Re: [PATCH v9 0/7] KVM PCIe/MSI passthrough on ARM/ARM64: kernel part 3/3: vfio changes

[Alex Williamson]

On Thu, 9 Jun 2016 09:55:37 +0200
Auger Eric <eric.auger@xxxxxxxxxx> wrote:

> Alex,
> > On Wed, 8 Jun 2016 10:29:35 +0200
> > Auger Eric <eric.auger@xxxxxxxxxx> wrote:
> >   
> >> Dear all,
> >> Le 20/05/2016 Ã 18:01, Eric Auger a Ãcrit :  
> >>> Alex, Robin,
> >>>
> >>> While my 3 part series primarily addresses the problematic of mapping
> >>> MSI doorbells into arm-smmu, it fails in :
> >>>
> >>> 1) determining whether the MSI controller is downstream or upstream to
> >>> the IOMMU,    
> >>> 	=> indicates whether the MSI doorbell must be mapped
> >>> 	=> participates in the decision about 2)    
> >>>
> >>> 2) determining whether it is safe to assign a PCIe device.
> >>>
> >>> I think we share this understanding with Robin. All above of course
> >>> stands for ARM.
> >>>
> >>> I get stuck with those 2 issues and I have few questions about iommu
> >>> group setup, PCIe, iommu dt/ACPI description. I would be grateful to you
> >>> if you could answer part of those questions and advise about the
> >>> strategy to fix those.    
> >>
> >> gentle reminder about the questions below; hope I did not miss any reply.
> >> If anybody has some time to spent on this topic...
> >>  
> >>>
> >>> Best Regards
> >>>
> >>> Eric
> >>>
> >>> QUESTIONS:
> >>>
> >>> 1) Robin, you pointed some host controllers which also are MSI
> >>> controllers
> >>> (http://thread.gmane.org/gmane.linux.kernel.pci/47174/focus=47268). In
> >>> that case MSIs never reach the IOMMU. I failed in finding anything about
> >>> MSIs in PCIe ACS spec. What should be the iommu groups in that
> >>> situation. Isn't the upstreamed code able to see some DMA transfers are
> >>> not properly isolated and alias devices in the same group? According to
> >>> your security warning, Alex, I would think the code does not recognize
> >>> it, can you confirm please?    
> >> my current understanding is end points would be in separate groups (assuming
> >> ACS support) although MSI controller frame is not properly protected.  
> > 
> > We don't currently consider MSI differently from other DMA and we don't
> > currently have any sort of concept of a device within the intermediate
> > fabric as being a DMA target.  We expect fabric devices to only be
> > transaction routers.  We use ACS to determine whether there's any
> > possibility of DMA being redirected before it reaches the IOMMU, but it
> > seems that a DMA being consumed by an interrupt controller before it
> > reaches the IOMMU would be another cause for an isolation breach.
> >    
> OK thank you for the confirmation
> >>> 2) can other PCIe components be MSI controllers?  
> > 
> > I'm not even entirely sure what this means.  Would a DMA write from an
> > endpoint target the MMIO space of an intermediate, fabric device?  
> With the example provided by Robin we have a host controller acting as
> an MSI controller. I wondered whether we could have some other fabric
> devices (downstream to the host controller in PCIe terminology) also
> likely to act as MSI controllers.
> >    
> >>> 3) Am I obliged to consider arbitrary topologies where an MSI controller
> >>> stands between the PCIe host and the iommu? in the PCIe space or
> >>> platform space? If this only relates to PCIe couldn' I check if an MSI
> >>> controller exists in the PCIe tree?    
> >> In my last series, I consider the assignment of platform device unsafe as
> >> soon as there is a GICv2m. This is a change in the user experience compared to
> >> what we have before.  
> > 
> > If the MSI controller is downstream of our DMA translation, it doesn't
> > seem like we have much choice but to mark it unsafe.  The endpoint is
> > fully able to attempt to exploit it.  
> OK the orginal question was related to non PCIe topologies:
> 
> - we know some PCIe fabric topologies where the PCIe host controller
> implements MSI controller.
> - Shall we be prepared to address the same kind of issues with platform
> MSI controllers. Are there some SOCs where we would put an unsafe MSI
> platform controller before IOMMU translation. Or do we consider it is a
> platform topology we don't support for assignment?

