Re: [RFC PATCH v5 0/5] vfio-pci: Add support for mmapping MSI-X table

[Benjamin Herrenschmidt <benh-XVmvHMARGAS8U2dJNN8I7kB+6BGkLq7r@public.gmane.org>]

On Tue, 2017-08-15 at 09:16 +0800, Jike Song wrote:
> > Taking a step back, though, why does vfio-pci perform this check in the
> > first place? If a malicious guest already has control of a device, any
> > kind of interrupt spoofing it could do by fiddling with the MSI-X
> > message address/data it could simply do with a DMA write anyway, so the
> > security argument doesn't stand up in general (sure, not all PCIe
> > devices may be capable of arbitrary DMA, but that seems like more of a
> > tenuous security-by-obscurity angle to me).

I tried to make that point for years, thanks for re-iterating it :-)

> Hi Robin,
> 
> DMA writes will be translated (thereby censored) by DMA Remapping hardware,
> while MSI/MSI-X will not. Is this different for non-x86?

There is no way your DMA remapping HW can differenciate. The only
difference between a DMA write and an MSI is ... the address. So if I
can make my device DMA to the MSI address range, I've defeated your
security.

The table obfuscating in qemu is only useful as an insecure way of
"making things sort-of-work" for HW that doesnt have proper remapping
or filtering.

On pseries we don't have that problem because:

 1) Our hypervisor (which is qemu) provide the DMA address for MSIs/X
so there is no need for "magic remapping" to give the guest a value
that works.

 2) Our HW (configured by VFIO/KVM) filters which device can DMA to
what address (including which MSIs/X) thus even if the guest doesn't
use the address passed and messes around, it can only shoot itself in
the foot.

So all we need is a way to tell qemu to stop doing that filtering on
our platform. This is *one bit* of information, it's taken 3 years of
arguments and we still don't have a solution. In the meantime,
workloads *are* being hurt by significant performance degradation due
to the MSI-X table sharing a 64K page (our page size) with other MMIOs.

Yay !

Ben.