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Re: Re: [Xen-devel] can dom0 modify Shadow PT of HVM domU?

At 10:30 +0100 on 26 Aug (1251282600), Wu Bingzheng wrote:
> I add a monitor in hypervisor to monitor some behaviors (like system call) of 
> HVM domU.
> If that behavior happens, the monitor 1) pauses this domU [1], 2) notify dom0.
> When dom0 gets the notification, it makes a decision, like killing the 
> process (who calls system-call) in domU, or not.
> Then hypervisor gets the killing request. To do the killing, I implement like 
> this:
>  1) set the domU's page, which contains the EIP before paused,  NX 
> (none-execute) in shadow page table;
>  2) then unpause domU
>  3) then domU will invoke a #PF. Hypervisor can catch this #PF, and then kill 
> the process in domU [2].

I see.  That seems like a pretty convoluted way of doing it.  Why don't
you just inject #GP (or whatever) straight into the guest from your dom0

Marking the guest page NX in the shadows isn't guaranteed to work anyway
since the shadow pagetables could get discarded and rebuilt before the
guest resumes running.



> So, I have to modify the domU's shadow PT in dom0's context, to make the domU 
> trap into hypervisor.
> Am I right? or is there a better way to make a paused domU trap into 
> hypervisor?
> [1] To pause domU, I call domain_pause_for_debugger();
> [2] To kill a process in domU, I call vmx_inject_hw_exception(v, 
> TRAP_gp_fault, 0);
> Thanks,
> Wu
> email:wubingzheng@xxxxxxx??2009-08-26??"Tim Deegan" <Tim.Deegan@xxxxxxxxxx> 
> ??????
> >Hi,
> >
> >At 09:35 +0100 on 26 Aug (1251279335), Wu Bingzheng wrote:
> >> Can Xen hypervisor modify HVM domU's Shadow page table, under the
> >> dom0's context, like trapped from dom0's hypercall?
> >
> >Yes, and it sometimes does (e.g. dom0 hypercalls that change domU's p2m
> >tables cause changes indirectly in the shadows).
> >
> >> I think it have to call 2 functions at least: guest_walk_tables() and
> >> flush_tlb_all(). Can these 2 functions called in dom0's context?
> >
> >Yes, but they're not nearly enough to safely modify the shadow
> >pagetables.  There's a lot of reference-counting and concurrency code in
> >there.  The paging_* function calls are really the only sensible way to
> >interact with the shadow pagetables code.
> >
> >> In my test, if hypervisor tries to modify HVM's shadow page table, it
> >> will bring down the whole system. I am not sure what's the reason.
> >
> >Why do you want to modify the shadow pagetables from dom0?  They're
> >probably the wrong place to be trying to do things since (a) they don't
> >exist on EPT/NPT-capable hardware, and (b) they can get discarded and
> >rebuilt by Xen at any time.
> >
> >Cheers,
> >
> >Tim.
> >
> >--
> >Tim Deegan <Tim.Deegan@xxxxxxxxxx>
> >Principal Software Engineer, Citrix Systems (R&D) Ltd.
> >[Company #02300071, SL9 0DZ, UK.]
> ________________________________
> ??????????????????????,www.yeah.net<http://www.yeah.net/?from=footer>

Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]

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