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Date: Fri, 17 May 2019 16:20:48 +0200
From: Michal Hocko <mhocko@...nel.org>
To: Alexander Potapenko <glider@...gle.com>
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 17-05-19 16:11:32, Alexander Potapenko wrote:
> 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.

This doesn't really explain why we need both.
-- 
Michal Hocko
SUSE Labs

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