[+cc Rajat; I still don't know what "shared memory with a hypervisor
in a confidential guest" means, but now we're talking about hardened
drivers and allow lists, which Rajat is interested in]

On Tue, Aug 24, 2021 at 10:20:44AM -0700, Andi Kleen wrote:

quoted

I see. Hmm. It's a bit of a random thing to do it at the map time
though. E.g. DMA is all handled transparently behind the DMA API.
Hardening is much more than just replacing map with map_shared
and I suspect what you will end up with is basically
vendors replacing map with map shared to make things work
for their users and washing their hands.

That concept exists too. There is a separate allow list for the drivers. So
just adding shared to a driver is not enough, until it's also added to the
allowlist

Users can of course chose to disable the allowlist, but they need to
understand the security implications.

quoted

I would say an explicit flag in the driver that says "hardened"
and refusing to init a non hardened one would be better.

We have that too (that's the device filtering)

But the problem is that device filtering just stops the probe functions, not
the initcalls, and lot of legacy drivers do MMIO interactions before going
into probe. In some cases it's unavoidable because of the device doesn't
have a separate enumeration mechanism it needs some kind of probing to even
check for its existence And since we don't want to change all of them it's
far safer to make the ioremap opt-in.


-Andi