Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Mar 19, 2021 at 11:22:21AM -0700, Jacob Pan wrote:
> Hi Jason,
> 
> On Fri, 19 Mar 2021 10:54:32 -0300, Jason Gunthorpe <jgg@nvidia.com> wrote:
> 
> > On Fri, Mar 19, 2021 at 02:41:32PM +0100, Jean-Philippe Brucker wrote:
> > > On Fri, Mar 19, 2021 at 09:46:45AM -0300, Jason Gunthorpe wrote:  
> > > > On Fri, Mar 19, 2021 at 10:58:41AM +0100, Jean-Philippe Brucker wrote:
> > > >   
> > > > > Although there is no use for it at the moment (only two upstream
> > > > > users and it looks like amdkfd always uses current too), I quite
> > > > > like the client-server model where the privileged process does
> > > > > bind() and programs the hardware queue on behalf of the client
> > > > > process.  
> > > > 
> > > > This creates a lot complexity, how do does process A get a secure
> > > > reference to B? How does it access the memory in B to setup the HW?  
> > > 
> > > mm_access() for example, and passing addresses via IPC  
> > 
> > I'd rather the source process establish its own PASID and then pass
> > the rights to use it to some other process via FD passing than try to
> > go the other way. There are lots of security questions with something
> > like mm_access.
> > 
> 
> Thank you all for the input, it sounds like we are OK to remove mm argument
> from iommu_sva_bind_device() and iommu_sva_alloc_pasid() for now?
> 
> Let me try to summarize PASID allocation as below:
> 
> Interfaces	| Usage	|  Limit	| bind¹ |User visible
> /dev/ioasid²	| G-SVA/IOVA	|  cgroup	| No	|Yes
> char dev³	| SVA		|  cgroup	| Yes	|No
> iommu driver	| default PASID|  no		| No	|No
> kernel		| super SVA	| no		| yes   |No
> 
> ¹ Allocated during SVA bind
> ² PASIDs allocated via /dev/ioasid are not bound to any mm. But its
>   ownership is assigned to the process that does the allocation.

What does "not bound to a mm" mean?

IMHO a use created PASID is either bound to a mm (current) at creation
time, or it will never be bound to a mm and its page table is under
user control via /dev/ioasid.

I thought the whole point of something like a /dev/ioasid was to get
away from each and every device creating its own PASID interface?

It maybe somewhat reasonable that some devices could have some easy
'make a SVA PASID on current' interface built in, but anything more
complicated should use /dev/ioasid, and anything consuming PASID
should also have an API to import and attach a PASID from /dev/ioasid.

> Currently, the proposed /dev/ioasid interface does not map individual PASID
> with an FD. The FD is at the ioasid_set granularity and bond to the current
> mm. We could extend the IOCTLs to cover individual PASID-FD passing case
> when use cases arise. Would this work?

Is it a good idea that the FD is per ioasid_set ? What is the set used
for?

Usually kernel interfaces work nicer with a one fd/one object model.

But even if it is a set, you could pass the set between co-operating
processes and the PASID can be created in the correct 'current'. But
there is all kinds of security questsions as soon as you start doing
anything like this - is there really a use case?

Jason