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

From: Lu Baolu <baolu.lu@linux.intel.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Stuart Yoder <stuyoder@gmail.com>,
	rafael@kernel.org, David Airlie <airlied@linux.ie>,
	linux-pci@vger.kernel.org,
	Thierry Reding <thierry.reding@gmail.com>,
	Diana Craciun <diana.craciun@oss.nxp.com>,
	Will Deacon <will@kernel.org>, Ashok Raj <ashok.raj@intel.com>,
	Jonathan Hunter <jonathanh@nvidia.com>,
	Christoph Hellwig <hch@infradead.org>,
	Jason Gunthorpe <jgg@nvidia.com>,
	Kevin Tian <kevin.tian@intel.com>,
	Chaitanya Kulkarni <kch@nvidia.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	kvm@vger.kernel.org, Bjorn Helgaas <bhelgaas@google.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Cornelia Huck <cohuck@redhat.com>,
	linux-kernel@vger.kernel.org, Li Yang <leoyang.li@nxp.com>,
	iommu@lists.linux-foundation.org,
	Jacob jun Pan <jacob.jun.pan@intel.com>,
	Daniel Vetter <daniel@ffwll.ch>,
	Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH v2 02/17] driver core: Add dma_unconfigure callback in bus_type
Date: Mon, 29 Nov 2021 12:03:30 +0800	[thread overview]
Message-ID: <0eca1892-a063-a695-ac35-0ac1e2de28e0@linux.intel.com> (raw)
In-Reply-To: <YaM3slBGozqxsQ+m@kroah.com>

On 11/28/21 4:02 PM, Greg Kroah-Hartman wrote:
> 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@nvidia.com>
>> Link: https://lore.kernel.org/linux-iommu/20210922123931.GI327412@nvidia.com/
>> Link: https://lore.kernel.org/linux-iommu/20210928115751.GK964074@nvidia.com/
>> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
>> ---
>>   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?

I agree with you. dma_cleanup() is more explicit and better here.

> 
> thanks,
> 
> greg k-h
> 

Best regards,
baolu
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