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

[Leon Romanovsky <leon@kernel.org>]

On Thu, Nov 20, 2025 at 05:04:13PM -0700, Alex Williamson wrote:
> On Thu, 20 Nov 2025 11:28:29 +0200
> Leon Romanovsky <leon@kernel.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,

It seems right to me.

Thanks

> 
> Alex