Re: [PATCH v4 11/15] pci: Add pci_iomap_shared{,_range}

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.

Right. So given that, why do we need to tweak a random API like the map?
If you just make all maps be shared then the user is in control.
Seems sensible to me.

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

Let's be frank, even without encryption disabling most drivers -
especially weird ones that poke at hardware before probe -
is far safer than keeping them, but one loses a bunch of features.
IOW all this hardening is nice but which security/feature tradeoff
to take it a policy decision, not something kernel should do
imho.

-- 
MST