Re: IOMMU Page faults when running DMA transfers from PCIe device

[Jerome Glisse <jglisse@redhat.com>]

On Thu, Apr 18, 2019 at 09:37:58AM +0000, David Laight wrote:
> From: Jerome Glisse
> > Sent: 16 April 2019 16:33
> ...
> > I am no expert but i am guessing your FPGA set the request field in the
> > PCIE TLP write packet to 00:00.0 and this might work when IOMMU is off but
> > might not work when IOMMU is on ie when IOMMU is on your device should set
> > the request field to the FPGA PCIE id so that the IOMMU knows for which
> > device the PCIE write or read packet is and thus against which IOMMU page
> > table.
> 
> Interesting.
> Does that mean that a malicious PCIe device can send write TLP
> that contain the 'wrong' id (IIRC that is bus:dev:fn) and so
> write to areas that it shouldn't access?

Yes it does, they are bunch of paper on that look for IOMMU DMA
attack.

> 
> For any degree of security the PCIe bridge nearest the target
> needs to verify the id as well.
> Actually all bridges need to verify the 'bus' part.
> Then boards with 'dodgy' bridges can only write to locations
> that other dev:fn on the same board can access.

Yes they should but it has a cost and AFAIK no bridges, not even
the root port, does that. PCIE bandwidth is big and it means a
lot of packets can go through a PCIE switch or PCIE bridge and
i believe that such PCIE packet inspection have been considered
too costly. Afterall if someone can plug a rogue device to your
computer (ignoring laptop) then he can do more harm with easier
method. FGPA accelerator as PCIE device, might open a door for
clever and _resourceful_ people to try to use them as a remote
vector attack.

Cheers,
Jérôme