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Date: Thu, 18 Apr 2019 18:50:18 +0200
From: Alexander Potapenko <glider@...gle.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Christoph Lameter <cl@...ux.com>, 
	Dmitriy Vyukov <dvyukov@...gle.com>, Kees Cook <keescook@...omium.org>, 
	Laura Abbott <labbott@...hat.com>, Linux Memory Management List <linux-mm@...ck.org>, 
	linux-security-module <linux-security-module@...r.kernel.org>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 1/3] mm: security: introduce the init_allocations=1 boot option

On Thu, Apr 18, 2019 at 6:43 PM Alexander Potapenko <glider@...gle.com> wrote:
>
> On Thu, Apr 18, 2019 at 6:35 PM Dave Hansen <dave.hansen@...el.com> wrote:
> >
> > On 4/18/19 8:42 AM, Alexander Potapenko wrote:
> > > This option adds the possibility to initialize newly allocated pages and
> > > heap objects with zeroes. This is needed to prevent possible information
> > > leaks and make the control-flow bugs that depend on uninitialized values
> > > more deterministic.
> >
> > Isn't it better to do this at free time rather than allocation time?  If
> > doing it at free, you can't even have information leaks for pages that
> > are in the allocator.
> I should have mentioned this in the patch description, as this
> question is being asked every time I send a patch :)
> If we want to avoid double initialization and take advantage of
> __GFP_NOINIT (see the second and third patches in the series) we need
> to do initialize the memory at allocation time, because free() and
> free_pages() don't accept GFP flags.

On a second thought, double zeroing on memory reclaim should be quite rare.
Most of the speedup we gain with __GFP_NOINIT is because we assume
it's safe to not initialize memory that'll be overwritten anyway.
I'll need to check how e.g. hackbench behaves if we choose to zero
memory on free() (my guess would be it'll be slower than with
__GFP_NOINIT hack, albeit a little safer)
>
>
> --
> Alexander Potapenko
> Software Engineer
>
> Google Germany GmbH
> Erika-Mann-Straße, 33
> 80636 München
>
> Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
> Registergericht und -nummer: Hamburg, HRB 86891
> Sitz der Gesellschaft: Hamburg



-- 
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg

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