Re: [Xen-devel] questions about the fast trap handler in Xen

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

> I learned from the Xen SOSP paper that Xen installs a fast trap handler to
> allow the system calls issued by guest app direct trap into the guest os
> system trap handler. I guess the procedure should be as follows:
> 
> Guest app issues a system call, because it runs at ring 3, the instruction
> will be trapped. Xem hypervisor, as a VMM, defines an exception handler
> for int 80. In the exception handler, there are an interrupt vector
> defined according to current running guest OS. That is, guest os reports
> its interrupt vector to vmm, and vmm save the vector for fast handling.
> Since VMM knows the interrupt vector of the specific guest os, it can
> directly jump to the right place, which is the entry of the guest os
> system call function, and start to process the system call.

The fast trap vector is installed directly into the IDT when
context-switchign between domains, so syscalls will trap directly to
teh guest OS in ring 1 --- Xen is not involved in the fast-trap path.

> Is my understanding right? If so, I have two questions:
> first, when will the guest os register the system call handler with the
> VMM?

Registers it when it boots, using HYPERVISOR_set_fast_trap().

> second, if a guest os is compromised, can intruders change the handler by
> registering another entry point at runtime?

If a guest OS root account is compromised then an intruder has full
control over that OS and can do things like registering a different
fast trap handler.

> I noticed that many people are trying to do more development on Xen. Can
> Xen group publish more design document? Although reading source code is
> usually a good way to understand Xen, it is not the fastest way, not to
> mention that people may misunderstand some codes. Maybe a simple flow
> chart of Xen will help us a lot!

More documentation is likely to appear in the future, now that the
interface to Xen has stabilised.

 -- Keir


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