Re: [ARM] Native application design and discussion (I hope)

[Julien Grall <julien.grall@arm.com>]

On 21/04/17 17:16, Volodymyr Babchuk wrote:
> Hi Julien,

Hi Volodymyr,

> On 21 April 2017 at 18:57, Julien Grall <julien.grall@arm.com> wrote:
>> Hello Volodymyr,
>>
>> On 20/04/17 21:20, Volodymyr Babchuk wrote:
>>>
>>> On 12 April 2017 at 22:17, Stefano Stabellini <sstabellini@kernel.org>
>>> wrote:
>>>>
>>>> On Wed, 12 Apr 2017, Dario Faggioli wrote:
>>>>>
>>>>> On Tue, 2017-04-11 at 13:32 -0700, Stefano Stabellini wrote:
>>>>>>
>>>>>> On Fri, 7 Apr 2017, Stefano Stabellini wrote:
>>>>
>>>> We would have one app per emulator. Each app would register an MMIO
>>>> range or instruction set to emulate. On a guest trap, Xen figures out
>>>> which app it needs to run.
>>>
>>> I't is not best approach, I think. For example we need one SMC handler for
>>> all domains. Because that SMC handler should track execution state of
>>> different
>>> guests to help TEE with scheduling. You know, TEE can't block in secure
>>> state,
>>> so it returns back and blocks in kernel driver. SMC handler need to know
>>> which guest it needs to wake up when times comes.
>>>
>>> The same story with virtual coprocessors, I think.
>>>
>>> On other hand, MMIO handler can be one per domain. So, it should be
>>> configurable. Or, maybe we need per-app MMIO handler and one global SMC
>>> handler.
>>> Perhaps, we need to think about all possible use cases.
>>
>>
>> Could you explain what would be the benefits to run this global SMC handler
>> in EL0?
>>
>> After all, it will require access to the host SMC. So what will you protect
>> against?
> Yes, it will require access to host SMC. Idea is not to protect (but,
> it can protect also).
> I want to allow different guests to work with one TEE. Imagine that
> multiple guests need
> protected storage, accelerated cryptography or other TEE services.
> All SMCs will be trapped to app, app will alter(or block) request and
> forward it to TEE. This is the most basic use case, which we want to
> implement.

I am sorry, but I don't understand it. I envision EL0 as a way to limit 
the attack vector to Xen and the host. If you give full access to SMC, 
then you cannot protect.

If the idea is not to protect, why do you want to move the code in EL0? 
What is the point to add an overhead (even if it is small) in this case?

Cheers,

-- 
Julien Grall

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