Re: [PATCH v5 9/9] vfio/pci: Add dma-buf export support for MMIO regions

Jason Gunthorpe
2025-10-27T12:09:10Z

On Sun, Oct 26, 2025 at 03:55:04PM +0800, Shuai Xue wrote:
>
>
> 在 2025/10/22 20:50, Jason Gunthorpe 写道:
> > On Mon, Oct 13, 2025 at 06:26:11PM +0300, Leon Romanovsky wrote:
> > > From: Leon Romanovsky <leonro@nvidia.com>
> > >
> > > Add support for exporting PCI device MMIO regions through dma-buf,
> > > enabling safe sharing of non-struct page memory with controlled
> > > lifetime management. This allows RDMA and other subsystems to import
> > > dma-buf FDs and build them into memory regions for PCI P2P operations.
> > >
> > > The implementation provides a revocable attachment mechanism using
> > > dma-buf move operations. MMIO regions are normally pinned as BARs
> > > don't change physical addresses, but access is revoked when the VFIO
> > > device is closed or a PCI reset is issued. This ensures kernel
> > > self-defense against potentially hostile userspace.
> >
> > Let's enhance this:
> >
> > Currently VFIO can take MMIO regions from the device's BAR and map
> > them into a PFNMAP VMA with special PTEs. This mapping type ensures
> > the memory cannot be used with things like pin_user_pages(), hmm, and
> > so on. In practice only the user process CPU and KVM can safely make
> > use of these VMA. When VFIO shuts down these VMAs are cleaned by
> > unmap_mapping_range() to prevent any UAF of the MMIO beyond driver
> > unbind.
> >
> > However, VFIO type 1 has an insecure behavior where it uses
> > follow_pfnmap_*() to fish a MMIO PFN out of a VMA and program it back
> > into the IOMMU. This has a long history of enabling P2P DMA inside
> > VMs, but has serious lifetime problems by allowing a UAF of the MMIO
> > after the VFIO driver has been unbound.
>
> Hi, Jason,
>
> Can you elaborate on this more?
>
> From my understanding of the VFIO type 1 implementation:
>
> - When a device is opened through VFIO type 1, it increments the
> device->refcount
> - During unbind, the driver waits for this refcount to drop to zero via
> wait_for_completion(&device->comp)
> - This should prevent the unbind() from completing while the device is
> still in use
>
> Given this refcount mechanism, I do not figure out how the UAF can
> occur.

A second vfio device can be opened and then use follow_pfnmap_*() to
read the first vfio device's PTEs. There is no relationship betweent
the first and second VFIO devices, so once the first is unbound it
sails through the device->comp while the second device retains the PFN
in its type1 iommu_domain.

Jason