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Re: CVE request: kernel: change in how tcp seq numbers are generated
From: Petr Matousek <pmatouse () redhat com>
Date: Tue, 23 Aug 2011 09:51:46 +0200

On Tue, Aug 23, 2011 at 03:47:37PM +0800, Eugene Teo wrote:
http://lwn.net/Articles/455135/
Dan Kaminsky pointed out that using partial MD4 and using that to
generate a sequence number, of which only 24-bits are truly unguessable,
seriously undermine the goals of random sequence number generation.

In particular, with only 24-bits being truly unguessable, packet
injection into a session using even something like brute force is a real
potential possibility.

We only use 24-bits because we regenerate the random number every 5
minutes "just in case."  But what does is trade a "we don't know" kind
of theoretical issue for a provably real one (brute force attack).

Therefore [Dave Miller] moving us more in line with RFC1948 (as well as
OpenBSD and Solaris), to use MD5 and a full 32-bit result in the
generated sequence number.

MD5 was selected as a compromise between performance loss and
theoretical ability to be compromised.  Willy Tarreau did extensive
testing and SHA1 was found to harm performance too much to be considered
seriously at this time.

We may later add a sysctl for various modes (ie. a "super secure" mode
that uses SHA1 if people want that, and an "insecure" mode that doesn't
use cryptographic hashing at all for people in protected environments
where that might be safe to do).

[Dave Miller] also moved the sequence number generators out of random.c
(they never really belonged there, and are only there due to historical
artifacts), and fixed a bug in DCCP sequence number generation (on ipv6
the 43-bit sequence number was truncated to 32-bits).

Upstream commits:
crypto: Move md5_transform to lib/md5.c
http://git.kernel.org/linus/bc0b96b54a21246e377122d54569eef71cec535f
net: Compute protocol sequence numbers and fragment IDs using MD5
http://git.kernel.org/linus/6e5714eaf77d79ae1c8b47e3e040ff5411b717ec

Thanks, Eugene

Please use CVE-2011-3188.

Thanks,
-- 
Petr Matousek / Red Hat Security Response Team


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