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NetBSD Security Advisory 2019-004: IPv6 neighbor cache leak on expiration

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		NetBSD Security Advisory 2019-004

Topic:		IPv6 neighbor cache leak on expiration

Version:	NetBSD-current:		affected up to 8.99.51
		NetBSD 8.1:		affected
		NetBSD 7.2:		not affected
		NetBSD 7.1.2:		not affected
		NetBSD 7.0.2:		not affected

Severity:	Remote DoS attack can cause the kernel to run out of memory

Fixed:		NetBSD-current:		July 26, 2019
		NetBSD-9 branch:	not affected
		NetBSD-8 branch:	July 26, 2019
		NetBSD-7 branch:	not affected
		NetBSD-7-2 branch:	not affected
		NetBSD-7-1 branch:	not affected
		NetBSD-7-0 branch:	not affected

Please note that NetBSD releases prior to 7.1 are no longer supported.
It is recommended that all users upgrade to a supported release.


An unprivileged user can cause kernel resource exhaustion by creating many
IPv6 neighbor entries, because the GC mechanism will not delete them due to
a reference counting bug. An explicit cache flush will delete them,
recovering the used memory.

Technical Details

The garbage collection (GC) mechanism of IPv6 neighbor caches has a bug in
that it doesn't free expired neighbor caches (llentry) because of a missing
refcount decrement.

The leak occurs when:
 1) giving up a neighbor discovery for a cache,
 2) giving up a reachability confirmation for a cache,
 3) a cache expires, or
 4) a forced GC runs.

A forced GC destroys cache entries that have overflowed its limit

The third and fourth cases can be triggered on receiving NS packets so it
can be used as a DoS attack that can originate from machines on the same link.

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), 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/netinet6/nd6.c
	# ./ 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:

Thanks To

Internet Initiative Japan Inc. for finding and fixing the vulnerability.

More Information

Advisories may be updated as new information becomes available.
The most recent version of this advisory (PGP signed) can be found at

Information about NetBSD and NetBSD security can be found at

Copyright 2019, The NetBSD Foundation, Inc.  All Rights Reserved.
Redistribution permitted only in full, unmodified form.

$NetBSD: NetBSD-SA2019-004.txt.asc,v 1.1 2019/08/08 10:18:29 christos Exp $


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