Re: Using Restricted DMA for virtio-pci

On Fri, 2025-03-21 at 14:32 -0400, Michael S. Tsirkin wrote:

On Fri, Mar 21, 2025 at 03:38:10PM +0000, David Woodhouse wrote:

quoted

On Tue, 2021-02-09 at 14:21 +0800, Claire Chang wrote:

quoted

This series implements mitigations for lack of DMA access control on
systems without an IOMMU, which could result in the DMA accessing the
system memory at unexpected times and/or unexpected addresses, possibly
leading to data leakage or corruption.

Replying to an ancient (2021) thread which has already been merged...

I'd like to be able to use this facility for virtio devices.

Virtio already has a complicated relationship with the DMA API, because
there were a bunch of early VMM bugs where the virtio devices where
magically exempted from IOMMU protection, but the VMM lied to the guest
and claimed they weren't.

With the advent of confidential computing, and the VMM (or whatever's
emulating the virtio device) not being *allowed* to arbitrarily access
all of the guest's memory, the DMA API becomes necessary again.

Either a virtual IOMMU needs to determine which guest memory the VMM
may access, or the DMA API is wrappers around operations which
share/unshare (or unencrypt/encrypt) the memory in question.

All of which is complicated and slow, if we're looking at a minimal
privileged hypervisor stub like pKVM which enforces the lack of guest
memory access from VMM.

I'm thinking of defining a new type of virtio-pci device which cannot
do DMA to arbitrary system memory. Instead it has an additional memory
BAR which is used as a SWIOTLB for bounce buffering.

The driver for it would look much like the existing virtio-pci device
except that it would register the restricted-dma region first (and thus
the swiotlb dma_ops), and then just go through the rest of the setup
like any other virtio device.

That seems like it ought to be fairly simple, and seems like a
reasonable way to allow an untrusted VMM to provide virtio devices with
restricted DMA access.

While I start actually doing the typing... does anyone want to start
yelling at me now? Christoph? mst? :)

I don't mind as such (though I don't understand completely), but since
this is changing the device anyway, I am a bit confused why you can't
just set the VIRTIO_F_ACCESS_PLATFORM feature bit?  This forces DMA API
which will DTRT for you, will it not?

That would be necessary but not sufficient. The question is *what* does
the DMA API do?

For a real passthrough PCI device, perhaps we'd have a vIOMMU exposed
to the guest so that it can do real protection with two-stage page
tables (IOVA→GPA under control of the guest, GPA→HPA under control of
the hypervisor). For that to work in the pKVM model though, you'd need
pKVM to be talking the guest's stage1 I/O page tables to see if a given
access from the VMM ought to be permitted?

Or for confidential guests there could be DMA ops which are an
'enlightenment'; a hypercall into pKVM to share/unshare pages so that
the VMM can actually access them, or SEV-SNP guests might mark pages
unencrypted to have the same effect with hardware protection.

Doing any of those dynamically to allow the VMM to access buffers in
arbitrary guest memory (when it wouldn't normally have access to
arbitrary guest memory) is complex and doesn't perform very well. And
exposes a full 4KiB page for any byte that needs to be made available.

Thus the idea of having a fixed range of memory to use for a SWIOTLB,
which is fairly much what the restricted DMA setup is all about.

We're just proposing that we build it in to a virtio-pci device model,
which automatically uses the extra memory BAR instead of the
restricted-dma-pool DT node.

It's basically just allowing us to expose through PCI, what I believe
we can already do for virtio in DT.