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

Ingo Molnar mingo at kernel.org
Wed Apr 17 17:09:18 UTC 2019

* 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.

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 

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?



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