Re: [PATCH 1/9] vfio/pci: Fix vfio_pci_dma_buf_cleanup() double-put

[Alex Williamson]

On Thu, 16 Apr 2026 06:17:44 -0700
Matt Evans <mattev@xxxxxxxx> wrote:

> vfio_pci_dma_buf_cleanup() assumed all VFIO device DMABUFs need to be
> revoked.  However, if vfio_pci_dma_buf_move() revokes DMABUFs before
> the fd/device closes, then vfio_pci_dma_buf_cleanup() would do a
> second/underflowing kref_put() then wait_for_completion() on a
> completion that never fires.  Fixed by predicating on revocation
> status.
> 
> This could happen if PCI_COMMAND_MEMORY is cleared before closing the
> device fd (but the scenario is more likely to hit when future commits
> add more methods to revoke DMABUFs).
> 
> Fixes: 1a8a5227f2299 ("vfio: Wait for dma-buf invalidation to complete")
> Signed-off-by: Matt Evans <mattev@xxxxxxxx>
> ---
> 
> (Just a fix, but later "vfio/pci: Convert BAR mmap() to use a DMABUF"
> and "vfio/pci: Permanently revoke a DMABUF on request" depend on this
> context, so including in this series.)

We really need a fix for this split out from this series, It's already
been shown[1] that this is trivially reachable.  Carlos proposed[2] a
similar solution to the one below.  I was concurrently working on the
issued and suggested an alternative[3].  Let's pick a solution for
7.1-rc.  Thanks,

Alex

[1]https://lore.kernel.org/all/GVXPR02MB12019AA6014F27EF5D773E89BFB372@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
[2]https://lore.kernel.org/all/20260429182736.409323-2-clopez@xxxxxxx/
[3]https://lore.kernel.org/all/20260429142242.70f746b4@xxxxxxxxxx/

 
> drivers/vfio/pci/vfio_pci_dmabuf.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
> index 281ba7d69567..04478b7415a0 100644
> --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
> +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
> @@ -395,20 +395,25 @@ void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev)
>  
>  	down_write(&vdev->memory_lock);
>  	list_for_each_entry_safe(priv, tmp, &vdev->dmabufs, dmabufs_elm) {
> +		bool was_revoked;
> +
>  		if (!get_file_active(&priv->dmabuf->file))
>  			continue;
>  
>  		dma_resv_lock(priv->dmabuf->resv, NULL);
>  		list_del_init(&priv->dmabufs_elm);
>  		priv->vdev = NULL;
> +		was_revoked = priv->revoked;
>  		priv->revoked = true;
>  		dma_buf_invalidate_mappings(priv->dmabuf);
>  		dma_resv_wait_timeout(priv->dmabuf->resv,
>  				      DMA_RESV_USAGE_BOOKKEEP, false,
>  				      MAX_SCHEDULE_TIMEOUT);
>  		dma_resv_unlock(priv->dmabuf->resv);
> -		kref_put(&priv->kref, vfio_pci_dma_buf_done);
> -		wait_for_completion(&priv->comp);
> +		if (!was_revoked) {
> +			kref_put(&priv->kref, vfio_pci_dma_buf_done);
> +			wait_for_completion(&priv->comp);
> +		}
>  		vfio_device_put_registration(&vdev->vdev);
>  		fput(priv->dmabuf->file);
>  	}