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Date: Fri, 17 May 2019 16:11:32 +0200
From: Alexander Potapenko <glider@...gle.com>
To: Michal Hocko <mhocko@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Christoph Lameter <cl@...ux.com>, 
	Kees Cook <keescook@...omium.org>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>, 
	Masahiro Yamada <yamada.masahiro@...ionext.com>, James Morris <jmorris@...ei.org>, 
	"Serge E. Hallyn" <serge@...lyn.com>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Kostya Serebryany <kcc@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>, Sandeep Patil <sspatil@...roid.com>, 
	Laura Abbott <labbott@...hat.com>, Randy Dunlap <rdunlap@...radead.org>, Jann Horn <jannh@...gle.com>, 
	Mark Rutland <mark.rutland@....com>, Linux Memory Management List <linux-mm@...ck.org>, 
	linux-security-module <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH v2 1/4] mm: security: introduce init_on_alloc=1 and
 init_on_free=1 boot options

On Fri, May 17, 2019 at 4:04 PM Michal Hocko <mhocko@...nel.org> wrote:
>
> On Tue 14-05-19 16:35:34, Alexander Potapenko wrote:
> > The new options are needed to prevent possible information leaks and
> > make control-flow bugs that depend on uninitialized values more
> > deterministic.
> >
> > init_on_alloc=1 makes the kernel initialize newly allocated pages and heap
> > objects with zeroes. Initialization is done at allocation time at the
> > places where checks for __GFP_ZERO are performed.
> >
> > init_on_free=1 makes the kernel initialize freed pages and heap objects
> > with zeroes upon their deletion. This helps to ensure sensitive data
> > doesn't leak via use-after-free accesses.
>
> Why do we need both? The later is more robust because even free memory
> cannot be sniffed and the overhead might be shifted from the allocation
> context (e.g. to RCU) but why cannot we stick to a single model?
init_on_free appears to be slower because of cache effects. It's
several % in the best case vs. <1% for init_on_alloc.

> --
> Michal Hocko
> SUSE Labs



-- 
Alexander Potapenko
Software Engineer

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