Re: [PATCH 2/2] mm/page_alloc.c: add config option to sanitize freed pages

["PaX Team" <pageexec@freemail.hu>]

On 27 Apr 2015 at 10:11, Anisse Astier wrote:

> >> +#ifdef CONFIG_SANITIZE_FREED_PAGES
> >> +     zero_pages(page, order);
> >> +#endif
> >
> > And not removing the clear on __GFP_ZERO by remembering that?
> >
> > That means all clears would be done twice.
> >
> > That patch is far too simple. Clearing is commonly the most
> > expensive kernel operation.
> >
> 
> I thought about this, but if you unconditionally remove the clear on
> __GFP_ZERO, you wouldn't be guaranteed to have a zeroed page when
> memory is first used (you would protect the kernel from its own info
> leaks though);

the PaX SANITIZE feature does exactly this in mm/page_alloc.c:prep_new_page:

#ifndef CONFIG_PAX_MEMORY_SANITIZE
	if (gfp_flags & __GFP_ZERO)
		prep_zero_page(page, order, gfp_flags);
#endif

> you'd need to clear memory on boot for example.

it happens automagically because on boot during the transition from the
boot allocator to the buddy one each page gets freed which will then go
through the page clearing path.

however there's a known problem/conflict with HIBERNATION (see
http://marc.info/?l=linux-pm&m=132871433416256&w=2) which i think would
have to be resolved before upstream acceptance.

cheers,
 PaX Team

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