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

[Michael S. Tsirkin]

On Mon, Aug 23, 2021 at 07:14:18PM -0700, Andi Kleen wrote:
> 
> On 8/23/2021 6:04 PM, Dan Williams wrote:
> > On Mon, Aug 23, 2021 at 5:31 PM Kuppuswamy, Sathyanarayanan
> > <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx> wrote:
> > > 
> > > 
> > > On 8/23/21 4:56 PM, Michael S. Tsirkin wrote:
> > > > > Add a new variant of pci_iomap for mapping all PCI resources
> > > > > of a devices as shared memory with a hypervisor in a confidential
> > > > > guest.
> > > > > 
> > > > > Signed-off-by: Andi Kleen<ak@xxxxxxxxxxxxxxx>
> > > > > Signed-off-by: Kuppuswamy Sathyanarayanan<sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> > > > I'm a bit puzzled by this part. So why should the guest*not*  map
> > > > pci memory as shared? And if the answer is never (as it seems to be)
> > > > then why not just make regular pci_iomap DTRT?
> > > It is in the context of confidential guest (where VMM is un-trusted). So
> > > we don't want to make all PCI resource as shared. It should be allowed
> > > only for hardened drivers/devices.
> > That's confusing, isn't device authorization what keeps unaudited
> > drivers from loading against untrusted devices? I'm feeling like
> > Michael that this should be a detail that drivers need not care about
> > explicitly, in which case it does not need to be exported because the
> > detail can be buried in lower levels.
> 
> We originally made it default (similar to AMD), but it during code audit we
> found a lot of drivers who do ioremap early outside the probe function.
> Since it would be difficult to change them all we made it opt-in, which
> ensures that only drivers that have been enabled can talk with the host at
> all and can't be attacked. That made the problem of hardening all these
> drivers a lot more practical.
> 
> Currently we only really need virtio and MSI-X shared, so for changing two
> places in the tree you avoid a lot of headache elsewhere.
> 
> Note there is still a command line option to override if you want to allow
> and load other drivers.
> 
> -Andi

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.

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

-- 
MST