NetBSD Security Advisory 2008-010: Malicious PPPoE discovery packet can overrun a kernel buffer

[NetBSD Security-Officer <security-officer%netbsd.org@localhost>]

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                 NetBSD Security Advisory 2008-010
                 =================================

Topic:          Malicious PPPoE discovery packet can overrun a kernel buffer

Version:        NetBSD-current:         affected
                NetBSD 4.0:             affected
                NetBSD 3.1.*:           affected
                NetBSD 3.1              affected
                NetBSD 3.0.*:           affected
                NetBSD 3.0:             affected

Severity:       Remote denial-of-service

Fixed:          NetBSD-current:         August 08, 2008
                NetBSD-4-0 branch:      August 08, 2008
                        (4.0.1 will include the fix)
                NetBSD-4 branch:        August 08, 2008
                        (4.1 will include the fix)
                NetBSD-3-1 branch:      August 08, 2008
                        (3.1.2 will include the fix)
                NetBSD-3-0 branch:      August 08, 2008
                        (3.0.4 will include the fix)
                NetBSD-3 branch:        August 08, 2008
                        (3.2 will include the fix)


Abstract
========

A problem has been identified in the pppoe(4) code.  A bug in range checking
allows a malicious packet to make the kernel access memory outside of the
allocated buffer and cause a kernel crash.  It is currently unclear if this
issue could be exploited any further than denial of service.


Technical Details
=================

The critical code deals with early states of a PPPoE connection, before
a session between client and access concentrator has been established.
Packets in this "discovery" phase may consist of multiple variable length
"tags" packed together in a pppoe packet. Each tag is checked and the length
validated against to total packet size. A bug in this length check allowed
packets to advance the next tag pointer to up to 4 bytes beyond the end
of the packet. This can cause a kernel crash.

The problematic code path is executed even without active pppoe(4) interfaces,
as long as at least one has been created with "ifconfig pppoe0 create". No
further configuration of the pppoe(4) interface is needed.

The attack is not routable, so attackers would have to have access to the
LAN of an affected machine - or the DSL side would need to be configured
like a shared ethernet segment. This is uncommon, but done by some providers.


Solutions and Workarounds
=========================

The pseudo-device pppoe is present in GENERIC kernels.  The system must
be configured with a pppoe(4) instance.

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), and                  
  KERNCONF with the name of your kernel configuration file.    

To update from CVS, re-build, and re-install the kernel:

        # cd src
        # cvs update -d -P sys/net/if_spppsubr.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
=========

Yasuoka Masahiko from Internet Initiative Japan Inc. for discovering the
problem and providing a fix.


Revision History
================

        2008-08-26      Initial release


More Information
================

Advisories may be updated as new information becomes available.
The most recent version of this advisory (PGP signed) can be found at 
  ftp://ftp.NetBSD.org/pub/NetBSD/security/advisories/NetBSD-SA2008-010.txt.asc

Information about NetBSD and NetBSD security can be found at
http://www.NetBSD.org/ and http://www.NetBSD.org/Security/.


Copyright 2008, The NetBSD Foundation, Inc.  All Rights Reserved.
Redistribution permitted only in full, unmodified form.

$NetBSD: NetBSD-SA2008-010.txt,v 1.1 2008/08/25 22:51:57 adrianp Exp $

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