Re: [PATCH v5 21/27] vfio/cxl: Exclude the HDM decoder registers from direct BAR access

[Richard Cheng]

On Thu, Sep 17, 2026 at 12:05:34AM +0800, mhonap@xxxxxxxxxx wrote:
> From: Manish Honap <mhonap@xxxxxxxxxx>
> 
> The HDM decoder register block lives in the component BAR that vfio-pci
> owns. The guest reads the decoder only through the trapped, read-only
> decoder region; it must not reach the physical registers through the BAR
> directly, and a host read of the range through a kernel mapping could
> abort as a fatal host SError.
> 
> Add the block to the generic excluded-range list with
> VFIO_PCI_EXCLUDE_MMAP | VFIO_PCI_EXCLUDE_READ | VFIO_PCI_EXCLUDE_WRITE
> similar to the MSI-X exclusion.
>

Hi Manish,

I think we need to block eventfd write and BAR DMA as well ?

Have some comments below.

 
> Assisted-by: LLM
> Signed-off-by: Manish Honap <mhonap@xxxxxxxxxx>
> ---
>  drivers/vfio/pci/cxl/vfio_cxl_core.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/drivers/vfio/pci/cxl/vfio_cxl_core.c b/drivers/vfio/pci/cxl/vfio_cxl_core.c
> index da04776356e4..37e8a3b54cfb 100644
> --- a/drivers/vfio/pci/cxl/vfio_cxl_core.c
> +++ b/drivers/vfio/pci/cxl/vfio_cxl_core.c
> @@ -435,8 +435,25 @@ static int vfio_cxl_add_region(struct vfio_pci_core_device *vdev, u32 subtype,
>  static int vfio_cxl_open_device(struct vfio_pci_core_device *vdev)
>  {
>  	struct vfio_cxl_state *cxl = vdev->cxl;
> +	struct pci_dev *pdev = vdev->pdev;
>  	int ret;
>  
> +	/*
> +	 * Keep the HDM decoder block out of the guest's direct BAR access: the
> +	 * guest reaches it only through the trapped decoder region, and a host
> +	 * read of the range through a kernel mapping could abort as an SError.
> +	 * Exclude it from mmap, fill reads with -1 and drop writes. The list is
> +	 * cleared on close, so re-add it per open.
> +	 */
> +	ret = vfio_pci_core_add_excluded_range(vdev, pdev->hdm->hdm_bar,
> +					       pdev->hdm->hdm_offset,
> +					       cxl->hdm_len,
> +					       VFIO_PCI_EXCLUDE_MMAP |
> +					       VFIO_PCI_EXCLUDE_READ |
> +					       VFIO_PCI_EXCLUDE_WRITE);

This adds the HDM reg to the exclusion list, but the eventfd and BAR DMA path doesn't
check that.

Maybe we need to block those 2 path as well ?

Best regards,
Richard Cheng.


> +	if (ret)
> +		return ret;
> +
>  	/*
>  	 * vfio_pci_core_disable() frees all dynamic regions on close, so register
>  	 * them here per open rather than at bind. A failed first open never
> -- 
> 2.25.1
> 
>