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Re: [Xen-devel] [PATCH 00/11] Alternate p2m: support multiple copies of host p2m

On Wed, Jan 14, 2015 at 8:04 AM, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>> Ed White <edmund.h.white@xxxxxxxxx> 01/13/15 10:32 PM >>>
>>On 01/13/2015 12:45 PM, Andrew Cooper wrote:
>>> On 13/01/15 20:02, Ed White wrote:
>>>> The set of mfn's is the same, but I do allow gfn->mfn mappings to be
>>>> modified under certain circumstances. One use of this is to point the
>>>> same VA to different physical pages (with different access permissions)
>>>> in different p2m's to hide memory changes.
>>> What is the practical use of being able to play paging tricks like this
>>> behind a VMs back?
>>I'm restricted in how much detail I can go into on a public mailing list,
>>but imagine that you want a data read to see one thing and an instruction
>>fetch to see something else.
> How would that work? There can only be one P2M in use at a time, and that's
> used for both translations. Or are you saying at least one of the two accesses
> would be emulated nevertheless?
> Jan

I can see it working by having data fetch access to a page trapped via
mem_access, while instruction fetch is not. This would be very handy
when doing stealthy debugging where the presence of breakpoints should
be hidden from the guest. With this technique it is possible to
present a copy of the page to the data fetch that has no breakpoints
in it, as done for example in this paper:


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