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SubjectRe: [RFC PATCH v9 03/13] mm: Add support for eXclusive Page Frame Ownership (XPFO)
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> On Apr 17, 2019, at 10:09 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Khalid Aziz <khalid.aziz@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.”

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