Re: [RFC PATCH v9 03/13] mm: Add support for eXclusive Page Frame Ownership (XPFO)

[Khalid Aziz]

On 4/17/19 1:49 PM, Andy Lutomirski wrote:
> On Wed, Apr 17, 2019 at 10:33 AM Khalid Aziz <khalid.aziz@xxxxxxxxxx> wrote:
>>
>> On 4/17/19 11:09 AM, Ingo Molnar wrote:
>>>
>>> * Khalid Aziz <khalid.aziz@xxxxxxxxxx> wrote:
>>>
>>>>> I.e. the original motivation of the XPFO patches was to prevent execution
>>>>> of direct kernel mappings. Is this motivation still present if those
>>>>> mappings are non-executable?
>>>>>
>>>>> (Sorry if this has been asked and answered in previous discussions.)
>>>>
>>>> Hi Ingo,
>>>>
>>>> That is a good question. Because of the cost of XPFO, we have to be very
>>>> sure we need this protection. The paper from Vasileios, Michalis and
>>>> Angelos - <http://www.cs.columbia.edu/~vpk/papers/ret2dir.sec14.pdf>,
>>>> does go into how ret2dir attacks can bypass SMAP/SMEP in sections 6.1
>>>> and 6.2.
>>>
>>> So it would be nice if you could generally summarize external arguments
>>> when defending a patchset, instead of me having to dig through a PDF
>>> which not only causes me to spend time that you probably already spent
>>> reading that PDF, but I might also interpret it incorrectly. ;-)
>>
>> Sorry, you are right. Even though that paper explains it well, a summary
>> is always useful.
>>
>>>
>>> The PDF you cited says this:
>>>
>>>   "Unfortunately, as shown in Table 1, the W^X prop-erty is not enforced
>>>    in many platforms, including x86-64.  In our example, the content of
>>>    user address 0xBEEF000 is also accessible through kernel address
>>>    0xFFFF87FF9F080000 as plain, executable code."
>>>
>>> Is this actually true of modern x86-64 kernels? We've locked down W^X
>>> protections in general.
>>>
>>> I.e. this conclusion:
>>>
>>>   "Therefore, by simply overwriting kfptr with 0xFFFF87FF9F080000 and
>>>    triggering the kernel to dereference it, an attacker can directly
>>>    execute shell code with kernel privileges."
>>>
>>> ... appears to be predicated on imperfect W^X protections on the x86-64
>>> kernel.
>>>
>>> Do such holes exist on the latest x86-64 kernel? If yes, is there a
>>> reason to believe that these W^X holes cannot be fixed, or that any fix
>>> would be more expensive than XPFO?
>>
>> Even if physmap is not executable, return-oriented programming (ROP) can
>> still be used to launch an attack. Instead of placing executable code at
>> user address 0xBEEF000, attacker can place an ROP payload there. kfptr
>> is then overwritten to point to a stack-pivoting gadget. Using the
>> physmap address aliasing, the ROP payload becomes kernel-mode stack. The
>> execution can then be hijacked upon execution of ret instruction. This
>> is a gist of the subsection titled "Non-executable physmap" under
>> section 6.2 and it looked convincing enough to me. If you have a
>> different take on this, I am very interested in your point of view.
> 
> My issue with all this is that XPFO is really very expensive.  I think
> that, if we're going to seriously consider upstreaming expensive
> exploit mitigations like this, we should consider others first, in
> particular CFI techniques.  grsecurity's RAP would be a great start.
> I also proposed using a gcc plugin (or upstream gcc feature) to add
> some instrumentation to any code that pops RSP to verify that the
> resulting (unsigned) change in RSP is between 0 and THREAD_SIZE bytes.
> This will make ROP quite a bit harder.
> 

Yes, XPFO is expensive. I have been able to reduce the overhead of XPFO
from 2537% to 28% (on large servers) but 28% is still quite significant.
Alternative mitigation techniques with lower impact would easily be more
acceptable as long as they provide same level of protection. If we have
to go with XPFO, we will continue to look for more performance
improvement to bring that number down further from 28%. Hopefully what
Tycho is working on will yield better results. I am continuing to look
for improvements to XPFO in parallel.

Thanks,
Khalid