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Date:	Mon, 22 Dec 2014 12:13:22 -0800
From:	Andy Lutomirski <>
To:	Jiri Kosina <>
Cc:	Hector Marco Gisbert <>,
	Cyrill Gorcunov <>,
	Pavel Emelyanov <>,
	Catalin Marinas <>,
	Heiko Carstens <>,
	Oleg Nesterov <>,
	Ingo Molnar <>,
	Anton Blanchard <>,
	Russell King - ARM Linux <>,
	"H. Peter Anvin" <>,
	David Daney <>,
	Andrew Morton <>,
	Arun Chandran <>,
	"" <>,
	Martin Schwidefsky <>,
	Ismael Ripoll <>,
	Christian Borntraeger <>,
	Thomas Gleixner <>,
	Hanno Böck <>,
	Will Deacon <>,
	Benjamin Herrenschmidt <>,
	Kees Cook <>,
	Reno Robert <>
Subject: Re: [PATCH] ASLRv3: randomize_va_space=3 preventing offset2lib attack

On Mon, Dec 22, 2014 at 12:03 PM, Jiri Kosina <> 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.

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