Re: [Xen-devel] Writes to guests' page table pages + mem_events

[Andres Lagar-Cavilla <andreslc@xxxxxxxxxxxxxx>]

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.

Aravindh Puthiyaparambil is trying to implement mem_event for PV: http://lists.xen.org/archives/html/xen-devel/2013-10/msg02623.html

A shadow PT based implementation can (hand waving) capture emulated PT writes.

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.

The corollary is that an HVM guest will generate mem_events when writing to its own page tables.

Andres

~Andrew

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