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Subject: Re: [PATCH 2/2] mm: restrict access to /proc/meminfo
From: Dave Hansen <dave@linux.vnet.ibm.com>
To: Vasiliy Kulikov <segoon@openwall.com>
Cc: David Rientjes <rientjes@google.com>, Christoph Lameter <cl@gentwo.org>,
        kernel-hardening@lists.openwall.com, Pekka Enberg <penberg@kernel.org>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
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Date: Thu, 29 Sep 2011 09:30:36 -0700
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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.  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.  A
MB-rounded meminfo is also fundamentally *BETTER* than a
PAGE_SIZE-rounded meminfo.  I find it hard to call this "nothing".

Anyway...  I'm working on a patch.  Will post soon.

-- Dave