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

On 8/23/2021 6:04 PM, Dan Williams wrote:

On Mon, Aug 23, 2021 at 5:31 PM Kuppuswamy, Sathyanarayanan
[off-list ref] wrote:

quoted

On 8/23/21 4:56 PM, Michael S. Tsirkin wrote:

quoted

quoted

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<redacted>
Signed-off-by: Kuppuswamy Sathyanarayanan<sathyanarayanan.kuppuswamy@linux.intel.com>

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We originally made it default (similar to AMD), but it during code audit 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

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