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Date: Thu, 29 Sep 2011 21:28:21 +0400
From: Vasiliy Kulikov <segoon@openwall.com>
To: David Rientjes <rientjes@google.com>
Cc: Christoph Lameter <cl@gentwo.org>, kernel-hardening@lists.openwall.com,
        Pekka Enberg <penberg@kernel.org>, Matt Mackall <mpm@selenic.com>,
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        Valdis.Kletnieks@vt.edu,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Alan Cox <alan@linux.intel.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] mm: restrict access to /proc/meminfo
Message-ID: <20110929172821.GA19601@albatros>
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On Thu, Sep 29, 2011 at 20:18 +0400, Vasiliy Kulikov wrote:
> On Tue, Sep 27, 2011 at 13:33 -0700, David Rientjes wrote:
> > I'd much rather just convert everything to use MB rather than KB so you 
> > can't determine things at a page level.  I think that gets us much closer 
> > to what the patch is intending to restrict.  But I also expect some 
> > breakage from things that just expect meminfo to be in KB units without 
> > parsing what the kernel is exporting.
> 
> I'm not convinced with rounding the information to MBs.  The attacker
> still may fill slabs with new objects to trigger new slab pages
> allocations.  He will be able to see when this MB-granularity barrier is
> overrun thus seeing how many kbs there were before:
> 
>     old = new - filled_obj_size_sum
> 
> As `new' is just increased, it means it is known with KB granularity,
> not MB.  By counting used slab objects he learns filled_obj_size_sum.

s/filled_obj_size_sum/old/ of course.

> So, rounding gives us nothing, but obscurity.
> 
> Thanks,

-- 
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments