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Date: Tue, 20 Jun 2017 11:08:33 -0700
From: Kees Cook <keescook@...omium.org>
To: Laura Abbott <labbott@...hat.com>
Cc: Christoph Lameter <cl@...ux.com>, Daniel Micay <danielmicay@...il.com>,
Pekka Enberg <penberg@...nel.org>, David Rientjes <rientjes@...gle.com>,
Joonsoo Kim <iamjoonsoo.kim@....com>, Andrew Morton <akpm@...ux-foundation.org>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Ingo Molnar <mingo@...nel.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>,
Rik van Riel <riel@...hat.com>, Linux-MM <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH] mm: Add SLUB free list pointer obfuscation
On Tue, Jun 20, 2017 at 11:05 AM, Laura Abbott <labbott@...hat.com> wrote:
> On 06/19/2017 08:01 PM, Kees Cook wrote:
>> This SLUB free list pointer obfuscation code is modified from Brad
>> Spengler/PaX Team's code in the last public patch of grsecurity/PaX based
>> on my understanding of the code. Changes or omissions from the original
>> code are mine and don't reflect the original grsecurity/PaX code.
>>
>> This adds a per-cache random value to SLUB caches that is XORed with
>> their freelist pointers. This adds nearly zero overhead and frustrates the
>> very common heap overflow exploitation method of overwriting freelist
>> pointers. A recent example of the attack is written up here:
>> http://cyseclabs.com/blog/cve-2016-6187-heap-off-by-one-exploit
>>
>> This is based on patches by Daniel Micay, and refactored to avoid lots
>> of #ifdef code.
>>
>> Suggested-by: Daniel Micay <danielmicay@...il.com>
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> ---
>> include/linux/slub_def.h | 4 ++++
>> init/Kconfig | 10 ++++++++++
>> mm/slub.c | 32 +++++++++++++++++++++++++++-----
>> 3 files changed, 41 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
>> index 07ef550c6627..0258d6d74e9c 100644
>> --- a/include/linux/slub_def.h
>> +++ b/include/linux/slub_def.h
>> @@ -93,6 +93,10 @@ struct kmem_cache {
>> #endif
>> #endif
>>
>> +#ifdef CONFIG_SLAB_HARDENED
>> + unsigned long random;
>> +#endif
>> +
>> #ifdef CONFIG_NUMA
>> /*
>> * Defragmentation by allocating from a remote node.
>> diff --git a/init/Kconfig b/init/Kconfig
>> index 1d3475fc9496..eb91082546bf 100644
>> --- a/init/Kconfig
>> +++ b/init/Kconfig
>> @@ -1900,6 +1900,16 @@ config SLAB_FREELIST_RANDOM
>> security feature reduces the predictability of the kernel slab
>> allocator against heap overflows.
>>
>> +config SLAB_HARDENED
>> + bool "Harden slab cache infrastructure"
>> + default y
>> + depends on SLAB_FREELIST_RANDOM && SLUB> + help
>> + Many kernel heap attacks try to target slab cache metadata and
>> + other infrastructure. This options makes minor performance
>> + sacrifies to harden the kernel slab allocator against common
>> + exploit methods.
>> +
>
> Going to bikeshed on SLAB_HARDENED unless this is intended to be used for
> more things. Perhaps SLAB_FREELIST_HARDENED?
Daniel's tree has a bunch of changes attached to that config name, but
it's unclear to me how many would be accepted upstream. I would be
fine with SLAB_FREELIST_HARDENED.
> What's the reason for the dependency on SLAB_FREELIST_RANDOM?
Looking at it again, I suspect the idea was to collect other configs
under SLAB_HARDENED. It should likely be either be a select or just
dropped.
-Kees
--
Kees Cook
Pixel Security
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