linux-kernel - Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND

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Date:	Tue, 3 Nov 2015 15:18:47 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Daniel Cashman <dcashman@...roid.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Jonathan Corbet <corbet@....net>,
	Don Zickus <dzickus@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Heinrich Schuchardt <xypron.glpk@....de>, jpoimboe@...hat.com,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	n-horiguchi@...jp.nec.com, Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mgorman@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Rientjes <rientjes@...gle.com>,
	Linux-MM <linux-mm@...ck.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Mark Salyzyn <salyzyn@...roid.com>,
	Jeffrey Vander Stoep <jeffv@...gle.com>,
	Nick Kralevich <nnk@...gle.com>, dcashman <dcashman@...gle.com>
Subject: Re: [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS.

On Tue, Nov 3, 2015 at 2:39 PM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Tue, Nov 03, 2015 at 11:19:44AM -0800, Kees Cook wrote:
>> On Tue, Nov 3, 2015 at 10:10 AM, Daniel Cashman <dcashman@...roid.com> wrote:
>> > From: dcashman <dcashman@...gle.com>
>> >
>> > arm: arch_mmap_rnd() uses a hard-code value of 8 to generate the
>> > random offset for the mmap base address.  This value represents a
>> > compromise between increased ASLR effectiveness and avoiding
>> > address-space fragmentation. Replace it with a Kconfig option, which
>> > is sensibly bounded, so that platform developers may choose where to
>> > place this compromise. Keep 8 as the minimum acceptable value.
>> >
>> > Signed-off-by: Daniel Cashman <dcashman@...gle.com>
>>
>> Acked-by: Kees Cook <keescook@...omium.org>
>>
>> Russell, if you don't see any problems here, it might make sense not
>> to put this through the ARM patch tracker since it depends on the 1/2,
>> and I think x86 and arm64 (and possibly other arch) changes are coming
>> too.
>
> Yes, it looks sane, though I do wonder whether there should also be
> a Kconfig option to allow archtectures to specify the default, instead
> of the default always being the minimum randomisation.  I can see scope
> to safely pushing our mmap randomness default to 12, especially on 3GB
> setups, as we already have 11 bits of randomness on the sigpage and if
> enabled, 13 bits on the heap.

My thinking is that the there shouldn't be a reason to ever have a
minimum that was below the default. I have no objection with it, but
it seems needless. Frankly minimum is "0", really, so I don't think it
makes much sense to have default != arch minimum. I actually view
"arch minimum" as "known good", so if we are happy with raising the
"known good" value, that should be the new minimum.

-Kees

-- 
Kees Cook
Chrome OS Security
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