Re: [PATCH v3 6/7] vfio: Wait for dma-buf invalidation to complete

Jason Gunthorpe
2026-01-22T23:44:09Z

On Thu, Jan 22, 2026 at 12:32:03PM +0100, Christian König wrote:
> >> What roughly happens is that each DMA-buf mapping through a couple
> >> of hoops keeps a reference on the device, so even after a hotplug
> >> event the device can only fully go away after all housekeeping
> >> structures are destroyed and buffers freed.
> >
> > A simple reference on the device means nothing for these kinds of
> > questions. It does not stop unloading and reloading a driver.
>
> Well as far as I know it stops the PCIe address space from being re-used.
>
> So when you do an "echo 1 > remove" and then an re-scan on the
> upstream bridge that works, but you get different addresses for your
> MMIO BARs!

That's pretty a niche scenario.. Most people don't rescan their PCI
bus. If you just do rmmod/insmod then it will be re-used, there is no
rescan to move the MMIO around on that case.

> Oh, well I never looked to deeply into that.
>
> As far as I know it doesn't block, but rather the last drm_dev_put()
> just cleans things up.
>
> And we have a CI test system which exercises that stuff over and
> over again because we have a big customer depending on that.

I doubt a CI would detect a UAF like we are discussing here..

Connect a RDMA pinned importer. Do rmmod. If rmmod doesn't hang the
driver has a UAF on some RAS cases. Not great, but is unlikely to
actually trouble any real user.

Jason