full-disclosure - [PRE-SA-2011-06] Linux kernel: ZERO_SIZE_PTR dereference for long symlinks in Be FS

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Date: Fri, 19 Aug 2011 17:50:31 +0200
From: Timo Warns <warns@...-sense.de>
To: bugtraq@...urityfocus.com, full-disclosure@...ts.grok.org.uk
Subject: [PRE-SA-2011-06] Linux kernel: ZERO_SIZE_PTR
 dereference for long symlinks in Be FS

PRE-CERT Security Advisory
==========================

* Advisory: PRE-SA-2011-06
* Released on: 19 August 2011
* Last updated on: 19 August 2011
* Affected product: Linux Kernel 2.4, 2.6, and 3.0
* Impact: denial-of-service
* Origin: Be file system
* Credit: Timo Warns (PRESENSE Technologies GmbH)
* CVE Identifier: CVE-2011-2928

Summary
-------

The Linux kernel contains a vulnerability in the driver for Be file systems
that may lead to a kernel oops via a corrupted Be file system.

In fs/befs/linuxvfs.c, befs_follow_link() reads a length attribute for a
long
symlink from a data stream of a Be file system.

    befs_data_stream *data = &befs_ino->i_data.ds;
    befs_off_t len = data->size;

The data->size / len value is not validated and can be 0 on a corrupted
file system.

befs_follow_link() allocates some memory based on len. Effectively, kmalloc
returns ZERO_SIZE_PTR in this case.

        link = kmalloc(len, GFP_NOFS);

Subsequently, an assignment dereferences ZERO_SIZE_PTR causing a kernel
oops:

			link[len - 1] = '\0';

Workaround
----------

Compile and use a kernel that does not support the Be file system. The
corresponding configuration key is CONFIG_BEFS_FS.

Solution
--------

A patch is available at
http://git.kernel.org/linus/338d0f0a6fbc82407864606f5b64b75aeb3c70f2

References
----------

When further information becomes available, this advisory will be
updated. The most recent version of this advisory is available at:

http://www.pre-cert.de/advisories/PRE-SA-2011-06.txt

Contact
--------

PRE-CERT can be reached under precert@...-secure.de. For PGP key
information, refer to http://www.pre-cert.de/.

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