Subject: chmod: race condition

From: Tobias Stoeckmann <[email-masked]>
Date: Sat, 16 Aug 2014 19:44:47 +0200

Hi,

chmod is vulnerable to a TOCTTOU (time of check to time of use) race
condition. Tested this on an OpenBSD system.  For people unfamiliar
with OpenBSD, /etc/master.passwd basically equals /etc/shadow:

# chmod --version | head -n 1
chmod (GNU coreutils) 8.23
# ls -l /etc/master.passwd
-rw-------  1 root  wheel  4244 Jul 23 21:14 /etc/master.passwd
# chmod -R g+w /usr/src
<second console interferes here>
# ls -l /etc/master.passwd
-rw-rw-r--  1 root  wheel  4244 Jul 23 21:14 /etc/master.passwd

$ rm /usr/src/Makefile
$ ln -s /etc/master.passwd /usr/src/Makefile

For the second console, the user belonged to the same group as
/usr/src (wsrc in this example).

The second console is able to modify Makefile because the directory
/usr/src was already made group-writable.

The race happens in src/chmod.c, around function process_file.  Before
it gets called, fts_read() retrieved information about the _file_
Makefile, i.e. before the second console removed it.

Then the file gets replaced by a symlink, pointing to a file we want
to get modified.  Now chmodat() resolves the path again and actually
evaluates the _symlink_.  The destination file /etc/master.passwd can
be happily parsed by the attacker now.

I won't supply a patch now, as I remember that GNU is a bit picky about
accepting patches from everyone.  But I will recommend to look into
the use of fchmodat() instead, supplying the argument
AT_SYMLINK_NOFOLLOW.  We discuss this solution at OpenBSD currently.

Tobias