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

[Baolin Wang]

Hi Jaroslav,
On Thu, 24 Jan 2019 at 21:43, Jaroslav Kysela <perex@xxxxxxxx> wrote:
>
> Dne 23.1.2019 v 13:46 Leo Yan napsal(a):
> > Hi all,
> >
> > On Wed, Jan 23, 2019 at 12:58:51PM +0100, Takashi Iwai wrote:
> >> On Tue, 22 Jan 2019 21:25:35 +0100,
> >> Mark Brown wrote:
> >>>
> >>> On Mon, Jan 21, 2019 at 03:15:43PM +0100, Jaroslav Kysela wrote:
> >>>> Dne 21.1.2019 v 13:40 Mark Brown napsal(a):
> >>>
> >>>>> It was the bit about adding more extended permission control that I was
> >>>>> worried about there, not the initial O_APPEND bit.  Indeed the O_APPEND
> >>>>> bit sounds like it might also work from the base buffer sharing point of
> >>>>> view, I have to confess I'd not heard of that feature before (it didn't
> >>>>> come up in the discussion when Eric raised this in Prague).
> >>>
> >>>> With permissions, I meant to make possible to restrict the file
> >>>> descriptor operations (ioctls) for the depending task (like access to
> >>>> the DMA buffer, synchronize it for the non-coherent platforms and maybe
> >>>> read/write the actual position, delay etc.). It should be relatively
> >>>> easy to implement using the snd_pcm_file structure.
> >>>
> >>> Right, that's what I understood you to mean.  If you want to have a
> >>> policy saying "it's OK to export a PCM file descriptor if it's only got
> >>> permissions X and Y" the security module is going to need to know about
> >>> the mechanism for setting those permissions.  With dma_buf that's all a
> >>> bit easier as there's less new stuff, though I've no real idea how much
> >>> of a big deal that actually is.
> >>
> >> There are many ways to implement such a thing, yeah.  If we'd need an
> >> implementation that is done solely in the sound driver layer, I can
> >> imagine to introduce either a new ioctl or an open flag (like O_EXCL)
> >> to specify the restricted sharing.  That is, a kind of master / slave
> >> model where only the master is allowed to manipulate the stream while
> >> the slave can mmap, read/write and get status.
> >
> > In order to support EXCLUSIVE mode, it is necessary to convert the
> > /dev/snd/ descriptor to an anon_inode:dmabuffer file descriptor.
> > SELinux allows that file descriptor to be passed to the client. It can
> > also be used by the AAudioService.
>
> Okay, so this is probably the only point which we should resolve for the
> already available DMA buffer sharing in ALSA (the O_APPEND flag).
>
> I had another glance to your dma-buf implementation and I see many
> things which might cause problems:
>
> - allow to call dma-buf ioctls only when the audio device is in specific
> state (stream is not running)

Right. Will fix.

>
> - as Takashi mentioned, if we return another file-descriptor (dma-buf
> export) to the user space and the server closes the main pcm
> file-descriptor (the client does not) - the result will be a crash (dma
> buffer will be freed, but referenced through the dma-buf interface)

Yes, will fix.

>
> - the attach function calls dma_buf_get(fd), but what if fd points to
> another dma-buf allocation from a different driver? the unexpected
> private data will cause crash - there should be a type checking in the
> dma-buf interface

There is a validation (is_dma_buf_file() ) in dma_buf_get() function
before getting the dma buffer.

> If I look to the dma_buf_fd() implementation:
>
>   fd = get_unused_fd_flags(flags);
>   fd_install(fd, dmabuf->file);
>
> .. what if we just add one new ioctl to the ALSA's PCM API which will
> return a new anonymous inode descriptor with the restricted access to
> the main PCM device to satisfy the SELinux requirements / security
> policies? It might be more nice and simple solution than to implement
> the full dma-buf interface for the ALSA's PCM devices.

I will do some investigation for your suggestion and talk with the
security people if it can work. Thanks for your suggestion.

-- 
Baolin Wang
Best Regards