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Date: Tue, 04 Dec 2012 21:41:54 -0700
From: Kurt Seifried <kseifried@...hat.com>
To: oss-security@...ts.openwall.com
CC: Greg KH <gregkh@...uxfoundation.org>, kargig@...d.gr
Subject: Re: Linux kernel handling of IPv6 temporary addresses

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On 11/14/2012 10:14 AM, Greg KH wrote:
> On Wed, Nov 14, 2012 at 10:43:22AM +0200, George Kargiotakis
> wrote:
>> Hello all,
>> 
>> Due to the way the Linux kernel handles the creation of IPv6
>> temporary addresses a malicious LAN user can remotely disable
>> them altogether which may lead to privacy violations and
>> information disclosure.
>> 
>> By default the Linux kernel uses the 'ipv6.max_addresses' option
>> to specify how many IPv6 addresses an interface may have. The 
>> 'ipv6.regen_max_retry' option specifies how many times the kernel
>> will try to create a new address.
>> 
>> Currently, in net/ipv6/addrconf.c,lines 898-910, there is no 
>> distinction between the events of reaching max_addresses for an 
>> interface and failing to generate a new address. Upon reaching
>> any of the above conditions the following error is emitted by the
>> kernel times 'regen_max_retry' (default value 3):
>> 
>> [183.793393] ipv6_create_tempaddr(): retry temporary address 
>> regeneration [183.793405] ipv6_create_tempaddr(): retry
>> temporary address regeneration [183.793411]
>> ipv6_create_tempaddr(): retry temporary address regeneration
>> 
>> After 'regen_max_retry' is reached the kernel completely
>> disables temporary address generation for that interface.
>> 
>> [183.793413] ipv6_create_tempaddr(): regeneration time exceeded
>> - disabled temporary address support
>> 
>> RFC4941 3.3.7 specifies that disabling temp_addresses MUST happen
>> upon failure to create non-unique addresses which is not the
>> above case. Addresses would have been created if the kernel had a
>> higher 'ipv6.max_addresses' limit.
>> 
>> A malicious LAN user can send a limited amount of RA prefixes and
>> thus disable IPv6 temporary address creation for any Linux host.
>> Recent distributions which enable the IPv6 Privacy extensions by
>> default, like Ubuntu 12.04 and 12.10, are vulnerable to such
>> attacks.
>> 
>> Due to the kernel's default values for valid (604800) and
>> preferred (86400) lifetimes, this scenario may even occur under
>> normal usage when a Router sends both a public and a ULA prefix,
>> which is not an uncommon scenario for IPv6. 16 addresses are not
>> enough with the current default timers when more than 1 prefix is
>> advertised.
>> 
>> The kernel should at least differentiate between the two cases
>> of reaching max_addresses and being unable to create new
>> addresses, due to DAD conflicts for example.
> 
> Have you discussed this with the upstream Linux kernel networking 
> developers?
> 
> thanks,
> 
> greg k-h


Sounds like this needs a CVE, is it correct that: an attacker can
create a bunch of RA prefixes thus filling up the # of allowed IPv6
addresses for an interface, preventing any more IPv6 addresses from
being assigned to that interface? In other words an attack over the
local network resulting in a DoS condition.

- -- 
Kurt Seifried Red Hat Security Response Team (SRT)
PGP: 0x5E267993 A90B F995 7350 148F 66BF 7554 160D 4553 5E26 7993

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