Re: [Xen-devel] fxsave, fnsave, ltr hang for guest OS.

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

At 18:01 +0000 on 19 Nov (1290189698), Keir Fraser wrote:
> On 19/11/2010 17:32, "alarson@xxxxxxxx" <alarson@xxxxxxxx> wrote:
> 
> > Just to report final resolution.  The problem turned out to be that
> > the destination of the fnsave was to a page specifying read only
> > access and the OS was depending on CR0.WP=0 to permit write access in
> > kernel mode.  Xen doesn't allow this configuration unless using HAP
> > (Hardware Assisted Paging).
> 
> To be precise, we have a go at emulating the instruction, but our emulator
> is incomplete. Running a whole OS in CR0.WP=0 mode is pretty much doomed if
> running on Xen without HAP. I think there is some supported OS that does do
> some small amount of stuff with CR0.WP=0, and we are complete enough to work
> in that case.

To be precise, the shadow code/emulator can run various flavours of
Windows when anti-virus software briefly turns off CR0.WP in order to
overwrite protected kernel structures (IDT &c).  That only involves
straightforward memory writes in an OS that otherwise obeys its own 
pagetable restrictions.

Cheers,

Tim.

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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