Re: How to intercept memory operation in Xen

[Anthony Liguori <aliguori@us.ibm.com>]

Abhinav Srivastava wrote:
> 
> Hi all,
> 
> I am new to the Xen and currently using Xen 3.0.3. My goal is to provide 
> memory logging facility in Dom0. If any guest domain (DomU) application 
> tries to access any memory area, I would like to know in dom0 what area 
> they are accessing. So that if any user space applications try to access 
> kernel memory then i can see in dom0 and using my detection system, i 
> can say that it is illegal memory request as user space applications are 
> not allowed to use kernel memory. I have three Qs related to this:

You would have a much easier time just modifying the Linux kernel.  Xen 
isn't going to help you much here.

> 1) Is this doable?

Exceptions do go through the hypervisor however page faults are not 
usually a sign of something bad happening.  Linux, for instance, uses 
copy-on-write and on-demand paging which means in many circumstances, 
page faults are not a sign of misuse of memory.

I don't think there's any generic way of identifying whether a page 
fault is a "good" page fault or a "bad" page fault at the hypervisor 
level.  The hypervisor merely forwards the fault to the guest and the 
guest then decides what action to take (update a page table, kill a 
process, etc.).  To complicate matters further, some apps catch SEGV and 
handle it themselves.  That makes the potential recovery behavior 
totally non-deterministic.  You could potentially try to track 
heuristically whether a PTE update occurs after a page fault at the 
hypervisor level but that would be easily defeatable (which means it 
isn't useful for an IDS system).  At the end of the day, you really have 
to modify the domU kernel.

You should look at some of the bug reporting tools in Linux.  They seem 
to be doing something to hook all process crashes.  Ubuntu has a new bug 
crash tool that you could probably start with.  This would put you in 
domU userspace though...

Regards,

Anthony Liguori

> 2) If yes, how should I start with? Do i need to intercept hypercalls? 
> If yes, how to do this?
> 
> 3) To intercept memory operation, do i need to change in the Xen code? 
> If yes, it would be great if you could point me exact file where changes 
> are to be made.
> 
> 4) Can it be done using some application or IDS in dom0 with some hooks 
> without changing the Xen code?
> 
> I would really appreciate your help.
> 
> Thanks,
> Abhi
> 
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