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Date: Thu, 29 Sep 2011 09:30:36 -0700
From: Dave Hansen <dave@...ux.vnet.ibm.com>
To: Vasiliy Kulikov <segoon@...nwall.com>
Cc: David Rientjes <rientjes@...gle.com>, 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>, 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, 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
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