Re: [RFC PATCH] ALSA: core: Add DMA share buffer support

From: Takashi Iwai
Date: Fri Jan 18 2019 - 14:39:41 EST

On Fri, 18 Jan 2019 20:08:05 +0100,
Mark Brown wrote:
> On Fri, Jan 18, 2019 at 10:35:44AM +0100, Jaroslav Kysela wrote:
> > the tinyalsa implementation does not show much - it's equal to the
> > standard mmap access for the PCM devices. Even considering the Mark's
> > text, there must be an arbiter (sound server) which communicates with
> > the producer or consumer to control the data flow. I really would like
> > to see a real usage for this.
> Right, the driving force behind implementing this is Android which had
> been using an out of tree version of this approach based on ION but
> that's run into trouble due to other outside changes.
> > It seems to me that the only point to implement this is the
> > permissions. We already have O_APPEND mode for the PCM file descriptor
> > which can reuse the PCM device multiple times (mmap the buffer to
> > multiple tasks). I would probably go in this way and add more extended
> > permission control for the PCM device, so permissions can be restricted
> > for the passed descriptor to the producer or the consumer task. In this
> > way, the restricted task might reuse other control mechanism offered buy
> > the PCM file descriptor without requesting the arbiter to do so (like
> > read the actual position in the DMA buffer, get the audio delay or so -
> > reduce context switches).
> One concern I have with doing some ALSA-specific custom permissions
> thing is integration with frameworks like SELinux (they'd presumably
> need to learn about the ALSA specific stuff to manage it). It also
> seems like it's adding a lot more security sensitive interfaces and
> code which which will require audit and review, one of the things I
> really like about this approach is that it's incredibly simple from
> the security point of view.

Well, I wonder what makes it more difficult by the approach Jaroslav
suggested. With O_APPEND, you can just call mmap() normally, and
that's all. What's the merit of dma-buf approach wrt the security?

BTW, the suggested patch seems to have a problem when the attached PCM
performs hw_free. Then the mapped target will be gone while another
process still mapping it. And the code looks pretty racy.