Re: [PATCH 0/3] mm: Randomize free memory

[Kees Cook]

On Mon, Sep 17, 2018 at 4:12 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Sat, 15 Sep 2018 09:23:02 -0700 Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
>> Data exfiltration attacks via speculative execution and
>> return-oriented-programming attacks rely on the ability to infer the
>> location of sensitive data objects. The kernel page allocator, has
>> predictable first-in-first-out behavior for physical pages. Pages are
>> freed in physical address order when first onlined. There are also
>> mechanisms like CMA that can free large contiguous areas at once
>> increasing the predictability of allocations in physical memory.
>>
>> In addition to the security implications this randomization also
>> stabilizes the average performance of direct-mapped memory-side caches.
>> This includes memory-side caches like the one on the Knights Landing
>> processor and those generally described by the ACPI HMAT (Heterogeneous
>> Memory Attributes Table [1]). Cache conflicts are spread over a random
>> distribution rather than localized.
>>
>> Given the performance sensitivity of the page allocator this
>> randomization is only performed for MAX_ORDER (4MB by default) pages. A
>> kernel parameter, page_alloc.shuffle_page_order, is included to change
>> the page size where randomization occurs.
>>
>> [1]: See ACPI 6.2 Section 5.2.27.5 Memory Side Cache Information Structure
>
> I'm struggling to understand the justification of all of this.  Are
> such attacks known to exist?  Or reasonably expected to exist in the
> future?  What is the likelihood and what is their cost?  Or is this all
> academic and speculative and possibly pointless?

While we already have a base-address randomization
(CONFIG_RANDOMIZE_MEMORY), attacks against the same hardware and
memory layouts would certainly be using the predictability of
allocation ordering (i.e. for attacks where the base address isn't
important: only the relative positions between allocated memory). This
is common in lots of heap-style attacks. They try to gain control over
ordering by spraying allocations, etc.

I'd really like to see this because it gives us something similar to
CONFIG_SLAB_FREELIST_RANDOM but for the page allocator. (This may be
worth mentioning in the series, especially as a comparison to its
behavior and this.)

> ie, something must have motivated you to do this work rather than
> <something-else>.  Please spell out that motivation.

I'd be curious to hear more about the mentioned cache performance
improvements. I love it when a security feature actually _improves_
performance. :)

Thanks for working on this!

-Kees

-- 
Kees Cook
Pixel Security