Re: [PATCH v4 1/2] iommu/sva: Tighten SVA bind API with explicit flags

Jason Gunthorpe
2021-05-13T19:00:46Z

On Thu, May 13, 2021 at 11:53:49AM -0700, Luck, Tony wrote:

> I'd like people to think of DSA as an extension to the instruction
> set. It implements asynchronous instructions like "MEMFILL" and
> "MEMCOPY". These can be limited in scope when executed in user
> processes or guests. But when executed by the host OS ring0 code
> they can have all the same access that ring0 code has when it
> dereferences a pointer.

If you want this then be explicit about what it is you are making when
building the API. Don't try to hide it under some generic idea of
"kernel PCI DMA SVA"

Add appropriately safe APIs that might have a chance of making it
secure, eg by confirming it is a physically trusted on-socket device.

It is not just Thunderbolt devices that could trigger this, many
devices with programmable firmware can spoof PCI DID/VID - having an
exploit chain that compromises a in-system FW device, chains it to
faking a IDXD HW, then accessing the all-memory PASID is pretty
alarming if the admin thought they had protection against DMA attacks.

I could easially see an admin option to "turn this off" entirely as
being too dangerous, especially if the users have no interest in IDXD.

Which is why being explicit of intent is really important.

Jason