Re: Writes to guests' page table pages + mem_events

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 04/11/13 17:32, Razvan Cojocaru wrote:
> Hello,
>
> looking at pages 6 and 7 of this document:
>
> http://www-archive.xenproject.org/files/summit_3/XenSummit_Shadow2.pdf
>
> I see that instructions writing to the page table pages of a guest are
> emulated by Xen.
>
> Does this mean that, assuming that I make a page table page read-only, I
> will _not_ receive a mem_event if the guest tries to write to said page?
>
>
> Thanks.

A PV guest never has write access to its pagetables.  A PV guest able to
modify its own pagetables without audit from Xen would be a serious
security vulnerability.

An HVM guest completely controls its own pagetables, and protection is
provided by HAP.  Shadow is a little more awkward where a guest has
pagetables which it believes it owns but doesn't.  A shadow guest will
fault on pagetable access but Xen will fix up.

~Andrew

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