SMC Calls in Trustzone VMM Scenario

Wed Apr 11 11:42:42 CEST 2018

Hi Mauricio,

thank you for sharing your intention.

On Tue, Apr 10, 2018 at 10:51:15PM -0700, Mauricio Gutierrez wrote:
> 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.

Ok, first I do not see how you can enforce the cache usage with
respect to the normal world. As long as the cache is enabled, how do
you confine memory access of the normal world when it's not asking
for permission (via your smc call)?
To me it looks like you want to close some covert channels by cache
data usage/displacement? In that case, maybe it is more reasonable to
circumvent usage of the cache by the secure world at all?

Anyway, concerning your technical issue, I am not sure whether you
count the cpu exception from 0 or 1, but however it does not
correspond to a SMC or prefetch-abort exception. It is much more
probable that you receive a data-abort when writing to access that
memory. The first obvious reason is that you are using the wrong
virtual address to access the normal world physical memory you like to
access. Our kernel/core is not mapped one-by-one physical-to-virtual.
Moreover, the normal world memory is not mapped at all into Genode's
kernel.

> 
> Right now I am trying to access the physical address by temporarily
> disabling the mmu so PA = VA.

This should not work in recent versions of our kernel, because as I
wrote before the kernel binary is linked to a different virtual
address region than its physical memory representation. That means,
the kernel will fault immediately when disabling the MMU.

> 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?

There is a designated kernel/core page-table that can be used to add
kernel mappings. Obviously, you are using some quite old Genode
version, so I'm not sure how to do that in your case. Nowadays, I
would add a designated static mapping to the core/kernel mappings in
the "bootstrap" code for the corresponding board, e.g.:

  https://github.com/genodelabs/genode/blob/master/repos/base-hw/src/bootstrap/spec/imx53_qsb/platform_trustzone.cc#L38

as an example for the i.MX53 Quickstart board when used with
TrustZone. The "core_mmio" regions here are static mappings always
present in the kernel. When the normal world memory is to big, it
might not fit into the virtual "core_mmio" region. Have a look at this
file to check the kernel virtual memory regions:

  https://github.com/genodelabs/genode/blob/master/repos/base-hw/src/lib/hw/spec/32bit/memory_map.cc

However, being in your position I would re-think the approach as a
whole, before digging further down the rabbit hole ;-).

Best regards
Stefan

> 
> 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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-- 
Stefan Kalkowski
Genode labs

https://github.com.skalk | https://genode.org