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SubjectRe: [PATCH v9 0/7] KVM PCIe/MSI passthrough on ARM/ARM64: kernel part 3/3: vfio changes
On Wed, 8 Jun 2016 10:29:35 +0200
Auger Eric <eric.auger@linaro.org> wrote:

> Dear all,
> Le 20/05/2016 à 18:01, Eric Auger a écrit :
> > 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.

> > 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?

> > 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.

> > 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.

> > 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. 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.

> > 6) can't we live with iommu/MSI controller respective location uncertainty?
> >
> > - in my current series, with the above Xilinx MSI controller, I would
> > see there is an arm-smmu requiring mapping behind the PCI host, would
> > query the characteristics of the MSI doorbell (not implemented by that
> > controller), so no mapping would be done. So it would work I think.
> > - However in case we have this topology: PCIe host -> MSI controller
> > generally used behind an IOMMU (so registering a doorbell) -> IOMMU,
> > this wouldn't work since the doorbell would be mapped.

I'm a little confused which direction "behind" is here, but it seems
like any time the MSI controller lives in the DMA address space of the
endpoint, both interfering with the available IOVA space to the user
and potentially an attack vector for the user, we need to call it out
as unsafe. Maybe some of them are for exclusive use of the device and
the attack vector is relatively contained, but they still affect the
IOVA space of the user. Such a configuration might be safe, but as I
said I'm not opposed to being pretty liberal in applying the unsafe
requirement if the platform has done something unfriendly. Thanks,

Alex

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