linux-kernel - Re: [PATCH] Make /proc/slabinfo 0400

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Message-ID: <alpine.LNX.2.00.1103060137410.6297@swampdragon.chaosbits.net>
Date:	Sun, 6 Mar 2011 01:42:46 +0100 (CET)
From:	Jesper Juhl <jj@...osbits.net>
To:	Dan Rosenberg <drosenberg@...curity.com>
cc:	Pekka Enberg <penberg@...nel.org>, Matt Mackall <mpm@...enic.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Theodore Tso <tytso@....edu>, cl@...ux-foundation.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Make /proc/slabinfo 0400

On Fri, 4 Mar 2011, Dan Rosenberg wrote:

> On Fri, 2011-03-04 at 22:58 +0200, Pekka Enberg wrote:
> > On Fri, Mar 4, 2011 at 10:37 PM, Dan Rosenberg <drosenberg@...curity.com> wrote:
> > > This patch makes these techniques more difficult by making it hard to
> > > know whether the last attacker-allocated object resides before a free or
> > > allocated object.  Especially with vulnerabilities that only allow one
> > > attempt at exploitation before recovery is needed to avoid trashing too
> > > much heap state and causing a crash, this could go a long way.  I'd
> > > still argue in favor of removing the ability to know how many objects
> > > are used in a given slab, since randomizing objects doesn't help if you
> > > know every object is allocated.
> > 
> > So if the attacker knows every object is allocated, how does that help
> > if we're randomizing the initial freelist?
> 
> If you know you've got a slab completely full of your objects, then it
> doesn't matter that they happened to be allocated in a random fashion -
> they're still all allocated, and by freeing one of them and
> reallocating, you'll still be next to your target.
> 

But still, if randomizing allocations makes life just a little harder for 
attackers in some scenarios, why not just do it?
Same with making /proc/slabinfo 0400, if it just makes things a little 
harder in a few cases, why not do it? It's not like a admin who needs 
/proc/slabinfo to have other permissions can't arrange for that.

Having been employed as a systems administrator for many years and having 
seen many a box cracked, my oppinion is that every little bit helps. The 
kernel is currently not a hard target and everything we can do to harden 
it is a good thing (within reason of course).

Why not just do both randomization and 0400 as a start? We can always 
harden further later.

-- 
Jesper Juhl <jj@...osbits.net>            http://www.chaosbits.net/
Plain text mails only, please.
Don't top-post http://www.catb.org/~esr/jargon/html/T/top-post.html

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