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AUSCERT External Security Bulletin Redistribution
ESB-2014.1543
Multiple vulnerabilities in the execve system call
10 September 2014
===========================================================================
AusCERT Security Bulletin Summary
---------------------------------
Product: kernel
Publisher: NetBSD
Operating System: NetBSD
Impact/Access: Denial of Service -- Existing Account
Resolution: Patch/Upgrade
Original Bulletin:
http://ftp.netbsd.org/pub/NetBSD/security/advisories/NetBSD-SA2014-009.txt.asc
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NetBSD Security Advisory 2014-009
=================================
Topic: Multiple vulnerabilities in the execve system call
Version: NetBSD-current: source prior to Fri, Feb 14th 2014
NetBSD 6.1 - 6.1.4: affected
NetBSD 6.0 - 6.0.4: affected
NetBSD 6.0.5: not affected
NetBSD 5.1 - 5.1.4: not affected
NetBSD 5.2 - 5.2.2: not affected
Severity: Local DoS
Fixed: NetBSD-current: Fri, Feb 14th 2014
NetBSD-6-0 branch: Fri, Feb 14th 2014
NetBSD-6-1 branch: Fri, Feb 14th 2014
NetBSD-6 branch: Fri, Feb 14th 2014
Teeny versions released later than the fix date will contain the fix.
Please note that NetBSD releases prior to 5.1 are no longer supported.
It is recommended that all users upgrade to a supported release.
Abstract
========
The execve system call is affected by two vulnerabilities:
1) A memory leak in the kernel could cause a local (un)privileged user
to use up kernel memory via a bogus ELF binary, and thus to freeze - or
eventually panic - the system.
2) A bug in the kernel could lead to a use-after-free condition when
loading a binary or a script, which would allow a local (un)privileged
user to crash the system.
Technical Details
=================
1) When trying to execute an ELF binary, the kernel looks up the
corresponding "interpreter" (in case of native dynamic ELF binaries: the
dynamic linker ld.elf_so). If this interpreter cannot be accessed
appropriately, or if it is bogus, a structure allocated to hold special
information on this interpreter was not freed.
If a standard toolchain is installed, a local user can easily create
such broken binaries by passing the -dynamic-linker switch to the linker.
2) When executing a binary via execve(), the kernel computes the new
user stack size, and returns an error if this size exceeds the maximum
architecture-defined stack size or the maximum stack size allowed by the
calling process through rlimit. However, the variable in charge of hold-
ing the error code returned was not properly initialised, causing the
kernel to keep setting up the new process environment and use data that
was already freed.
Both the new stack size and the rlimit stack size are approximately
user-controllable, which makes it easy to trigger from a local user.
Solutions and Workarounds
=========================
For all NetBSD versions, you need to obtain fixed kernel sources,
rebuild and install the new kernel, and reboot the system.
The fixed source may be obtained from the NetBSD CVS repository.
The following instructions briefly summarise how to upgrade your
kernel. In these instructions, replace:
ARCH with your architecture (from uname -m),
KERNCONF with the name of your kernel configuration file and
VERSION with the file version below
File versions containing the fixes:
FILE HEAD netbsd-6 netbsd-6-1 netbsd-6-0
---- ---- -------- ---------- ----------
sys/kern/exec_elf.c
1.55 1.37.2.2 1.37.2.1.6.1 1.37.2.1.4.1
sys/kern/kern_exec.c
1.403 1.339.2.9 1.339.2.6.2.2 1.339.2.5.4.3
To update from CVS, re-build, and re-install the kernel:
# cd src
# cvs update -d -P -r VERSION sys/kern/exec_elf.c
# cvs update -d -P -r VERSION sys/kern/kern_exec.c
# ./build.sh kernel=KERNCONF
# mv /netbsd /netbsd.old
# cp sys/arch/ARCH/compile/obj/KERNCONF/netbsd /netbsd
# shutdown -r now
For more information on how to do this, see:
http://www.NetBSD.org/guide/en/chap-kernel.html
Thanks To
=========
Thanks to Maxime Villard, who found the issues and provided fixes.
Revision History
================
2014-08-27 Initial release
2014-09-08 Updated affected versions
More Information
================
Advisories may be updated as new information becomes available.
The most recent version of this advisory (PGP signed) can be found at
http://ftp.NetBSD.org/pub/NetBSD/security/advisories/NetBSD-SA2014-009.txt.asc
Information about NetBSD and NetBSD security can be found at
http://www.NetBSD.org/ and http://www.NetBSD.org/Security/ .
Copyright 2014, The NetBSD Foundation, Inc. All Rights Reserved.
Redistribution permitted only in full, unmodified form.
$NetBSD: NetBSD-SA2014-009.txt,v 1.4 2014/09/02 14:26:27 spz Exp $
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