Re: [PATCH 03/11] PCI: pci_stub: Suppress kernel DMA ownership auto-claiming

Jason Gunthorpe
2021-11-15T16:18:09Z

On Mon, Nov 15, 2021 at 03:14:49PM +0000, Robin Murphy wrote:

> > If userspace has control of device A and can cause A to issue DMA to
> > arbitary DMA addresses then there are certain PCI topologies where A
> > can now issue peer to peer DMA and manipulate the MMMIO registers in
> > device B.
> >
> > A kernel driver on device B is thus subjected to concurrent
> > manipulation of the device registers from userspace.
> >
> > So, a 'safe' kernel driver is one that can tolerate this, and an
> > 'unsafe' driver is one where userspace can break kernel integrity.
>
> You mean in the case where the kernel driver is trying to use device B in a
> purely PIO mode, such that userspace might potentially be able to interfere
> with data being transferred in and out of the kernel?

s/PIO/MMIO, but yes basically. And not just data trasnfer but
userspace can interfere with the device state as well.

> Perhaps it's not so clear to put that under a notion of "DMA
> ownership", since device B's DMA is irrelevant and it's really much
> more equivalent to /dev/mem access or mmaping BARs to userspace
> while a driver is bound.

It is DMA ownership because device A's DMA is what is relevant
here. device A's DMA compromises device B. So device A asserts it has
USER ownership for DMA.

Any device in a group with USER ownership is incompatible with a
kernel driver.

> > The second issue is DMA - because there is only one iommu_domain
> > underlying many devices if we give that iommu_domain to userspace it
> > means the kernel DMA API on other devices no longer works.
>
> Actually, the DMA API itself via iommu-dma will "work" just fine in the
> sense that it will still successfully perform all its operations in the
> unattached default domain, it's just that if the driver then programs the
> device to access the returned DMA address, the device is likely to get a
> nasty surprise.

A DMA API that returns an dma_ddr_t that does not result in data
transfer to the specified buffers is not working, in my book - it
breaks the API contract.

> > So no kernel driver doing DMA can work at all, under any PCI topology,
> > if userspace owns the IO page table.
>
> This isn't really about userspace at all - it's true of any case where a
> kernel driver wants to attach a grouped device to its own unmanaged
> domain.

This is true for the dma api issue in isolation.

I think if we have a user someday it would make sense to add another
API DMA_OWNER_DRIVER_DOMAIN that captures how the dma API doesn't work
but DMA MMIO attacks are not possible.

> The fact that the VFIO kernel driver uses its unmanaged domains to map user
> pages upon user requests is merely a VFIO detail, and VFIO happens to be the
> only common case where unmanaged domains and non-singleton groups intersect.
> I'd say that, logically, if you want to put policy on mutual driver/usage
> compatibility anywhere it should be in iommu_attach_group().

It would make sense for iommu_attach_group() to require that the
DMA_OWNERSHIP is USER or DRIVER_DOMAIN.

That has a nice symmetry with iommu_attach_device() already requiring
that the group has a single device. For a driver to use these APIs it
must ensure security, one way or another.

That is a good idea, but requires understanding what tegra is
doing. Maybe tegra is that DMA_OWNER_DRIVER_DOMAIN user?

I wouldn't want to see iommu_attach_group() change the DMA_OWNERSHIP,
I think ownership is cleaner as a dedicated API. Adding a file * and
probably the enum to iommu_attach_group() feels weird.

We need the dedicated API for the dma_configure op, and keeping
ownership split from the current domain makes more sense with the
design in the iommfd RFC.

Thanks,
Jason