Re: [RFC][PATCH 0/5 v2] DMA-BUF Heaps (destaging ION)

[Benjamin Gaignard <benjamin.gaignard@linaro.org>]

Le mer. 20 mars 2019 à 15:54, Andrew F. Davis <afd@ti.com> a écrit :
>
> On 3/20/19 4:16 AM, Benjamin Gaignard wrote:
> > Le mar. 19 mars 2019 à 23:36, John Stultz <john.stultz@linaro.org> a écrit :
> >>
> >> On Tue, Mar 19, 2019 at 2:58 PM Rob Clark <robdclark@gmail.com> wrote:
> >>>
> >>> On Tue, Mar 19, 2019 at 1:00 PM Andrew F. Davis <afd@ti.com> wrote:
> >>>>
> >>>> On 3/19/19 11:54 AM, Benjamin Gaignard wrote:
> >>>>> Le mer. 13 mars 2019 à 23:31, John Stultz <john.stultz@linaro.org> a écrit :
> >>>>>>
> >>>>>> On Wed, Mar 13, 2019 at 1:11 PM Liam Mark <lmark@codeaurora.org> wrote:
> >>>>>>> On Tue, 5 Mar 2019, John Stultz wrote:
> >>>>>>>>
> >>>>>>>> Eventual TODOS:
> >>>>>>>> * Reimplement page-pool for system heap (working on this)
> >>>>>>>> * Add stats accounting to system/cma heaps
> >>>>>>>> * Make the kselftest actually useful
> >>>>>>>> * Add other heaps folks see as useful (would love to get
> >>>>>>>>   some help from actual carveout/chunk users)!
> >>>>>>>
> >>>>>>> We use a modified carveout heap for certain secure use cases.
> >>>>>>
> >>>>>> Cool! It would be great to see if you have any concerns about adding
> >>>>>> such a secure-carveout heap to this framework. I suspect it would be
> >>>>>> fairly similar to how its integrated into ION, but particularly I'd be
> >>>>>> interested in issues around the lack of private flags and other
> >>>>>> allocation arguments like alignment.
> >>>>>>
> >>>>>>> Although there would probably be some benefit in discssing how the dma-buf
> >>>>>>> heap framework may want to support
> >>>>>>> secure heaps in the future it is a large topic which I assume you don't
> >>>>>>> want to tackle now.
> >>>>>>
> >>>>>> So I suspect others (Benjamin?) would have a more informed opinion on
> >>>>>> the details, but the intent is to allow secure heap implementations.
> >>>>>> I'm not sure what areas of concern you have for this allocation
> >>>>>> framework in particular?
> >>>>>
> >>>>> yes I would be great to understand how you provide the information to
> >>>>> tell that a dmabuf
> >>>>> is secure (or not) since we can't add flag in dmabuf structure itself.
> >>>>> An option is manage
> >>>>> the access rights when a device attach itself to the dmabuf but in
> >>>>> this case you need define
> >>>>> a list of allowed devices per heap...
> >>>>> If you have a good solution for secure heaps you are welcome :-)
> >>>>>
> >>>>
> >>>> Do we really need any of that? A secure buffer is secured by the
> >>>> hardware firewalls that keep out certain IP (including often the
> >>>> processor running Linux). So the only thing we need to track internally
> >>>> is that we should not allow mmap/kmap on the buffer. That can be done in
> >>>> the per-heap layer, everything else stays the same as a standard
> >>>> carveout heap.
> >>>
> >>> For at least some hw the importing driver needs to configure things
> >>> differently for secure buffers :-/
> >>
> >> Does the import ioctl need/use a flag for that then? Userland already
> >> has to keep meta-data about dmabufs around.
> >
> > To secure a buffer you need to know who is allowed to write/read it and
> > hardware block involved in the dataflow may need to know that the buffer
> > is secure to configure themself.
> > As example for a video decoding you allow hw video decoder to read in
> > a buffer and display to read it. You can also allow cpu to write on the buffer
> > to add subtitles. For that we need to be able to mmap/kmap the buffer.
> > Using a carveout heap for secure buffer mean that you reserved a large
> > memory region only for this purpose, that isn't possible on embedded device
> > where we are always limited in memory so we use CMA.
> > In the past I have used dmabuf's attach function to know who write into
> > the buffer and then configure who will be able to read it. It was working well
> > but the issue was how to in generic way this behavior.
> >
>
> Okay, I think I see what you are saying now.
>
> The way we handle secure playback is to firewall everything upfront and
> it is up to the application to inform the hardware about what it can and
> cannot do to the buffer, or simply not ask anything not allowed (E.g.
> writeback the decrypted stream) else it will get a firewall exception.
> The buffer itself doesn't have to carry any information.
>
> It sounds like you want the hardware driver to be able to detect the
> use-case based on the buffer itself and configure itself accordingly? Or
> the exporter at attach time to check access permissions?

Both are needed, the buffer client must know that it is a secure buffer and heap
will have to configure the permissions.

>
> The first would need a change to DMA-BUF framework, maybe an added flag.

Sumit will NACK that because dmabuf have to remain neutral and not embedded
flags for every possible usage.

> The second would just need a heap exporter with the system wide smarts,
> but as you say that is not very generic..

yes it is difficult to find a good solution for that.

>
> Andrew
>
> >>
> >> thanks
> >> -john