-vixie-port (open-input-string input-string))
+           (unless (file-exists? config-spool-dir)
+             (mkdir config-spool-dir #o700))
+           (with-output-to-file crontab-file
+             (λ () (display input-string))))
+          (hit-server crontab-user)))
+
+       ;; In the list personality, we simply open the crontab and copy it
+       ;; character-by-character to the standard output. If anything goes
+       ;; wrong, it can only mean that this user does not have a crontab
+       ;; file.
+       (else ;; --list or no action specified
+        (catch #t
+          (λ ()
+            (with-input-from-file crontab-file
+              (λ ()
+                (do ((input (read-char) (read-char)))
+                    ((eof-object? input))
+                  (display input)))))
+          (λ (key . args)
+            (mcron-error 17 "No crontab for " crontab-user " exists.\n"))))))))