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SubjectRe: [PATCH v5 12/16] PCI: Add pci_iomap_host_shared(), pci_iomap_host_shared_range()
On Sat, Oct 9, 2021 at 2:53 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Fri, Oct 08, 2021 at 05:37:07PM -0700, Kuppuswamy Sathyanarayanan wrote:
> > From: Andi Kleen <ak@linux.intel.com>
> >
> > For Confidential VM guests like TDX, the host is untrusted and hence
> > the devices emulated by the host or any data coming from the host
> > cannot be trusted. So the drivers that interact with the outside world
> > have to be hardened by sharing memory with host on need basis
> > with proper hardening fixes.
> >
> > For the PCI driver case, to share the memory with the host add
> > pci_iomap_host_shared() and pci_iomap_host_shared_range() APIs.
> >
> > Signed-off-by: Andi Kleen <ak@linux.intel.com>
> > Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
>
> So I proposed to make all pci mappings shared, eliminating the need
> to patch drivers.
>
> To which Andi replied
> One problem with removing the ioremap opt-in is that
> it's still possible for drivers to get at devices without going through probe.
>
> To which Greg replied:
> https://lore.kernel.org/all/YVXBNJ431YIWwZdQ@kroah.com/
> If there are in-kernel PCI drivers that do not do this, they need to be
> fixed today.
>
> Can you guys resolve the differences here?

I agree with you and Greg here. If a driver is accessing hardware
resources outside of the bind lifetime of one of the devices it
supports, and in a way that neither modrobe-policy nor
device-authorization -policy infrastructure can block, that sounds
like a bug report. Fix those drivers instead of sprinkling
ioremap_shared in select places and with unclear rules about when a
driver is allowed to do "shared" mappings. Let the new
device-authorization mechanism (with policy in userspace) be the
central place where all of these driver "trust" issues are managed.

> And once they are resolved, mention this in the commit log so
> I don't get to re-read the series just to find out nothing
> changed in this respect?
>
> I frankly do not believe we are anywhere near being able to harden
> an arbitrary kernel config against attack.
> How about creating a defconfig that makes sense for TDX then?
> Anyone deviating from that better know what they are doing,
> this API tweaking is just putting policy into the kernel ...

Right, userspace authorization policy and select driver fixups seems
to be the answer to the raised concerns.

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