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

[Laura Abbott <labbott@redhat.com>]

On 01/25/2016 02:05 PM, Kees Cook wrote:
> On Mon, Jan 25, 2016 at 12:16 PM, Dave Hansen <dave.hansen@intel.com> 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).
>

I'll look into the approach kASLR uses to disable hibernation although
having the hibernation code zero the memory could be useful as well.
We can see if there are actual complaints.
  
>> 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().
>>

Yes that sounds much better

>>>   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.

Interesting, do you have any benchmarks I could test?

>
> Oh, good point!
>
> -Kees
>