Re: [RFC 06/20] iommu: Add iommu_device_init[exit]_user_dma interfaces

David Gibson
2021-09-30T04:19:37Z

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On Wed, Sep 29, 2021 at 09:57:16AM -0300, Jason Gunthorpe wrote:
> On Wed, Sep 29, 2021 at 04:35:19PM +1000, David Gibson wrote:
>
> > Yes, exactly. And with a group interface it's obvious it has to
> > understand it. With the non-group interface, you can get to this
> > stage in ignorance of groups. It will even work as long as you are
> > lucky enough only to try with singleton-group devices. Then you try
> > it with two devices in the one group and doing (3) on device A will
> > implicitly change the DMA environment of device B.
>
> The security model here says this is fine.

I'm not making a statement about the security model, I'm making a
statement about surprisingness of the programming interface. In your
program you have devices A & B, you perform an operation that
specifies only device A and device B changes behaviour.

> This idea to put the iommu code in charge of security is quite clean,
> as I said in the other mail drivers attached to 'struct devices *'
> tell the iommu layer what they are are doing:
>
> iommu_set_device_dma_owner(dev, DMA_OWNER_KERNEL, NULL)
> iommu_set_device_dma_owner(dev, DMA_OWNER_SHARED, NULL)
> iommu_set_device_dma_owner(dev, DMA_OWNER_USERSPACE, group_file/iommu_file)
>
> And it decides if it is allowed.
>
> If device A is allowed to go to userspace then security wise it is
> deemed fine that B is impacted. That is what we have defined already
> today.
>
> This proposal does not free userpace from having to understand this!
> The iommu_group sysfs is still there and still must be understood.
>
> The *admin* the one responsible to understand the groups, not the
> applications. The admin has no idea what a group FD is - they should
> be looking at the sysfs and seeing the iommu_group directories.

Not just the admin. If an app is given two devices in the same group
to use *both* it must understand that and act accordingly.

--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson

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