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

Shuai Xue
2025-10-26T07:55:15Z

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.

Thanks.