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Re: [Xen-devel] [RFC 0/4] TEE mediator framework + OP-TEE mediator

Hi Volodymyr,
Let me begin the e-mail with I am not totally adversed to putting the TEE mediator in Xen. At the moment, I am trying to understand the whole picture. 

On 20/10/17 18:37, Volodymyr Babchuk wrote:

On Fri, Oct 20, 2017 at 02:11:14PM +0100, Julien Grall wrote:

On 17/10/17 16:59, Volodymyr Babchuk wrote:

On Mon, Oct 16, 2017 at 01:00:21PM +0100, Julien Grall wrote:

On 11/10/17 20:01, Volodymyr Babchuk wrote:

I want to present TEE mediator, that was discussed earlier ([1]).

I selected design with built-in mediators. This is easiest way,
it removes many questions, it is easy to implement and maintain
(at least I hope so).


Well, it may close the technical questions but still leave the security
impact unanswered. I would have appreciated a summary of each approach and
explain the pros/cons.

This is the most secure way also. In terms of trust between guests and
Xen at least. I'm worked with OP-TEE guys mostly, so when I hear about
"security", my first thoughts are "Can TEE OS trust to XEN as a
mediator? Can TEE client trust to XEN as a mediator?". And with
current approach answer is "yes, they can, especially if XEN is a part
of a chain of trust".

But you probably wanted to ask "Can guest compromise whole system by
using TEE mediator or TEE OS?". This is an interesting question.
First let's discuss requirements for a TEE mediator. So, mediator
should be able to:

  * Receive request to handle trapped SMC. This request should include
    user registers + some information about guest (at least domain id).
  * Pin/unpin domain memory pages.
  * Map domain memory pages into own address space with RW access.
  * Issue real SMC to a TEE.
  * Receive information about guest creation and destruction.
  * (Probably) inject IRQs into a domain (this can be not a requester domain,
    but some other domain, that also called to TEE).

This is a minimal list of requirements. I think, this should be enough to
implement mediator for OP-TEE. But I can't say for sure for other TEEs.

Let's consider possible approaches:

1. Mediator right in XEN, works at EL2.
    Pros:
     * Mediator can use all XEN APIs
     * As mediator resides in XEN, it can be checked together with XEN
       for a validity (trusted boot).
     * Mediator is initialized before Dom0. Dom0 can work with a TEE.
     * No extra context switches, no special ABI between XEN and mediator.

    Cons:
     * Because it lives in EL2, it can compromise whole hypervisor,
       if there is a security bug in mediator code.
     * No support for closed source TEEs.


Another cons is you assume TEE API is fully stable and will not change.
Imagine a new function is added, or a vendor decided to hence with a new set
of API. How will you know Xen is safe to use it?

With whitelisting, as you correctly suggested below. XEN will process
only know requests. Anything that looks unfimiliar should be rejected.
Let's imagine the guest is running on a platform with a newer version of TEE. This guest will probe the version of OP-TEE and knows the new function is present.
If as you said Xen is using a whitelist, this means the hypervisor will return unimplemented. 

How do you expect the guest to behave in that case?
Note that I think a whitelist is a good idea, but I think we need to think a bit more about the implication. 




If it is not safe, this means you have a whitelist solution and therefore
tie Xen to a specific OP-TEE version. So if you need to use a new function
you would need to upgrade Xen making the code of using new version
potentially high.

Yes, any ABI change between OP-TEE and its clients will require mediator
upgrade. Luckilly, OP-TEE maintains ABI backward-compatible, so if you'll
install old XEN and new OP-TEE, OP-TEE will use only that subset of ABI,
which is known to XEN.


Also, correct me if I am wrong, OP-TEE is a BSD 2-Clause. This means you
impose anyone wanted to modify OP-TEE for their own purpose can make a
closed version of the TEE. But if you need to introspect/whitelist call, you
impose the vendor to expose their API.

Basically yes. Is this bad? OP-TEE driver in Linux is licensed under GPL v2.
If vendor modifies interface between OP-TEE and Linux, they anyways obligued
to expose API.


Pardon me for potential stupid questions, my knowledge of OP-TEE is limited.
My understanding is the OP-TEE will provide a generic way to access different Trusted Application. While OP-TEE API may be generic, the TA API is custom. AFAICT the latter is not part of Linux driver. 

So here my questions:
1) Are you planning allow all the guests to access every Trusted Applications? 
        2) Will you ever need to introspect those messages?



2. Mediator in a stubdomain. Works at EL1.
    Pros:
     * Mediator is isolated from hypervisor (but it still can do potentially
       dangerous things like mapping domain memory or pining pages).
     * One can legally create and use mediator for a closed-source TEE.


        * Easier to upgrade to a new version of OP-TEE.

Yes, this is true. But what about interface between XEN and mediator?
This is a new entity that should be maintained. Will I abe able to use
new XEN with old mediator? Or new mediator with old XEN?


Why would you need to specific interface for the mediator? (see more below)




     Cons:

     * Overhead in XEN<->Mediator communication.
     * XEN needs to be modified to boot mediator domain before Dom0.


