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

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Message-ID: <20260122234404.GB1589888@ziepe.ca>
Date: Thu, 22 Jan 2026 19:44:04 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Christian König <christian.koenig@....com>
Cc: Leon Romanovsky <leon@...nel.org>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Alex Deucher <alexander.deucher@....com>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Gerd Hoffmann <kraxel@...hat.com>,
	Dmitry Osipenko <dmitry.osipenko@...labora.com>,
	Gurchetan Singh <gurchetansingh@...omium.org>,
	Chia-I Wu <olvaffe@...il.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Lucas De Marchi <lucas.demarchi@...el.com>,
	Thomas Hellström <thomas.hellstrom@...ux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@...el.com>,
	Kevin Tian <kevin.tian@...el.com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	Felix Kuehling <Felix.Kuehling@....com>,
	Alex Williamson <alex@...zbot.org>,
	Ankit Agrawal <ankita@...dia.com>,
	Vivek Kasireddy <vivek.kasireddy@...el.com>,
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
	amd-gfx@...ts.freedesktop.org, virtualization@...ts.linux.dev,
	intel-xe@...ts.freedesktop.org, linux-rdma@...r.kernel.org,
	iommu@...ts.linux.dev, kvm@...r.kernel.org
Subject: Re: [PATCH v3 6/7] vfio: Wait for dma-buf invalidation to complete

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