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[SLAB] NetBSD / OpenBSD kernfs_xread patch evasion
From: SecurityLab Research <SLAB_research () securitylab net>
Date: Thu, 02 Feb 2006 15:40:47 -0500


--- SecurityLab Technologies, Inc.
--- Security Advisory
--- http://www.securitylab.net


Advisory Name: NetBSD / OpenBSD kernfs_xread patch evasion
Release Date: February 02, 2006
Application: kernfs
Platform: NetBSD / OpenBSD
Severity: Severe
Author: SLAB Research
Vendor Status: Patched
Reference: http://www.securitylab.net/research/
<http://www.securitylab.net/research/>

Overview:

Due to a flaw in the original patch implemented by the NetBSD team in
release 2.0.3 the kernfs_xread function was still vulnerable to
exploitation. The original patch failed to manage the truncation of
64bit integers. Prior to the 2.0.3 patch kernfs_read neglected to test
for a negative file offset value. The 2.0.3 patch enforced the testing
of negative offsets but failed to test for negative 32bit values. Since
the kernfs_xread function truncates the 64bit offset to a 32bit value it
was possible to have a negative 32bit offset bypass the security
employed. This negative offset flaw made continued disclosure of kernel
memory possibly.

OpenBSD's 3.8 kernel release contained the same vulnerability and the
same type of patch as NetBSD 2.0.3. It checked for the negative value in
a 64bit read offset. However, kernfs is no longer included in the
current OpenBSD generic kernel.


Vendor response:

OpenBSD:
OpenBSD believes this issue is not a vulnerability, because kernfs was
not linked into the GENERIC kernel by default. The OpenBSD team has
chosen to remove the kernfs tree from the current kernel code, rather
than implementing a patch.

NetBSD:
In response to this advisory the NetBSD team patched kernfs_vnops.c
version 1.114. The fix is available in the current source tree. NetBSD
3.0 recently released is not affected by this flaw. The NetBSD team has
issued an advisory:

ftp://ftp.netbsd.org/pub/NetBSD/security/advisories/NetBSD-SA2006-001.txt.asc


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Copyright 2006 SecurityLab Technologies, Inc. You may distribute freely
without modification.




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