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

[Julien Grall <julien.grall@xxxxxxx>]

Hi Volodymyr,

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:
> > > Hello all,
> > Hi Volodymyr,
> Hi Julien
> > > 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?
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.
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.
> 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.

> >     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.
>      * Some extra entity required to check validity of a mediator.
>
> 3. Mediator in an EL0 app.
>     The same pros and cons as for mediator in a stubdomain + extra code for EL0
>     apps, which is needed to be supported and maintained.
>
> Now, back to security questions. There are two possible attacks:
> attack at XEN and attack at TEE itself.
>
> If your TEE is vulnerable, then your whole system is compromised anyways.
> AFAIK, this approach was used to hack Samsung phones. Some flaw in Qualcom's
> TZ implementation.
>
> If your TEE mediator is vulnerable, then your hypervisor and all guests are
> compromised. Yes TEE mediator increases attack surface, but the same does
> any other XEN<->Guest interface. TEE mediators are expected to be simple
> and straightforward, without complex logic. So, I think that they are not
> more dangerous than vGIC driver or PL011 emulator.
To be honest when I read the list that a mediator should be able to do, 
I don't think it is possible to say it will not have complex logic.
For instance, for memory pinning. You need to know the buffers which 
likely means introspection of the calls if there are nested buffers.
This implies that you may tie into a specific version of TEE for a 
specific version of Xen.
So how do you expect OP-TEE evolving with Xen support? For example, if 
there is a new function do you expect to work on previous version of 
Xen? Or shall it wait the next release?
> 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?
> 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.
> > 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.
Cheers,

--
Julien Grall

--
Julien Grall

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