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Message-ID: <alpine.DEB.2.20.1707070844100.11769@east.gentwo.org>
Date: Fri, 7 Jul 2017 08:50:33 -0500 (CDT)
From: Christoph Lameter <cl@...ux.com>
To: Kees Cook <keescook@...omium.org>
cc: Rik van Riel <riel@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>,
Pekka Enberg <penberg@...nel.org>, David Rientjes <rientjes@...gle.com>,
Joonsoo Kim <iamjoonsoo.kim@....com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Ingo Molnar <mingo@...nel.org>, Josh Triplett <josh@...htriplett.org>,
Andy Lutomirski <luto@...nel.org>,
Nicolas Pitre <nicolas.pitre@...aro.org>, Tejun Heo <tj@...nel.org>,
Daniel Mack <daniel@...que.org>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Helge Deller <deller@....de>, Linux-MM <linux-mm@...ck.org>,
Tycho Andersen <tycho@...ker.com>, LKML <linux-kernel@...r.kernel.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v3] mm: Add SLUB free list pointer obfuscation
On Thu, 6 Jul 2017, Kees Cook wrote:
> Right. This is about blocking the escalation of attack capability. For
> slab object overflow flaws, there are mainly two exploitation methods:
> adjacent allocated object overwrite and adjacent freed object
> overwrite (i.e. a freelist pointer overwrite). The first attack
> depends heavily on which slab cache (and therefore which structures)
> has been exposed by the bug. It's a very narrow and specific attack
> method. The freelist attack is entirely general purpose since it
> provides a reliable way to gain arbitrary write capabilities.
> Protecting against that attack greatly narrows the options for an
> attacker which makes attacks more expensive to create and possibly
> less reliable (and reliability is crucial to successful attacks).
The simplest thing here is to vary the location of the freelist pointer.
That way you cannot hit the freepointer in a deterministic way
The freepointer is put at offset 0 right now. But you could put it
anywhere in the object.
Index: linux/mm/slub.c
===================================================================
--- linux.orig/mm/slub.c
+++ linux/mm/slub.c
@@ -3467,7 +3467,8 @@ static int calculate_sizes(struct kmem_c
*/
s->offset = size;
size += sizeof(void *);
- }
+ } else
+ s->offset = s->size / sizeof(void *) * <insert random chance logic here>
#ifdef CONFIG_SLUB_DEBUG
if (flags & SLAB_STORE_USER)
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