Re: [PATCH v8 1/2] mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options

From: Alexander Potapenko <glider@google.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Christoph Lameter <cl@linux.com>,
	Kees Cook <keescook@chromium.org>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	James Morris <jmorris@namei.org>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Kostya Serebryany <kcc@google.com>,
	Dmitry Vyukov <dvyukov@google.com>,
	Sandeep Patil <sspatil@android.com>,
	Laura Abbott <labbott@redhat.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	Jann Horn <jannh@google.com>, Mark Rutland <mark.rutland@arm.com>,
	Marco Elver <elver@google.com>, Qian Cai <cai@lca.pw>,
	Linux Memory Management List <linux-mm@kvack.org>,
	linux-security-module <linux-security-module@vger.kernel.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>
Subject: Re: [PATCH v8 1/2] mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options
Date: Thu, 27 Jun 2019 15:09:06 +0200	[thread overview]
Message-ID: <CAG_fn=V4SZwu50LCZq+2Fa-zAZmQ+X-80vxzN-MGJZdjpFpjhw@mail.gmail.com> (raw)
In-Reply-To: <20190626154237.GZ17798@dhcp22.suse.cz>

On Wed, Jun 26, 2019 at 5:42 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Wed 26-06-19 17:00:43, Alexander Potapenko wrote:
> > On Wed, Jun 26, 2019 at 4:49 PM Michal Hocko <mhocko@kernel.org> wrote:
> [...]
> > > > @@ -1142,6 +1200,8 @@ static __always_inline bool free_pages_prepare(struct page *page,
> > > >       }
> > > >       arch_free_page(page, order);
> > > >       kernel_poison_pages(page, 1 << order, 0);
> > > > +     if (want_init_on_free())
> > > > +             kernel_init_free_pages(page, 1 << order);
> > >
> > > same here. If you don't want to make this exclusive then you have to
> > > zero before poisoning otherwise you are going to blow up on the poison
> > > check, right?
> > Note that we disable initialization if page poisoning is on.
>
> Ohh, right. Missed that in the init code.
>
> > As I mentioned on another thread we can eventually merge this code
> > with page poisoning, but right now it's better to make the user decide
> > which of the features they want instead of letting them guess how the
> > combination of the two is going to work.
>
> Strictly speaking zeroying is a subset of poisoning. If somebody asks
> for both the poisoning surely satisfies any data leak guarantees
> zeroying would give. So I am not sure we have to really make them
> exclusive wrt. to the configuraion. I will leave that to you but it
> would be better if the code didn't break subtly once the early init
> restriction is removed for one way or another. So either always make
> sure that zeroying is done _before_ poisoning or that you do not zero
> when poisoning. The later sounds the best wrt. the code quality from my
> POV.
I somewhat liked the idea of always having zero-initialized page/heap
memory if init_on_{alloc,free} is on.
But in production mode we won't have page or slab poisoning anyway,
and for debugging this doesn't really matter much.
I've sent v9 with poisoning support added.
> --
> Michal Hocko
> SUSE Labs


-- 
Alexander Potapenko
Software Engineer

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