Re: [Tee-dev] TEE with XEN

[Stefano Stabellini <sstabellini@kernel.org>]

On Thu, 18 Jun 2020, Julien Grall wrote:
> +Bertrand and Stefano

Thanks for CC'ing me


> On 16/06/2020 02:24, Volodymyr Babchuk wrote:
> > Hi Peng,
> > 
> > On Mon, 15 Jun 2020 at 05:07, Peng Fan <peng.fan@nxp.com> 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?
> 
> > > 
> > > 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).

Given that Secure World has access to Normal World but not vice versa,
it is more natural to run Trusty in one of the Secure execution levels.
The expectation is that Trusty has higher privilege, thus it is more
"trusted" than anything running in Normal World, including Xen.

Of course no client should be able to crash Trusty, so the reality
sometimes can be very different from the theory :-)

If I were you, I would consider running the TEE "inside" the VM only if
the service that the TEE provides is strongly coupled with the VM and
doesn't actually need a high level of privilege to function (i.e.
doesn't access secure devices or at least not often.)


> 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.

This would most probably be the best compromise.