[PATCH v2,04/21] v4l: add documentation for secure memory flag

[Jeffrey Kardatzke]

Mediatek,

What happened to the RFC cover letter that explained more overall for
what this is for?  That should be included in the 0th patch for each
of the series.

Pavel,
This is for secure video playback where the memory is 'secure'
(TrustZone in this case) and is only accessible in the TEE and
specific HW blocks. Userspace has FDs that reference the memory, but
kernel/userspace can't actually map/access that memory.  And yes, this
is for supporting DRM (Digital Rights Management) playback.

Cheers,
Jeff


On Sat, Nov 11, 2023 at 11:06 AM Pavel Machek <pavel at ucw.cz> wrote:
>
> Hi!
>
> > From: Jeffrey Kardatzke <jkardatzke at google.com>
> >
> > Adds documentation for V4L2_MEMORY_FLAG_SECURE.
>
> > --- a/Documentation/userspace-api/media/v4l/buffer.rst
> > +++ b/Documentation/userspace-api/media/v4l/buffer.rst
> > @@ -696,7 +696,7 @@ enum v4l2_memory
> >
> >  .. _memory-flags:
> >
> > -Memory Consistency Flags
> > +Memory Flags
> >  ------------------------
> >
> >  .. raw:: latex
> > @@ -728,6 +728,12 @@ Memory Consistency Flags
> >       only if the buffer is used for :ref:`memory mapping <mmap>` I/O and the
> >       queue reports the :ref:`V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS
> >       <V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS>` capability.
> > +    * .. _`V4L2-MEMORY-FLAG-SECURE`:
> > +
> > +      - ``V4L2_MEMORY_FLAG_SECURE``
> > +      - 0x00000002
> > +      - DMA bufs passed into the queue will be validated to ensure they were
> > +     allocated from a secure dma-heap.
>
> Could we get some more information somewhere? Why would userspace want
> to work with "secure" DMA heaps? How exactly are they different from
> others? What attacks are these secure against? What is goal of all
> this? DRM?
>
> BR,
>                                                                 Pavel
> --
> People of Russia, stop Putin before his war on Ukraine escalates.