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Date: Fri, 17 May 2019 16:11:32 +0200
From: Alexander Potapenko <>
To: Michal Hocko <>
Cc: Andrew Morton <>, Christoph Lameter <>, 
	Kees Cook <>, 
	Kernel Hardening <>, 
	Masahiro Yamada <>, James Morris <>, 
	"Serge E. Hallyn" <>, Nick Desaulniers <>, 
	Kostya Serebryany <>, Dmitry Vyukov <>, Sandeep Patil <>, 
	Laura Abbott <>, Randy Dunlap <>, Jann Horn <>, 
	Mark Rutland <>, Linux Memory Management List <>, 
	linux-security-module <>
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 <> 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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