Re: [Xen-devel] [RFC v2 3/6] xen/arm: Allow platform_hvc to handle guest SMC calls

["Edgar E. Iglesias" <edgar.iglesias@xxxxxxxxxx>]

On Thu, Feb 09, 2017 at 10:12:41AM +0100, Edgar E. Iglesias wrote:
> On Wed, Feb 08, 2017 at 05:20:44PM -0800, Stefano Stabellini wrote:
> > On Thu, 9 Feb 2017, Julien Grall wrote:
> > > On 08/02/2017 23:28, Tamas K Lengyel wrote:
> > > > On Wed, Feb 8, 2017 at 3:04 PM, Julien Grall <julien.grall@xxxxxxx> 
> > > > wrote:
> > > > > Hi Tamas,
> > > > > 
> > > > > Can you please try to configure your e-mail client to use '>' rather 
> > > > > than
> > > > > '
> > > > > '? It makes quite hard to read the e-mail.
> > > > 
> > > > Hm, not sure why it switched but should be fine now.
> > > > 
> > > > > On 08/02/2017 20:15, Tamas K Lengyel wrote:
> > > > > > 
> > > > > > 
> > > > > > 
> > > > > > On Wed, Feb 8, 2017 at 12:40 PM, Edgar E. Iglesias
> > > > > > <edgar.iglesias@xxxxxxxxx <mailto:edgar.iglesias@xxxxxxxxx>> wrote:
> > > > > >     On Wed, Feb 08, 2017 at 06:29:13PM +0100, Edgar E. Iglesias 
> > > > > > wrote:
> > > > > 
> > > > > 
> > > > > >     If platform_hvc() consumes an SMC, it's considered part of the 
> > > > > > Xen
> > > > > > API.
> > > > > >     Visible but not filterable by a monitor.
> > > > > > 
> > > > > > 
> > > > > >     Platforms can then dictate which SMC calls are better handled 
> > > > > > within
> > > > > >     Xen and
> > > > > >     which ones can be exposed to dom0 user-space.
> > > > > > 
> > > > > >     In addition, there could be a hypercall to disable platform 
> > > > > > specific
> > > > > >     handling
> > > > > >     in Xen alltogether for a given guest. Then everything goes to 
> > > > > > dom0
> > > > > >     user-space.
> > > > > > 
> > > > > >     It's a little messy...
> > > > > > 
> > > > > > 
> > > > > > 
> > > > > > That is messy and I would not want any SMCs reaching the firmware 
> > > > > > when
> > > > > > the monitor application is in use. The monitor interface is 
> > > > > > disabled by
> > > > > > default and there aren't any known usecases where the SMC has to 
> > > > > > reach
> > > > > > both the firmware and the monitor application as well. So I think 
> > > > > > it is
> > > > > > safe to just make the two things mutually exclusive.
> > > > > 
> > > > > 
> > > > > If you look at the SMC Calling Convention [1] both HVC and SMC are
> > > > > considered a conduit for service call to the secure firmware or
> > > > > hypervisor.
> > > > > It would be up to the hypervisor deciding what to do.
> > > > > 
> > > > > Lets imagine the guest is deciding to use HVC to access the secure
> > > > > firmware
> > > > > (AFAIU this patch series is adding that), are you going to monitor 
> > > > > all the
> > > > > HVCs (including hypercall)?
> > > > 
> > > > There are some fundamental differences between HVC and SMC calls
> > > > though. An HVC can only land in the hypervisor, so as a hypercall, I
> > > > would expect it to be something I can deny via XSM. That is a
> > > > sufficient option for now to block the path to the firmware. If we end
> > > > up needing to support an application that uses that hypercall for
> > > > something critical, then yes, it would also need to be hooked into the
> > > > monitor system. At the moment this is not necessary.
> > > 
> > > My point is not about what is necessary at the moment. But what is right
> > > things to do. If you look at the spec, HVC are not only for hypercall, 
> > > but any
> > > other kind of services. Why would you deny something that is valid from 
> > > the
> > > specification (see 5.2.1)?
> > > 
> > > "The SMC calling convention, however, does not specify which instruction
> > > (either SMC or HVC) to use to invoke a
> > > particular service."
> > 
> > To have a generic solution, we need a way to specify a set of HVC/SMC
> > calls that get monitored and a set that get handled in Xen (platform
> > specific or otherwise). I think it is OK not to do both, at least at the
> > beginning, but we might want to add that feature in the future.
> > 
> > As much as I would like to see that, in respect to this series, I don't
> > think we should ask Edgar to introduce such a mechanism. However, we do
> > need to decide what Xen should do when platform_hvc is implemented and
> > monitor is also enabled.
> > 
> > I think the default should be to only call platform_hvc, because there
> > are many valid monitoring use-cases which don't require those few
> > platform specific SMC/HVC calls to be forwarded to the monitor.
> > 
> > However, if we did that, we would break Tamas' scenario. Thus, I suggest
> > we also introduce a simple compile time option or Xen command line
> > option to forward all platform_hvc calls to the monitor instead of
> > implementing them in Xen. Something like "MONITOR_OVERRIDE". In the
> > future, we can replace it with a more generic framework to dynamically
> > configure at runtime which SMC/HVC calls get forwarded.
> > 
> > What do you think?
> 
> This could work in some scenarios, but for example on the ZynqMP,
> dom0 needs access to Firmware as it boots, otherwise a lot of I/O
> will end up non-functional (with recent kernels). Anyway, I think it
> would give us a path forward. Future patches could either implement
> finer control or something else.

