Re: [PATCH v6 10/11] vfio: Remove iommu group notifier

[Alex Williamson]

On Fri, 18 Feb 2022 08:55:20 +0800
Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx> wrote:

> The iommu core and driver core have been enhanced to avoid unsafe driver
> binding to a live group after iommu_group_set_dma_owner(PRIVATE_USER)
> has been called. There's no need to register iommu group notifier. This
> removes the iommu group notifer which contains BUG_ON() and WARN().
> 
> The commit 5f096b14d421b ("vfio: Whitelist PCI bridges") allowed all
> pcieport drivers to be bound with devices while the group is assigned to
> user space. This is not always safe. For example, The shpchp_core driver
> relies on the PCI MMIO access for the controller functionality. With its
> downstream devices assigned to the userspace, the MMIO might be changed
> through user initiated P2P accesses without any notification. This might
> break the kernel driver integrity and lead to some unpredictable
> consequences. As the result, currently we only allow the portdrv driver.
> 
> For any bridge driver, in order to avoiding default kernel DMA ownership
> claiming, we should consider:
> 
>  1) Does the bridge driver use DMA? Calling pci_set_master() or
>     a dma_map_* API is a sure indicate the driver is doing DMA
> 
>  2) If the bridge driver uses MMIO, is it tolerant to hostile
>     userspace also touching the same MMIO registers via P2P DMA
>     attacks?
> 
> Conservatively if the driver maps an MMIO region at all, we can say that
> it fails the test.

IIUC, there's a chance we're going to break user configurations if
they're assigning devices from a group containing a bridge that uses a
driver other than pcieport.  The recommendation to such an affected user
would be that the previously allowed host bridge driver was unsafe for
this use case and to continue to enable assignment of devices within
that group, the driver should be unbound from the bridge device or
replaced with the pci-stub driver.  Is that right?

Unfortunately I also think a bisect of such a breakage wouldn't land
here, I think it was actually broken in "vfio: Set DMA ownership for
VFIO" since that's where vfio starts to make use of
iommu_group_claim_dma_owner() which should fail due to
pci_dma_configure() calling iommu_device_use_default_domain() for
any driver not identifying itself as driver_managed_dma.

If that's correct, can we leave a breadcrumb in the correct commit log
indicating why this potential breakage is intentional and how the
bridge driver might be reconfigured to continue to allow assignment from
within the group more safely?  Thanks,

Alex