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SubjectRe: [RFC PATCH v9 03/13] mm: Add support for eXclusive Page Frame Ownership (XPFO)
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On 4/17/19 10:15 AM, Ingo Molnar wrote:
>
> [ Sorry, had to trim the Cc: list from hell. Tried to keep all the
> mailing lists and all x86 developers. ]
>
> * Khalid Aziz <khalid.aziz@oracle.com> wrote:
>
>> From: Juerg Haefliger <juerg.haefliger@canonical.com>
>>
>> This patch adds basic support infrastructure for XPFO which protects
>> against 'ret2dir' kernel attacks. The basic idea is to enforce
>> exclusive ownership of page frames by either the kernel or userspace,
>> unless explicitly requested by the kernel. Whenever a page destined for
>> userspace is allocated, it is unmapped from physmap (the kernel's page
>> table). When such a page is reclaimed from userspace, it is mapped back
>> to physmap. Individual architectures can enable full XPFO support using
>> this infrastructure by supplying architecture specific pieces.
>
> I have a higher level, meta question:
>
> Is there any updated analysis outlining why this XPFO overhead would be
> required on x86-64 kernels running on SMAP/SMEP CPUs which should be all
> recent Intel and AMD CPUs, and with kernel that mark all direct kernel
> mappings as non-executable - which should be all reasonably modern
> kernels later than v4.0 or so?
>
> 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.

Thanks,
Khalid


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