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

Nadav Amit nadav.amit at gmail.com
Wed Apr 17 17:44:56 UTC 2019

> On Apr 17, 2019, at 10:26 AM, Ingo Molnar <mingo at kernel.org> wrote:
> * Nadav Amit <nadav.amit at gmail.com> wrote:
>>> On Apr 17, 2019, at 10:09 AM, Ingo Molnar <mingo at kernel.org> wrote:
>>> * Khalid Aziz <khalid.aziz at oracle.com> 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. ;-)
>>> 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.
>> As I was curious, I looked at the paper. Here is a quote from it:
>> "In x86-64, however, the permissions of physmap are not in sane state.
>> Kernels up to v3.8.13 violate the W^X property by mapping the entire region
>> as “readable, writeable, and executable” (RWX)—only very recent kernels
>> (≥v3.9) use the more conservative RW mapping.”
> But v3.8.13 is a 5+ years old kernel, it doesn't count as a "modern" 
> kernel in any sense of the word. For any proposed patchset with 
> significant complexity and non-trivial costs the benchmark version 
> threshold is the "current upstream kernel".
> So does that quote address my followup questions:
>> 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?
> ?
> What you are proposing here is a XPFO patch-set against recent kernels 
> with significant runtime overhead, so my questions about the W^X holes 
> are warranted.

Just to clarify - I am an innocent bystander and have no part in this work.
I was just looking (again) at the paper, as I was curious due to the recent
patches that I sent that improve W^X protection.

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