Actually, MONITOR_OVERRIDE could allow dom0 full access to the Firmware
and only block guests. That would work better on the ZynqMP. I probably
overlooked this in your suggestion.

Cheers,
Edgar


> 
> Perhaps a hypercall to allow dom0 to turn off any platform_hvc handling for
> a given guest would help. If that mode is enabled, Xen is hands off on
> any platform specific HVC/SMC. It still leaves the problem open for
> other calls but I must say I find it strange to emulate the Xen Hypercalls
> in dom0 user-space.
> 
> AFAIK, the monitor is not looking at HVC today. So it is allowing PSCI
> firmware calls through Xen's PSCI mediator/emulator. Some of these calls
> may sometimes hit firmware, so the point of isolating a guest completely
> from Firmware access is not valid today.
> 
> Cheers,
> Edgar
> 
> 
> > > > So if we are landing in do_trap_smc from an HVC call, I think it would
> > > > be better to introduce a separate function for it rather then just
> > > > bunching the two together here.
> > > > 
> > > > > 
> > > > > Similarly, non-modified baremetal app could use SMC to power on/off 
> > > > > the
> > > > > vCPU
> > > > > (see PSCI spec). Will you emulate that in the monitor app?
> > > > 
> > > > Yes, the underlying setup requires that everything that is expected
> > > > from the firmware to be performed either by the monitor app, or have
> > > > the monitor app further delegate it somewhere that can perform the
> > > > task. That can be either the firmware itself (if its safe), or an
> > > > isolated VM if it is possible to perform the task there. I wouldn't
> > > > call this emulation necessarily btw.
> > > 
> > > You haven't understood my point. Xen is currently emulating PSCI call for 
> > > the
> > > guest to allow powering up and down the CPUs and other stuff. If you 
> > > decide to
> > > trap all the SMCs, you would have to handle them.
> > > 
> > > And yes it is emulation as you don't seem to be willing passing those SMC 
> > > to
> > > the firmware or even back to Xen. If we expect a VM to emulate a trusted
> > > firmware, then you have a security problem. Some hardware may be only
> > > accessible through the secure world and I doubt some trusted app vendor 
> > > will
> > > be willing to move cryptography stuff in non secure world. I would highly
> > > recommend to skim through the OP-TEE thread, it will provide you some 
> > > insights
> > > of the constraints.
> > 
> > Each platform is different. It seems unlikely to me too, and it might
> > always remain a niche use-case, but it is still a valid scenario to
> > consider.
> 
> 

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