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Date: Thu, 29 Sep 2011 20:43:41 +0400
From: Vasiliy Kulikov <>
Cc: David Rientjes <>, Christoph Lameter <>,
	Pekka Enberg <>, Matt Mackall <>,
	Andrew Morton <>,,
	Kees Cook <>,,
	Linus Torvalds <>,
	Alan Cox <>,
Subject: Re: Re: [PATCH 2/2] mm: restrict access to

On Thu, Sep 29, 2011 at 09:30 -0700, Dave Hansen wrote:
> On Thu, 2011-09-29 at 20:18 +0400, Vasiliy Kulikov wrote:
> > 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.
> > 
> > So, rounding gives us nothing, but obscurity. 
> I'll agree that it doesn't fundamentally fix anything.  But, it does
> make an attack more difficult in the real world.

No, it doesn't.  An attacker is able to simply add/remove objects from
slab and get the precise numbers.  The only thing it takes some time,
but the delay is negligible.  It neither eliminates the whole attack
vector in specific cases nor makes the attacks probabilistic.

>  There's a reason that
> real-world attackers are going after slabinfo: it's a fundamentally
> *BETTER* than meminfo as a tool with which to aim an attack.

Agreed, it gives much more information.

>  A
> MB-rounded meminfo is also fundamentally *BETTER* than a
> PAGE_SIZE-rounded meminfo.  I find it hard to call this "nothing".

Could you elaborate?  As I've tried to describe above, an attacker still
may recover the numbers.


Vasiliy Kulikov - bringing security into open computing environments

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