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GNU fileutils - recursive directory removal race condition
From: Wojciech Purczynski <cliph () isec pl>
Date: Mon, 11 Mar 2002 00:21:23 +0100 (CET)

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Name:           fileutils
Version:        4.1 stable and 4.1.6 development version
Homepage:       http://www.gnu.org/software/fileutils/fileutils.html
Author:         Wojciech Purczynski <cliph () isec pl>
Date:           March 10, 2002


Issue:
======

Race condition in various utilities from fileutils GNU package may cause
root user to delete the whole filesystem.


Description:
============

The GNU File Utilities are the basic file-manipulation utilities of the
GNU operating system.


Details:
========

An insecure chdir("..") syscall is done after removing content of a
subdirectory in order to get back to the upper directory during recursive
removal of directory tree.

Example of 'rm -fr /tmp/a' removing '/tmp/a/b/c' directory tree:

(strace output simplified for better readability)

chdir("/tmp/a")                         = 0
chdir("b")                              = 0
chdir("c")                              = 0
chdir("..")                             = 0
rmdir("c")                              = 0
chdir("..")                             = 0
rmdir("b")                              = 0
fchdir(3)                               = 0
rmdir("/tmp/a")                         = 0

After current directory is changed to /tmp/a/b/c a race condition occurs.
If we then move /tmp/a/b/c directory to the /tmp/c two subsequent
chdir("..") syscalls will move to the root directory / and rm will start
removing files from the whole file systems if it has enough privileges
(i.e. if called by root user).

Timeframe of this race condition depends on how complicated directory
structure is.

The same issue affects also mv utility when source and destination
directory lie on different filesystems and they are removed after
creating copy on destination.


Impact:
=======

Unprivileged users may launch daemon program that will detect the removal
operation of user's directories and exploit race condition leading to
Denial of Service.


Fix:
====

On March 7, 2002 we have contacted with developers of GNU fileutils
package. On March 9, 2002 a patch fixing this vulnerability has been
released for the latest 4.1.6 development version:

http://mail.gnu.org/pipermail/bug-fileutils/2002-March/002440.html


- -- 
Wojciech Purczynski
iSEC Security Research
http://isec.pl/
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