Re: [PATCH v2 02/17] driver core: Add dma_unconfigure callback in bus_type

[Greg Kroah-Hartman]

On Sun, Nov 28, 2021 at 10:50:36AM +0800, Lu Baolu wrote:
> The bus_type structure defines dma_configure() callback for bus drivers
> to configure DMA on the devices. This adds the paired dma_unconfigure()
> callback and calls it during driver unbinding so that bus drivers can do
> some cleanup work.
> 
> One use case for this paired DMA callbacks is for the bus driver to check
> for DMA ownership conflicts during driver binding, where multiple devices
> belonging to a same IOMMU group (the minimum granularity of isolation and
> protection) may be assigned to kernel drivers or user space respectively.
> 
> Without this change, for example, the vfio driver has to listen to a bus
> BOUND_DRIVER event and then BUG_ON() in case of dma ownership conflict.
> This leads to bad user experience since careless driver binding operation
> may crash the system if the admin overlooks the group restriction. Aside
> from bad design, this leads to a security problem as a root user, even with
> lockdown=integrity, can force the kernel to BUG.
> 
> With this change, the bus driver could check and set the DMA ownership in
> driver binding process and fail on ownership conflicts. The DMA ownership
> should be released during driver unbinding.
> 
> Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Link: https://lore.kernel.org/linux-iommu/20210922123931.GI327412@xxxxxxxxxx/
> Link: https://lore.kernel.org/linux-iommu/20210928115751.GK964074@xxxxxxxxxx/
> Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> ---
>  include/linux/device/bus.h | 3 +++
>  drivers/base/dd.c          | 7 ++++++-
>  2 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/device/bus.h b/include/linux/device/bus.h
> index a039ab809753..ef54a71e5f8f 100644
> --- a/include/linux/device/bus.h
> +++ b/include/linux/device/bus.h
> @@ -59,6 +59,8 @@ struct fwnode_handle;
>   *		bus supports.
>   * @dma_configure:	Called to setup DMA configuration on a device on
>   *			this bus.
> + * @dma_unconfigure:	Called to cleanup DMA configuration on a device on
> + *			this bus.

"dma_cleanup()" is a better name for this, don't you think?

thanks,

greg k-h