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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20251027120904.GA896317@nvidia.com>
Date: Mon, 27 Oct 2025 09:09:04 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Shuai Xue <xueshuai@...ux.alibaba.com>
Cc: Leon Romanovsky <leon@...nel.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	Leon Romanovsky <leonro@...dia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Christian König <christian.koenig@....com>,
	dri-devel@...ts.freedesktop.org, iommu@...ts.linux.dev,
	Jens Axboe <axboe@...nel.dk>, Joerg Roedel <joro@...tes.org>,
	kvm@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-media@...r.kernel.org, linux-mm@...ck.org,
	linux-pci@...r.kernel.org, Logan Gunthorpe <logang@...tatee.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Vivek Kasireddy <vivek.kasireddy@...el.com>,
	Will Deacon <will@...nel.org>
Subject: Re: [PATCH v5 9/9] vfio/pci: Add dma-buf export support for MMIO
 regions

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@...dia.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