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From: Kees Cook <keescook@chromium.org>
Date: Fri, 23 Jun 2017 12:20:25 -0700
Message-ID: <CAGXu5jJ8SD8hsMDfZ9qJHQbJ3iSTXTq81PpiG+kbnXwx=akDKg@mail.gmail.com>
Subject: Re: [PATCH v2] mm: Allow slab_nomerge to be set at build time
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To: Michal Hocko <mhocko@kernel.org>
Cc: Christoph Lameter <cl@linux.com>, Jonathan Corbet <corbet@lwn.net>, Daniel Micay <danielmicay@gmail.com>, David Windsor <dave@nullcore.net>, Eric Biggers <ebiggers3@gmail.com>, Pekka Enberg <penberg@kernel.org>, David Rientjes <rientjes@google.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Andrew Morton <akpm@linux-foundation.org>, "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>, Mauro Carvalho Chehab <mchehab@kernel.org>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, Arnd Bergmann <arnd@arndb.de>, Andy Lutomirski <luto@kernel.org>, Nicolas Pitre <nicolas.pitre@linaro.org>, Tejun Heo <tj@kernel.org>, Daniel Mack <daniel@zonque.org>, Sebastian Andrzej Siewior <bigeasy@linutronix.de>, Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, Helge Deller <deller@gmx.de>, Rik van Riel <riel@redhat.com>, "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>

On Fri, Jun 23, 2017 at 7:06 AM, Michal Hocko <mhocko@kernel.org> wrote:
> On Tue 20-06-17 16:09:11, Kees Cook wrote:
>> Some hardened environments want to build kernels with slab_nomerge
>> already set (so that they do not depend on remembering to set the kernel
>> command line option). This is desired to reduce the risk of kernel heap
>> overflows being able to overwrite objects from merged caches and changes
>> the requirements for cache layout control, increasing the difficulty of
>> these attacks. By keeping caches unmerged, these kinds of exploits can
>> usually only damage objects in the same cache (though the risk to metadata
>> exploitation is unchanged).
>
> Do we really want to have a dedicated config for each hardening specific
> kernel command line? I believe we have quite a lot of config options
> already. Can we rather have a CONFIG_HARDENED_CMD_OPIONS and cover all
> those defauls there instead?

There's not been a lot of success with grouped Kconfigs in the past
(e.g. CONFIG_EXPERIMENTAL), but one thing that has been suggested is a
defconfig-like make target that would collect all the things together.
I haven't had time for that, but that would let us group the various
configs.

Additionally, using something like CONFIG_CMDLINE seems a little clunky to me.

-Kees

-- 
Kees Cook
Pixel Security

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