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Date: Wed, 14 Nov 2012 09:14:11 -0800
From: Greg KH <gregkh@...uxfoundation.org>
To: oss-security@...ts.openwall.com
Subject: Re: Linux kernel handling of IPv6 temporary addresses

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

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