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Re: [Xen-devel] [ARM] SMC (and HVC) handling in hypervisor

On Mon, 13 Feb 2017, Tamas K Lengyel wrote:
> On Mon, Feb 13, 2017 at 11:06 AM, Julien Grall <julien.grall@xxxxxxx> wrote:
> >
> >
> > On 13/02/17 16:59, Tamas K Lengyel wrote:
> >>
> >> On Mon, Feb 13, 2017 at 9:37 AM, Julien Grall <julien.grall@xxxxxxx>
> >> wrote:
> >>>
> >>> Hi Tamas,
> >>>
> >>>
> >>> On 13/02/17 16:20, Tamas K Lengyel wrote:
> >>>>
> >>>>
> >>>> On Fri, Feb 10, 2017 at 5:14 PM, Volodymyr Babchuk
> >>>> <vlad.babchuk@xxxxxxxxx> wrote:
> >>>>>
> >>>>>
> >>>>> Hello,
> >>>>>
> >>>>> This e-mail is sort of follow-up to the two threads: [1] (my thread
> >>>>> about TEE interaction) and [2] (Edgar's thread regarding handling SMC
> >>>>> calls in platform_hvc). I want to discuss more broad topic there.
> >>>>>
> >>>>> Obviously, there are growing number of SMC users and current state of
> >>>>> SMC handling in Xen satisfies nobody. My team wants to handle SMCs in
> >>>>> secure way, Xilinx wants to forward some calls directly to Secure
> >>>>> Monitor, while allowing to handle other in userspace, etc.
> >>>>>
> >>>>> My proposition is to gather all requirements to SMC (and HVC) handling
> >>>>> in one place (e.g. in this mail thread). After we' will have clear
> >>>>> picture of what we want, we will be able to develop some solution,
> >>>>> that will satisfy us all. At least, I hope so :)
> >>>>>
> >>>>> Also I want to remind, that there are ARM document called "SMC Calling
> >>>>> Convention" [3]. According to it, any aarch64 hypervisor "must
> >>>>> implement the Standard Secure and Hypervisor Service calls". At this
> >>>>> moment XEN does not conform to this.
> >>>>>
> >>>>> So, lets get started with the requirements:
> >>>>> 0. There are no much difference between SMC and HVC handling (at least
> >>>>> according to SMCCC).
> >>>>> 1. Hypervisor should at least provide own UUID and version while
> >>>>> called by SMC/HVC
> >>>>> 2. Hypervisor should forward some calls from dom0 directly to Secure
> >>>>> Monitor (Xilinx use case)
> >>>>> 3. Hypervisor should virtualize PSCI calls, CPU service calls, ARM
> >>>>> architecture service calls, etc.
> >>>>> 4. Hypervisor should handle TEE calls in a secure way (e.g. no
> >>>>> untrusted handlers in Dom0 userspace).
> >>>>> 5. Hypervisor should support multiple TEEs (at least at compilation
> >>>>> time).
> >>>>> 6. Hypervisor should do this as fast as possible (DRM playback use
> >>>>> case).
> >>>>> 7. All domains (including dom0) should be handled in the same way.
> >>>>> 8. Not all domains will have right to issue certain SMCs.
> >>>>> 9. Hypervisor will issue own SMCs in some cases.
> >>>>
> >>>>
> >>>>
> >>>> 10. Domains on which the monitor privileged call feature is enabled
> >>>> (which is by default disabled for all domains) should not be able to
> >>>> issue SMCs such that it reaches the firmware directly. Xen should not
> >>>> bounce such calls to the firmware on behalf of the domain. Xen should
> >>>> not alter the state of the domain automatically (ie. incrementing PC).
> >>>> These calls should be exclusively transfered to the monitor subscriber
> >>>> for further processing. HVC calls need not be included in the monitor
> >>>> forwarding as long as the HVC call can be governed by XSM.
> >>>
> >>>
> >>>
> >>> This should not be a strong requirement. Whilst in your use case you want
> >>> to
> >>> forward all the SMCs to the monitor app, there are use case where Xen
> >>> would
> >>> need to emulate SMCs on the behalf of the guest. For instance see PSCI
> >>> (arch/arm/vpsci.c).
> >>
> >>
> >> In my usecases it is a strong requirement. What happens when the
> >> monitor system is disabled is beyond my concerns - Xen can emulate or
> >> forward the call as it wishes. But when the monitor application is in
> >> use - in my usecase - it needs to be in exclusive control. If that
> >> breaks an in-guest application, that is acceptable in my usecase. As
> >> soon as there is another usecase that would need to support such an
> >> application while the monitor system is enabled, the monitor system
> >> can be fine-tuned for those needs to allow Xen to emulate. I've said
> >> it many times, I have nothing against doing that, but as I don't need
> >> it I won't be able to spend time implementing it.

Right, as I wrote in the other thread, I think we should be able to find
a way to satisfy Tamas' requirement as well as all the others. Of
course, once you enable the "forward all SMCs to monitor" mode, some of
the other requirements might not be met anymore, but that's acceptable
because they are for different use-cases.

> > Let me remind you that this discussion is not about what you implemented but
> > what is a sensible design to fit everyone. I also never ask you to implement
> > anything.
> >>
> >>>
> >>> Another valid use case is Xen handling power management for device
> >>> assigned
> >>> to the guest and having the monitor app acting as a "Trusted App".
> >>>
> >>> Regarding the HVC call governed by XSM. I think this is the wrong way to
> >>> g.
> >>> As it was mentioned a couple of time HVC is a valid conduit for Secure
> >>> monitor call. You should not deny them on the basis that your monitor app
> >>> is
> >>> not able to handle it properly. A better way would be to have a list of
> >>> Secure Monitor Call (HVC/SMC) that should be forwarded to the monitor
> >>> app.
> >>
> >>
> >> I disagree. XSM needs to be in complete control of all hypercalls.
> >> Whether denying some of them will break an application or not is not
> >> Xen's concern. That is up to me as a user of Xen and XSM. If Xen
> >> overrides a XSM policy because we hard-coded HVCs that pass-through,
> >> that is a huge security policy violation. So even if we make a list of
> >> HVCs that should also fall under the monitor privileged call umbrella,
> >> it should still not override XSM. 

I agree with you on this.

> >> So since I would not be looking to
> >> emulate anything that gets forwarded as a result of an HVC call, XSM
> >> works for me just fine as the only thing I would do anyway is deny
> >> them. So why would that list help when I might as well just make my
> >> list more efficiently using XSM?
> >
> >
> > Again, why do you want to handle SMC and HVC differently given they are both
> > a conduit for Secure Call?
> AFAIU HVCs are used for hypercalls. If some HVCs will be used to go
> and issue a firmware call from Xen, it still doesn't change the fact
> that it was a hypercall in the first place. So it should be governed
> by XSM. Am I missing something?

The problem is the following: on some platforms an operating system
might issue a firmware call via HVC instead of SMC. It's outside of our
control. The monitor won't be able to receive that event, because the
infrastructure doesn't allow to forward HVC calls to the monitor. It
might break your use-case.

To properly support firmware calls monitoring, we should make no
distinctions between SMC and HVC, as specified by the SMC calling

To do this right, we need to be able to specify which SMCs/HVCs get
forwarded to the monitor. Forwarding all SMCs, but no HVCs, is not a
complete solution, although it should work in the vast majority of

I would put down as a requirement that we want to be able to monitor any
HVC/SMC calls, while accepting the compromise that as an initial
implementation only SMCs can actually be monitored.

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