Re: [Tee-dev] TEE with XEN

[Volodymyr Babchuk <vlad.babchuk@xxxxxxxxx>]

On Fri, 19 Jun 2020 at 12:50, Peng Fan <peng.fan@xxxxxxx> wrote:
>
> > Subject: Re: [Tee-dev] TEE with XEN
> >
> > Hi Peng,
> >
> > On Fri, 19 Jun 2020 at 12:06, Peng Fan <peng.fan@xxxxxxx> wrote:
> > >
> > > > Subject: Re: [Tee-dev] TEE with XEN
> > > >
> > > >
> > > >
> > > > > On 19 Jun 2020, at 09:52, Peng Fan <peng.fan@xxxxxxx> wrote:
> > > > >
> > > > > Hi Bertrand,
> > > > >
> > > > >> Subject: Re: [Tee-dev] TEE with XEN
> > > > >>
> > > > >> Hi,
> > > > >>
> > > > >>> On 18 Jun 2020, at 19:05, Julien Grall <julien@xxxxxxx> wrote:
> > > > >>>
> > > > >>> +Bertrand and Stefano
> > > > >>>
> > > > >>> On 16/06/2020 02:24, Volodymyr Babchuk wrote:
> > > > >>>> Hi Peng,
> > > > >>>> On Mon, 15 Jun 2020 at 05:07, Peng Fan <peng.fan@xxxxxxx>
> > wrote:
> > > > >>>>>
> > > > >>>>> Hi All,
> > > > >>>>>
> > > > >>>>> While enabling trusty os with xen, I took same approach as
> > > > >>>>> OP-TEE, with OP-TEE running in secure world. But I am also
> > > > >>>>> thinking this might introduce potential issue is that secure
> > > > >>>>> world OS communicate with
> > > > >> DomU.
> > > > >>>>> If there are some misbehavior in secure world OS, it might let
> > > > >>>>> XEN hypervisor not work proper.
> > > > >>>>>
> > > > >>>>> In my setup, trusty os sometimes panic in secure world, xen
> > > > >>>>> will not able to control the panic core anymore.
> > > > >>>
> > > > >>> May I ask in which case Trusty is panicking?
> > > > >>
> > > > >> In any case, optee should protect itself against this and it
> > > > >> should be considered that optee is more priviledged then Xen.
> > > > >> So if optee is crashing we cannot expect that Xen can recover or fix 
> > > > >> it.
> > > > >>
> > > > >> I would more consider this as a bug that optee needs to be robust
> > against.
> > > > >
> > > > > ok. I am not using OP-TEE, currently I use google trusty OS.
> > > >
> > > > Sorry i should have been more generic.
> > > > Please read this as “Anything running in secure world”, being optee or
> > trusty.
> > > >
> > > > >
> > > > > I have two OS, Dom0 linux + DomU android auto.
> > > > >
> > > > > DomU android auto needs trusty OS, Dom0 Linux not need that.
> > > >
> > > > But i would guess your Dom0 is more “critical” then your DomU.
> > > > In this case you must make sure that any resource given to your DomU
> > > > cannot affect your Dom0.
> > > > For example: if the DomU is starting a very heavy operation in
> > > > blocked in trusty, any interrupt for non-secure could be blocked,
> > > > thus affecting the ability of your Dom0.
> > > >
> > > > >
> > > > > I not wanna trusty OS for DomU could bring any detect to Dom0 or xen.
> > > > >
> > > > > One more case is if dom0 linux needs OP-TEE, DomU needs google
> > > > > trusty, how we handle this in one SoC?
> > > >
> > > > You have a shared resource in this case, someone more or as trusted
> > > > as the clients needs to decide how the resource can be shared.
> > > > In this case could be Dom0 or Xen or Trusty or op-Tee (if i should
> > > > make an order).
> > > >
> > > > >
> > > > >>
> > > > >>>
> > > > >>>>>
> > > > >>>>> So I am thinking whether we need to emulating secure world in
> > > > >>>>> a XEN VM which is the VM running DomU. Just like what ACRN did
> > > > >>>>> to run trusty os.
> > > > >>>> Well, it depends on whom you are trusting more. Both XEN and
> > > > >>>> TEE are minimal OS implementations with aim at security. I'm
> > > > >>>> speaking about generic TEE OS, not about particular OS like OP-TEE
> > or Trusty.
> > > > >>>> Problem is that, if TEE is running inside VM, it will be
> > > > >>>> susceptible to a hypervisor misbehaviour. You need to
> > > > >>>> understand that Xen and privileged domain (dom0, mostly) can
> > > > >>>> access memory of
> > > > any guest.
> > > > >>>> At least, in default configuration. There are means to harden
> > > > >>>> this setup. But anyways, Xen can't be stopped from reading TEE's
> > secrets.
> > > > >>>
> > > > >>> IIRC, we discussed this approach for OP-TEE in the past. There
> > > > >>> was other
> > > > >> potential pitfalls with it. For instance, you wouldn't be able to
> > > > >> directly access any secure device from that guest (it is running
> > > > >> in
> > > > non-secure world).
> > > > >>>
> > > > >>> There are also issues in term of latency as you may have the
> > > > >>> following
> > > > >> model:
> > > > >>>
> > > > >>> domU -> Xen -> domU TEE -> (Xen -> host TEE -> Xen -> domU TEE)
> > > > >>> -> Xen ->
> > > > >> domU.
> > > > >>>
> > > > >>> The bit in () is if you require to call the host TEE.
> > > > >>>
> > > > >>> One possibility would be to use Secure-EL2 for your Trusty OS.
> > > > >>> But I don't
> > > > >> know whether your platform supports it.
> > > > >>>
> > > > >>> Depending on whether you can modify Trusty OS, alternative would
> > > > >>> be to
> > > > >> make itvirtualization aware as OP-TEE did. The core would need to
> > > > >> be resilient and the panic only affect a given client.
> > > > >>
> > > > >> I do not have right a clear idea of what is the status of optee
> > > > >> and xen but in theory I would see 2 possible ways to handle this:
> > > > >> - without optee modification, something in a guest (Dom0 or an
> > > > >> other priviledged one) needs to have access to optee and emulate
> > > > >> optee access for others.
> > > > >> - with optee modifications, optee needs to have a concept of
> > > > >> client and Xen would need to passthrough optee requests but being
> > > > >> responsible of adding a “client” identifier. Maybe also informing
> > > > >> Optee when a new client is created/removed.
> > > > >>
> > > > >> The second scenario could then be somehow splitted in the
> > > > >> previous one from Julien if some parts would need to be emulated
> > > > >> somewhere in some kind of combination of the 2 models.
> > > > >>
> > > > >> In any case i would always consider that anything running on
> > > > >> optee (or in general in the secure world) is more trusted then Xen.
> > > > >
> > > > > Ok, this means optee runs on all cores in secure world, but this
> > > > > would not work when we need to support multiple OSes with their own
> > TEE.
> > > >
> > > > I would think you have one TEE running on all cores (or runnable in this
> > case).
> > > > So the Tee needs to handle several contexts or someone needs to
> > virtualize it.
> > >
> > > This back to my original question, should I virtualize TEE in a XEN 
> > > dedicated
> > VM?
> > > or I need to emulate secure world to let one VM could have secure and
> > > non-secure world?
> > >
> >
> > Well, I think that the best approach is that we did in the OP-TEE: make 
> > Trusty
> > virtualization-aware, so it can handle multiple VMs.
>
> Trusty virtualization-aware, you mean android Linux trusty driver communicate
> with OP-TEE or else?

