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

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

Hi Volodymyr,

On 21/04/17 18:04, Volodymyr Babchuk wrote:
> On 21 April 2017 at 19:47, Julien Grall <julien.grall@arm.com> wrote:
>> On 21/04/17 17:16, Volodymyr Babchuk wrote:
>>> 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.
> In any case it will limit the attack surface. Filtered SMC request is
> not as destructive as
> arbitrary SMC from a guest.

I agree with that. But why in EL0? I think you answer partly below.

>
>> 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?
> There are many reasons:
> 1. Community is reluctant to add OP-TEE (or any other TEE) handler
> right into hypervisor codebase.

Well, I think I was the only one to be reluctant. And I asked you to 
look at different solutions and come up with suggestion are saying why 
you solution is better.

Whilst I agree that EL0 app is a solution for a lot of emulation. We 
should be careful before moving code to EL0 and evaluating the impact. I 
am expecting to see the interface very small and the application to be 
standalone (e.g not requiring much interaction with Xen or the host 
hardware). But you seem to have a different view (see your e-mail with: 
"Probably, we can try another approach: allow application to register 
hooks in hypervisor: i.e. hook on MMIO, hook on SMC, hook on timer and 
so on.").

If you introduce EL0 but require a big interface, then I believe you 
don't limit the surface attack.

> 2. Modularity. You can detect running TEE during boot and load
> appropriate TEE handler app (honestly, it is not a big deal, because
> you know on which system will work your hypervisor and TEE type can be
> hardcoded in build).

Well, you could make Xen modular like Linux and still run everything in 
EL2. (Disclaimer, I am not saying we should do that...)

> 3. Some degree of protection. Bug in EL0 handler will not bring down
> whole hypervisor.

If you have a single app handling all the domains using SMC, then you 
will bring down all thoses domains. I agree it does not take down the 
hypervisor, but it will render unusable a part of the platform. In this 
case, how do you plan to restore the services?

Also, a bug in the EL0 handler may give the opportunity, in your use 
case, to get access to the firmware or data from another guest. How this 
will bring more protection?

If you handle only one guest per app, then it is very easy to kill that 
app and domain. It will only harm itself.

Cheers,

-- 
Julien Grall

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