SMC Calls in Trustzone VMM Scenario

Wed Apr 11 07:51:15 CEST 2018

Hello Stefan,

Thank you for your reply.

If you really need to execute code in monitor mode (I wonder why), I
> think it will be best to create an explicit interface on the
> kernel/core level that can be called from the VMM component, maybe an
> extensions of the VM session interface.
>
> I think it somehow depends on what you are trying to do. If your
> routine has to be called every time a secure monitor call was
> executed, it is better to handle that directly within the
> Vm::exception function. If you have a very special device that needs
> to be accessed from secure, privileged/monitor mode you should extend the
> interface of kernel/core.
>

Indeed this is what I ended up doing. I have added some exception handling
in the default case
of the Vm::exception function for my smc call (#4) before it switches over
to user-level privilege.

Your are welcome. Maybe, you can give us some insights why do you need
> to enter monitor mode at all?
>

I am trying to make it so that the secure world can lock some normal world
physical memory into the cache.
For security reasons, I do not want to allow the normal word to do it so I
make an smc into secure world
passing the virtual address to load into cache. The secure world then uses
the VA to PA registers to get the
physical address and load the memory into the cache. Of course, the only
way this would work would be for
the cache entry to be tagged with an NS bit = 1. In order to do this, I
need to enter monitor mode so that I can
change the NS bit to 1 while remaining is secure world. This way I perform
the cache loading and locking on
behalf of the normal world while being able to check for consistency. The
issue I am facing now is that I get a
cpu exception 3 (Breakpoint) when I try to write to the given normal world
memory from secure world,
and I am not sure why.

Right now I am trying to access the physical address by temporarily
disabling the mmu so PA = VA.
However, another method would be to create/edit a page table entry in the
secure world such that it
maps to the specified physical address, essentially creating world shared
memory. However, I am not
sure how I would do this in Genode. For example, in the Normal World linux
I could edit the paging global
directory, but in the Genode OS how could I make a PTE map to a given
physical address?

Thank you,

Mauricio

On Mon, Apr 9, 2018 at 2:36 AM, Stefan Kalkowski <
stefan.kalkowski at ...1...> wrote:

> Hi Mauricio,
>
> On Wed, Mar 28, 2018 at 07:48:43PM -0700, Mauricio Gutierrez wrote:
> > Hello,
> >
> > I have been doing some work with the Genode Trustzone VMM scenario on my
> > i.MX53 development board and I am having a bit of trouble understanding
> how
> > the SMC calls work between the normal and secure world. Online you talk
> > about how you implemented 6 calls in the modified normal world linux
> kernel
> > but in the main for the tz_vmm I only found 4 different cases in the
> > _handle_smc() function. In any case, I wanted to add my own call and was
> > able to add it and check that the required arguments are passed correctly
> > and everything so that part I think I understand.
> >
> > However, I need to do some of the handling in Monitor Mode and my
> > understanding was that an SMC would throw your into monitor mode but it
> > seems the handler operates in user mode? Since it is not privileged then
> I
> > am not able to call a "cps #22" to switch to monitor mode.  In an earlier
> > thread I know you refer to the
> >
> > section "World switch between non-secure world and secure
> > > world" in http://genode.org/documentation/articles/trustzone.
> >
> > But I am still uncertain as to how I could get my case in _handle_smc()
> to
> > enter monitor mode so that I can play around with the NS bit without
> > leaving secure world.
>
> You are right, our virtual-machine monitor is an unprivileged
> user-level component. Because driving the normal world is not
> crucial to other components inside the secure world, there is no need
> to make them dependent on complex emulation/para-virtualization code inside
> the kernel. That is why the kernel contains a slim
> exception-vector-only functionality that is used to copy over the
> normal world state to the VMM user-level component. This
> exception-vector code is entered, e.g., when doing a "smc" call and
> can be found here:
>
>   repos/base-hw/src/core/spec/arm_v7/trustzone/exception_vector.s
>
> That assembly code is the only code executed in monitor mode. In the
> end it switches to secure world's supervisor mode and enters the
> normal kernel routine. However, it hits subsequently some C++ VMM
> kernel object routine here:
>
>   repos/base-hw/src/core/spec/arm_v7/trustzone/kernel/vm.cc
>
> Namely, the Vm::exception function, which informs the kernel scheduler
> to exclude the normal world for now, and signals the VMM user-level
> component that the normal world's state changed.
>
> > I have been studying what happens when I call an smc, say "smc #4" from
> > normal world. But I have not been able to exactly pin point, where is the
> > entry point for such an exception in the Genode secure world call? What
> > exactly happens once I make that call to secure world and where I can I
> > find/follow the code? Is this covered somewhere in your book?
> >
> > I know about the mode_transitions.s file as well as the
> exception_vector.s
> > and vm.cc files in repos/base-hw/src/core/spec/arm_v7/trustzone, it
> seems
> > this is the entry point? But where does it go after we call the
> > nonsecure_to_secure transition?
> >
> > Most importantly, I need to understand where is the code operating in
> > monitor mode? Where does it end and where does it start? How can I tell?
> If
> > I needed to write at least some part of my smc handler in monitor mode
> > before it switches out, what is the best approach to doing that?
> >
>
> If you really need to execute code in monitor mode (I wonder why), I
> think it will be best to create an explicit interface on the
> kernel/core level that can be called from the VMM component, maybe an
> extensions of the VM session interface.
>
> I think it somehow depends on what you are trying to do. If your
> routine has to be called every time a secure monitor call was
> executed, it is better to handle that directly within the
> Vm::exception function. If you have a very special device that needs
> to be accessed from secure, privileged/monitor mode you should extend the
> interface of kernel/core.
>
> > I apologize for all the questions and would appreciate any help and
> > guidance you can provide.
>
> Your are welcome. Maybe, you can give us some insights why do you need
> to enter monitor mode at all?
>
> Best regards
> Stefan
>
> >
> > Thank you,
> >
> > Mauricio
>
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>
> --
> Stefan Kalkowski
> Genode labs
>
> https://github.com.skalk | https://genode.org
>
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