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Re: [Security] [PATCH] proc: avoid information leaks to non-privileged processes

From:  Matt Mackall <mpm-AT-selenic.com>
To:  Linus Torvalds <torvalds-AT-linux-foundation.org>
Subject:  Re: [Security] [PATCH] proc: avoid information leaks to non-privileged processes
Date:  Wed, 6 May 2009 19:50:16 -0500
Message-ID:  <20090507005016.GJ31071@waste.org>
Cc:  Ingo Molnar <mingo-AT-elte.hu>, "Eric W. Biederman" <ebiederm-AT-xmission.com>, Arjan van de Ven <arjan-AT-infradead.org>, Jake Edge <jake-AT-lwn.net>, security-AT-kernel.org, Linux Kernel Mailing List <linux-kernel-AT-vger.kernel.org>, James Morris <jmorris-AT-namei.org>, linux-security-module-AT-vger.kernel.org, Eric Paris <eparis-AT-redhat.com>, Alan Cox <alan-AT-lxorguk.ukuu.org.uk>, Roland McGrath <roland-AT-redhat.com>, mingo-AT-redhat.com, Andrew Morton <akpm-AT-linux-foundation.org>, Greg KH <greg-AT-kroah.com>, Dave Jones <davej-AT-redhat.com>
Archive-link:  Article

On Wed, May 06, 2009 at 12:57:17PM -0500, Matt Mackall wrote:
> On Wed, May 06, 2009 at 09:48:20AM -0700, Linus Torvalds wrote:
> > 
> > Matt, are you willing to ack my suggested patch which adds history to the 
> > mix? Did somebody test that? I have this memory of there being an 
> > "exploit" program to show the non-randomness of the values, but I can't 
> > recall details, and would really want to get a second opinion from 
> > somebody who cares about PRNG's.
> 
> I still don't like it. I bounced it off some folks on the adversarial
> side of things and they didn't think it looked strong enough either.
> Full MD5 collisions can be generated about as fast as they can be
> checked, which makes _reduced strength_ MD4 not much better than an
> LFSR in terms of attack potential. So I suggest we either:
> 
> a) take my original patch 
> b) respin your patch using at least SHA1 rather than halfMD4 and
> changing the name to get_random_u32
> 
> If you'd prefer (b), I'll do the legwork.

I've done some basic benchmarks on the primitives here in userspace:

halfMD4 get_random_int: about .326us per call or 12.2MB/s
sha1 get_random_int: about .660us per call or 6.1MB/s
dd /dev/urandom: 3.6MB/s

So I think the SHA1 solution is quite competitive on the performance
front with far fewer concerns about its strength. I'll spin a proper
patch tomorrow.

-- 
Mathematics is the supreme nostalgia of our time.
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