linux-kernel - Re: [PATCH 2/2] mm: restrict access to /proc/meminfo

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Date:	Thu, 29 Sep 2011 21:28:21 +0400
From:	Vasiliy Kulikov <segoon@...nwall.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Christoph Lameter <cl@...two.org>,
	kernel-hardening@...ts.openwall.com,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	Kees Cook <kees@...ntu.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>, Valdis.Kletnieks@...edu,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alan Cox <alan@...ux.intel.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: restrict access to /proc/meminfo

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
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