Linux Kernel cpuset tasks Information Disclosure Vulnerability

iDefense Security Advisory 06.07.07
http://labs.idefense.com/intelligence/vulnerabilities/ 
Jun 07, 2007

I. BACKGROUND

Linux is a clone of the UNIX operating system, written from scratch by
Linus Torvalds with assistance from a loosely-knit team of hackers
across the Internet. The cpuset functionality allows process to be
assigned to processors on multi-processor machines.

II. DESCRIPTION

Local exploitation of an information disclosure vulnerability within the
Linux Kernel allows attackers to obtain sensitive information from
kernel memory.

This vulnerability specifically exists in the "cpuset_tasks_read"
function. This function is responsible for supplying user-land
processes with data when they read from the /dev/cpuset/tasks file. The
code excerpt below shows the problem area.

1754 if (*ppos + nbytes > ctr->bufsz)
1755 nbytes = ctr->bufsz - *ppos;
1756 if (copy_to_user(buf, ctr->buf + *ppos, nbytes))

By reading from an offset (*ppos) larger than the contents of the file,
an attacker can cause an integer underflow to occur in the subtraction
on line 1755. This will result in the "copy_to_user" function on line
1756 to be called with a memory address located at a lower address than
the start of the intended buffer. This memory could potentially contain
sensitive information such as security tokens or passwords.

III. ANALYSIS

Exploitation of this vulnerability allows attackers to obtain sensitive
information from kernel memory.

In order to exploit this vulnerability, an attacker would need access to
open the /dev/cpuset/tasks file. It is important to note that this file
does not exist unless the cpuset file system has been mounted.
Additionally, this functionality is not included by default in a
vanilla kernel build.

Furthermore, because of checks at the VFS layer and in the
'copy_to_user()' function, an attacker cannot use arbitrary values.
However, on 32-bit systems it is easily exploitable.

IV. DETECTION

iDefense has confirmed the existence of this vulnerability in version
2.6.20 of the Linux Kernel as installed with Fedora CORE 6. It is
suspected that previous versions, at least until 2.6.12, are also
vulnerable.

V. WORKAROUND

In order to prevent exploitation of this vulnerability, discontinue use
of the cpuset file system. This can be accomplished by un-mounting the
file system using the "umount" command.

VI. VENDOR RESPONSE

The Linux kernel team has released versions 2.6.20.13 and 2.6.21.4 to
address this vulnerability. More information can be found via the
following URLs.

http://kernel.org/pub/linux/kernel/v2.6/ChangeLog-2.6.20.13 

http://kernel.org/pub/linux/kernel/v2.6/ChangeLog-2.6.21.4 

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2007-2875 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org/), which standardizes names for 
security problems.

VIII. DISCLOSURE TIMELINE

04/27/2007 Initial vendor notification
06/04/2007 Second vendor notification
06/04/2007 Initial vendor response
06/07/2007 Coordinated public disclosure

IX. CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

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X. LEGAL NOTICES

Copyright (c) 2007 iDefense, Inc.

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Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
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There are no warranties with regard to this information. Neither the
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