Re: [kernel-hardening] [RFC][PATCH 3/3] mm/page_poisoning.c: Allow for zero poisoning

[Kees Cook]

On Mon, Jan 25, 2016 at 12:16 PM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> Thanks for doing this!  It all looks pretty straightforward.
>
> On 01/25/2016 08:55 AM, Laura Abbott wrote:
>> By default, page poisoning uses a poison value (0xaa) on free. If this
>> is changed to 0, the page is not only sanitized but zeroing on alloc
>> with __GFP_ZERO can be skipped as well. The tradeoff is that detecting
>> corruption from the poisoning is harder to detect. This feature also
>> cannot be used with hibernation since pages are not guaranteed to be
>> zeroed after hibernation.
>
> Ugh, that's a good point about hibernation.  I'm not sure how widely it
> gets used but it does look pretty widely enabled in distribution kernels.
>
> Is this something that's fixable?  It seems like we could have the
> hibernation code run through and zero all the free lists.  Or, we could
> just disable the optimization at runtime when a hibernation is done.

We can also make hibernation run-time disabled when poisoning is used
(similar to how kASLR disables it).

> Not that we _have_ to do any of this now, but if a runtime knob (like a
> sysctl) could be fun too.  I would be nice for folks to turn it on and
> off if they wanted the added security of "real" poisoning vs. the
> potential performance boost from this optimization.
>
>> +static inline bool should_zero(void)
>> +{
>> +     return !IS_ENABLED(CONFIG_PAGE_POISONING_ZERO) ||
>> +             !page_poisoning_enabled();
>> +}
>
> I wonder if calling this "free_pages_prezeroed()" would make things a
> bit more clear when we use it in prep_new_page().
>
>>  static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
>>                                                               int alloc_flags)
>>  {
>> @@ -1401,7 +1407,7 @@ static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
>>       kernel_map_pages(page, 1 << order, 1);
>>       kasan_alloc_pages(page, order);
>>
>> -     if (gfp_flags & __GFP_ZERO)
>> +     if (should_zero() && gfp_flags & __GFP_ZERO)
>>               for (i = 0; i < (1 << order); i++)
>>                       clear_highpage(page + i);
>
> It's probably also worth pointing out that this can be a really nice
> feature to have in virtual machines where memory is being deduplicated.
>  As it stands now, the free lists end up with gunk in them and tend not
> to be easy to deduplicate.  This patch would fix that.

Oh, good point!

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security