Okay, OP-TEE is implemented right as Bertran has suggested: OP-TEE can work
with multiple VMs at the same time. It has special calls to create and
destroy VM
contextes.

So, when a new VM/guest is spawned, Xen says "Hey, OP-TEE, there is a new
VM with ID NNN". OP-TEE then initializes internal state for the new VM.
At any moment Xen can say "Oops, that VM with ID NNN is dead". OP-TEE will
immediately destroy the internal state for that VM.

Also, Xen needs to perform certain actions every time VM calls OP-TEE: translate
addresses, tell OP-TEE id of that VM, lock guest pages...
And no changes to the OP-TEE client in Linux are needed. It thinks
that it communicates
directly with the OP-TEE. OP-TEE is running in the Secure World as usual.

The same can be done for any other TEE. Trusty, for example. Downside is that
you can't run two different TEEs in the Secure world.

Actually, ARM added virtualization support in Secure mode, but AFAIK, i.MX8 does
not support it.

To sum it up:
1. If you want to run only Trusty on your system, and you want to run
it in Secure World,
 you need to make Trusty virtualization-aware, and write mediator in
Xen. This is what
I did for OP-TEE.

2. If you want to run Trusty AND OP-TEE in the Secure World, then you
need to do p1.
and then implement some paravirtualization support in Secure Monitor,
Trusty and OP-TEE

3. If your use case does not require high security, you can try to run
Trusty as a VM.
But, I am almost certain that this will not pass Google's
requirements. So no Google Pay,
no E-SIM, no DRM for this setup. But, you should clarify this with
Google. I'm not a Google
representative :)