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

* Khalid Aziz <> 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 - <>,
> 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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