linux-kernel - Re: [PATCH v2] mm: Allow slab_nomerge to be set at build time

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Date:   Fri, 23 Jun 2017 12:20:25 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Christoph Lameter <cl@...ux.com>, Jonathan Corbet <corbet@....net>,
        Daniel Micay <danielmicay@...il.com>,
        David Windsor <dave@...lcore.net>,
        Eric Biggers <ebiggers3@...il.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Arnd Bergmann <arnd@...db.de>,
        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-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] mm: Allow slab_nomerge to be set at build time

On Fri, Jun 23, 2017 at 7:06 AM, Michal Hocko <mhocko@...nel.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