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

Ingo Molnar mingo at kernel.org
Wed Apr 17 17:26:32 UTC 2019

* 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.



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