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Date:	Mon, 22 Dec 2014 12:13:22 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Hector Marco Gisbert <hecmargi@....es>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Anton Blanchard <anton@...ba.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"H. Peter Anvin" <hpa@...or.com>,
	David Daney <ddaney.cavm@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arun Chandran <achandran@...sta.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ismael Ripoll <iripoll@...ca.upv.es>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Hanno Böck <hanno@...eck.de>,
	Will Deacon <will.deacon@....com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Kees Cook <keescook@...omium.org>,
	Reno Robert <renorobert@...il.com>
Subject: Re: [PATCH] ASLRv3: randomize_va_space=3 preventing offset2lib attack

On Mon, Dec 22, 2014 at 12:03 PM, Jiri Kosina <jkosina@...e.cz> wrote:
> On Mon, 22 Dec 2014, Andy Lutomirski wrote:
>
>> It could be worth having a mode that goes all out: randomize every
>> single allocation independently in, say, a 45 or 46-byte range.  That
>> would be about as strong ASLR as we could possibly have, it would
>> result in guard intervals around mmap data allocations (which has real
>> value), and it would still leave plenty of space for big address space
>> hogs like the Chromium sandbox.
>>
>> The main downside would be lots of memory used for page tables.
>
> Plus get_random_int() during every mmap() call.

If that's actually a problem, then I think we should fix
get_random_int.  Chacha20 can generate 64 bits in a few cycles.

> Plus the resulting VA
> space fragmentation.

I think the main cost of fragmentation would be the page tables and
vmas.  2^45 bytes is a lot of bytes.

We could tone it down a bit if we dedicated a range to mmapped data
and tried to pack it reasonably densely.  We could even do a fair
amount of merging for data-heavy applications if we gave MAP_PRIVATE |
MAP_ANONYMOUS, PROT_READ | PROT_WRITE mappings a decent chance of
ending up next to each other.

Anyway, this would be a knob.  The database people would presumably turn it off.

--Andy
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