Is it a really cons? In the past, we had discussion to allow Xen creating
multiple domain, avoiding the overhead of Dom0. This could also benefits
here.

As I understand, this is a significant change in XEN. What are the chances,
that community will accept this change? As I can see, immediate benefit
of this is only TEE mediator support. Looks like no one except us
interested in this topic.
The GSOC project was not added because of TEE mediator. We had companies showing interest to start multiple domains at the same time. This would significantly shrink down the boot time of the whole platform. 



BTW, I checked "Xen on ARM: create multiple guests from device
tree" at [1]. This is close, to what we need, but not exactly. You see,
TEE mediator should be created *before* Dom0. So actually TEE mediator
will receive domid 0. I suspect that this only change will break
many things.


Can you please give example?
Technically none of the hypervisor, Linux and the toolstack should rely on dom0 to be domid 0.
AFAIK, the hypervisor and Linux are free of them. It might be possible to have few hardcoded in the toolstack, but they should really disappear.
However, I can't see why you require the mediator to use domid 0. You could for example keep the hardware domain paused until the mediator has started. 

[...]





And yes, it seems obvious, but I want to say this explicitly: generic
TEE mediator framework should and will use XSM to control which domain
can work with TEE. So, if you don't trust your guest - don't let it
to call TEE at all.


Correct me if I am wrong. TEE could be used by Android guest which likely
run the user apps... right? So are you saying you fully trust that guest and
obviously the user installing rogue app?

I don't think that app downloaded from Play Marget can access OP-TEE directly.
OP-TEE can be used by Android itself as a key storage or to access to a SE,
for example. But 3rd app that issues TEE calls... I don't think so.
You didn't get my point here. That rogue app may be able to break into kernel via an exploit or have enough privilege to break the guest. Who knows what it will be able to do after...
The whole point of using an hypervisor is to isolate guest from each other. So what is the isolation model with OP-TEE and the mediator? 




This feature is not implemented in this RFC only because
currently only Dom0 calls are supported.


This would help to understand that maybe it is an easy way but also still
secure...

In previous discussion we considered only two variants: in XEN or outside
XEN. Stubdomain approach looks more secure, but I'm not sure that it is true.
Such stubdomain will need access to all guests memory. If you managed to
gain control on mediator stubdomain, you can do anything you want with all
guests.


That's slightly untrue. The stubdomain will only be able to mess with
domains using TEE.

Yes, this is more strict. Then either you are not allowing your privileged
domain to use TEE, or your system may be compromised anyways.
Can you give an example of privilege domain for you? Do you consider Android a privilege domain? 




To be clear, this series don't look controversial at least for OP-TEE. What
I am more concerned is about DomU supports.

Your concern is that rogue DomU can compromise whole system, right?


Yes. You seem to assume that DomU using TEE will always be trusted, I think
this is the wrong approach if the use is able to interact directly with
those guests. See above.

No, I am not assuming that DomU that calls TEE should be trusted. Why do you
think so? It should be able to use TEE services, but this does not mean that
XEN should trust it.
In a previous answer you said: "So, if you don't trust your guest - don't let it". For me, this clearly means you consider that DomU using TEE are trusted. 

So can you clarify by what you mean by trust then?


Even now, XEN processes requests from DomUs without
trusting them. Why do you think, that TEE mediator usage will differ?
I guess you are comparing with vGIC and PL011? IHMO, the main difference is Xen is taking care alone of the isolation between guest. Here in the TEE case, you rely on a combination of both TEE and Xen to do the isolation. 



Look, I generally not against idea of TEE mediator in stubdoms. But this
approach require many changes in existing XEN code:

1. Load domains before Dom0.

2. Add special API for mediator. Or alter existing ones. You can't use
    existing APIs as it, because you need to enforce stricter XSM rules
    on them.
Mind giving more explanation....? Xen has a default policy for XSM and indeed may not fit your use case. But you can write your own policy and load it. 



3. Changes in scheduling to allow TEE mediator use credits/slices of
    calling guest.

4. Support boilerplate code in stubdom. You know, you can't simply
    write mediator in stubdom. You need a kernel. You need to
    maintain it.
Well, in a way or another someone will have to maintain the mediator... The kernel does not need to be specific to TEE, it could be a unikernel.
And before you say again no-one in the community seem to be interested. I should remind you that Arm is working on it (see development update). 



This is a lot of a work. It requires changes in generic parts of XEN.
I fear it will be very hard to upstream such changes, because no one
sees an immediate value in them. How do you think, what are my chances
to upstream this?
It is fairly annoying to see you justifying back most of this thread with "no one sees an immediate value in them".
I am not the only maintainers in Xen, so effectively can't promise whether it is going to be upstreamed. But I believe the community has been very supportive so far, a lot of discussions happened (see [2]) because of the OP-TEE support. So what more do you expect from us? 



Approach in this RFC is much simpler. Few hooks in arch code + additional
subsystem, which can be easily turned off.


Stefano do you have any opinion on this discussion?

Regards,



[1] https://wiki.xenproject.org/wiki/Outreach_Program_Projects
[2] https://lists.xenproject.org/archives/html/xen-devel/2017-05/msg01931.html 

--
Julien Grall

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