Re: [Xen-devel] [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Tue, Jul 28, 2015 at 10:17 AM, Jan Kiszka <jan.kiszka@xxxxxxxxxxx> wrote:
> On 2015-07-28 19:10, Andy Lutomirski wrote:
>> The trouble is that this is really a property of the bus and not of
>> the device.  If you build a virtio device that physically plugs into a
>> PCIe slot, the device has no concept of an IOMMU in the first place.
>
> If one would build a real virtio device today, it would be broken
> because every IOMMU would start to translate its requests. Already from
> that POV, we really need to introduce a feature flag "I will be
> IOMMU-translated" so that a potential physical implementation can carry
> it unconditionally.
>

Except that, with my patches, it would work correctly.  ISTM the thing
that's broken right now is QEMU and the virtio_pci driver.  My patches
fix the driver.  Last year that would have been the end of the story
except for PPC.  Now we have to deal with QEMU.

>> Similarly, if you take an L0-provided IOMMU-supporting device and pass
>> it through to L2 using current QEMU on L1 (with Q35 emulation and
>> iommu enabled), then, from L2's perspective, the device is 1:1 no
>> matter what the device thinks.
>>
>> IOW, I think the original design was wrong and now we have to deal
>> with it.  I think the best solution would be to teach QEMU to fix its
>> ACPI tables so that 1:1 virtio devices are actually exposed as 1:1.
>
> Only the current drivers are broken. And we can easily tell them apart
> from newer ones via feature flags. Sorry, don't get the problem.

I still don't see how feature flags solve the problem.  Suppose we
added a feature flag meaning "respects IOMMU".

Bad case 1:  Build a malicious device that advertises
non-IOMMU-respecting virtio.  Plug it in behind an IOMMU.  Host starts
leaking physical addresses to the device (and the device doesn't work,
of course).  Maybe that's only barely a security problem, but still...

Bad case 2:  Use current QEMU w/ IOMMU enabled.  Assign a virtio
device provided by L0 QEMU to L2.  L1 crashes.  I consider *that* to
be a security problem, although in practice no one will configure
their system that way because it has zero chance of actually working.
Nonetheless, the device does work if L1 accesses it directly?  The
issue is vfio doesn't notice that the device doesn't respect the IOMMU
because "respects-IOMMU" is a property of the PCI bus and the platform
IOMMU, and vfio assumes it works correctly.

Bad case 2: Some hypothetical well-behaved new QEMU provides a virtio
device that *does* respect the IOMMU and sets the feature flag.  They
emulate Q35 with an IOMMU.  They boot Linux 4.1.  Data corruption in
the guest.

We could make the rule that *all* virtio-pci devices (except on PPC)
respect the bus rules.  We'd have to fix QEMU so that virtio devices
on Q35 iommu=on systems set up a PCI topology where the devices
*aren't* behind the IOMMU or are protected by RMRRs or whatever.  Then
old kernels would work correctly on new hosts, new kernels would work
correctly except on old iommu-providing hosts, and Xen would work.

In fact, on Xen, it's impossible without colossal hacks to support
non-IOMMU-respecting virtio devices because Xen acts as an
intermediate IOMMU between the Linux dom0 guest and the actual host.
The QEMU host doesn't even know that Xen is involved.  This is why Xen
and virtio don't currently work together (without my patches): the
device thinks it doesn't respect the IOMMU, the driver thinks the
device doesn't respect the IOMMU, and they're both wrong.

TL;DR: I think there are only two cases.  Either a device respects the
IOMMU or a device doesn't know whether it respects the IOMMU.  The
latter case is problematic.

--Andy

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