On Wed, Jun 26, 2019 at 5:42 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>
> On Wed 26-06-19 17:00:43, Alexander Potapenko wrote:
> > On Wed, Jun 26, 2019 at 4:49 PM Michal Hocko <mhocko@xxxxxxxxxx> 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
Google Germany GmbH
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