I was trying to answer as generically, non-PCI as I could, but I guess
I still slipped an "endpoint" in there.  So if we define an MSI
controller generically as a DMA write target which generates platform
specific interrupts in response to those writes, and that MSI
controller is fixed in the address space of the interrupt generating
device (ie. there's no IOMMU translation applied), then I think:

a) The part of the address space consumed by the MSI controller needs
to be described to the user

b) Depending on the scope of the interrupts the MSI controller is able
to generate, the interrupts need to be marked unsafe

This is really not all that different from x86 with interrupt
remapping.  We have an address space hole where the MSI controller
lives at 0xfee00000.  We would like to describe this, but we currently
don't have a strong need to do so because it's architecturally fixed.
With interrupt remapping, the scope of the interrupts the device is
able to generate is only the interrupts intended for the device, which
we consider safe.  Without interrupt remapping, the IOMMU does not do
translation of this range (so it doesn't matter whether the MSI
controller is upstream, downstream, or coincident with the IOMMU), and
the scope of the interrupts the device can target are sufficiently
large to make it unsafe.

I believe what you're trying to account for is an MSI controller that
might arbitrarily consume a fixed portion of the IOVA space for a
device and for which it's ability to generate interrupts may or may not
be limited in scope.

I don't currently have a strong preference whether we do or don't
support assignment of such devices, but if we do and we have no ability
to manage the device's access to the MSI controller's DMA address space
(ie. no IOMU in the DMA path) and no guarantees of the scope of the
interrupts that the controller can generate through that DMA window, it
needs to be marked unsafe.  If we don't plan to support devices behind
such topologies then we need to prevent their use through vfio.

> >>> 4) Robin suggested in a private thread to enumerate through a list of
> >>> "registered" doorbells and if any belongs to an unsafe MSI controller,
> >>> consider the assignment is unsafe. This would be a first step before
> >>> doing something more complex. Alex, would that be acceptable to you for
> >>> issue #2?    
> >> I implemented this technique in my last series waiting for more discussion
> >> on 4, 5.  
> > 
> > Seems sufficient.  I don't mind taking a broad swing versus all the
> > extra complexity of defining which devices are safe vs unsafe.  
> OK
> >    
> >>> 5) About issue #1: don't we miss tools in dt/ACPI to describe the
> >>> location of the iommu on ARM? This is not needed on x86 because
> >>> irq_remapping and IOMMU are at the same place but my understanding is
> >>> that it is on ARM where
> >>> - there is no connection between the MSI controller - which implements
> >>> irq remapping - and the iommu
> >>> - MSI are conveyed on the same address space as standard memory
> >>> transactions.  
> > 
> > It seems pretty dubious to me to have fixed address, unprotected MSI
> > controllers sitting in the DMA space of a device before IOMMU
> > translation.  
> same for me ;-)
>   Seems like you not only need to mark interrupts as
> > unsafe, but exclude the address space of the MSI controller from the
> > available IOVA space to the user.  
> I currently do not see how to achieve that. The guest can program the
> assigned device DMA target address with the MSI frame PA. there is no
> IOMMU to protect. How can we make it if we don't trap on DMA programming?

All we can do regarding "excluding the address space of the MSI
controller" is to create a vfio interface to describe that exclusion.
Without an IOMMU we can't prevent access to it.  Therefore if we go
back to b) above, a user should only be allowed access to the device if
either the scope is sufficiently limited to be considered safe or we
have an opt-in for unsafe interrupts.

Still you have the issue of how the device actually gets programmed to
hit their DMA target to generate interrupts.  This is a virtualization
question though, a userspace driver needs to know both the available
IOVA space and the MSI target addresses.  It's userspace drivers like
QEMU that need to provide further virtualization to intercept guest
programming of the device and "do the right thing".  For this you
either need to rely on standards-based interfaces, like PCI MSI/X or
you get to write device specific virtualization code that traps the
interrupt programming.  Thanks,

Alex