linux-kernel - Re: [PATCH v9 10/11] vfio/pci: Add dma-buf export support for MMIO regions

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Message-ID: <20251120170413.050ccbb5.alex@shazbot.org>
Date: Thu, 20 Nov 2025 17:04:13 -0700
From: Alex Williamson <alex@...zbot.org>
To: Leon Romanovsky <leon@...nel.org>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>,
 Logan Gunthorpe <logang@...tatee.com>, Jens Axboe <axboe@...nel.dk>,
 Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>,
 Marek Szyprowski <m.szyprowski@...sung.com>,
 Jason Gunthorpe <jgg@...pe.ca>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Jonathan Corbet <corbet@....net>, Sumit Semwal <sumit.semwal@...aro.org>,
 Christian König <christian.koenig@....com>,
 Kees Cook <kees@...nel.org>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 Ankit Agrawal <ankita@...dia.com>, Yishai Hadas <yishaih@...dia.com>,
 Shameer Kolothum <skolothumtho@...dia.com>,
 Kevin Tian <kevin.tian@...el.com>, Krishnakant Jaju <kjaju@...dia.com>,
 Matt Ochs <mochs@...dia.com>, linux-pci@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
 iommu@...ts.linux.dev, linux-mm@...ck.org, linux-doc@...r.kernel.org,
 linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linaro-mm-sig@...ts.linaro.org, kvm@...r.kernel.org,
 linux-hardening@...r.kernel.org,
 Vivek Kasireddy <vivek.kasireddy@...el.com>
Subject: Re: [PATCH v9 10/11] vfio/pci: Add dma-buf export support for MMIO
 regions

On Thu, 20 Nov 2025 11:28:29 +0200
Leon Romanovsky <leon@...nel.org> wrote:
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index 142b84b3f225..51a3bcc26f8b 100644
> --- a/drivers/vfio/pci/vfio_pci_core.c
> +++ b/drivers/vfio/pci/vfio_pci_core.c
...
> @@ -2487,8 +2500,11 @@ static int vfio_pci_dev_set_hot_reset(struct vfio_device_set *dev_set,
>  
>  err_undo:
>  	list_for_each_entry_from_reverse(vdev, &dev_set->device_list,
> -					 vdev.dev_set_list)
> +					 vdev.dev_set_list) {
> +		if (__vfio_pci_memory_enabled(vdev))
> +			vfio_pci_dma_buf_move(vdev, false);
>  		up_write(&vdev->memory_lock);
> +	}

I ran into a bug here.  In the hot reset path we can have dev_sets
where one or more devices are not opened by the user.  The vconfig
buffer for the device is established on open.  However:

bool __vfio_pci_memory_enabled(struct vfio_pci_core_device *vdev)
{
        struct pci_dev *pdev = vdev->pdev;
        u16 cmd = le16_to_cpu(*(__le16 *)&vdev->vconfig[PCI_COMMAND]);
	...

Leads to a NULL pointer dereference.

I think the most straightforward fix is simply to test the open_count
on the vfio_device, which is also protected by the dev_set->lock that
we already hold here:

--- a/drivers/vfio/pci/vfio_pci_core.c
+++ b/drivers/vfio/pci/vfio_pci_core.c
@@ -2501,7 +2501,7 @@ static int vfio_pci_dev_set_hot_reset(struct vfio_device_set *dev_set,
 err_undo:
        list_for_each_entry_from_reverse(vdev, &dev_set->device_list,
                                         vdev.dev_set_list) {
-               if (__vfio_pci_memory_enabled(vdev))
+               if (vdev->vdev.open_count && __vfio_pci_memory_enabled(vdev))
                        vfio_pci_dma_buf_move(vdev, false);
                up_write(&vdev->memory_lock);
        }

Any other suggestions?  This should be the only reset path with this
nuance of affecting non-opened devices.  Thanks,

Alex