Re: [PATCH v2 2/3] DRM: Create new Content Protection connector property

[Dave Stevenson]

Hi Mark (and Dmitry)

On Fri, 21 Apr 2023 at 18:07, Dmitry Baryshkov
<dmitry.baryshkov@xxxxxxxxxx> wrote:
>
> On 21/04/2023 19:27, Mark Yacoub wrote:
> > From: Mark Yacoub <markyacoub@xxxxxxxxxxxx>
>
> Nit: is there a reason for this header? My first impression is that it
> matches your outgoing name & email address and as such is not necessary.
>
> Nit#2: subject should mention 'Key', as you are creating a property for
> the key.
>
> >
> > [Why]
> > To enable Protected Content, some drivers require a key to be injected
> > from user space to enable HDCP on the connector.
> >
> > [How]
> > Create new "Content Protection Property" of type "Blob"
>
> Generic observation is that the ability to inject HDCP keys manually
> seems to be quite unique to your hardware. As such, I think the debugfs
> or sysfs suits better in comparison to the DRM property.

I was about to reply to v1 with a very similar comment over the
requirement to keep HDCP keys secret.

v2 has added WRITE_ONLY blobs so at least another process can't just
read the blob back out again, but it feels like there are still
numerous ways to grab those keys. For an unsecured userspace to have
the keys in the first place seems like a bad move, and IMHO they
should only be held in either a secure environment, or only held in
hardware (passed direct from OTP to HDCP block).


There's also the DRM uAPI requirement for having reviewed patches for
an open source project to go alongside any uAPI change. Do such
patches exist? https://docs.kernel.org/gpu/drm-uapi.html#open-source-userspace-requirements

  Dave

> >
> > Signed-off-by: Mark Yacoub <markyacoub@xxxxxxxxxxxx>
> > ---
> >   drivers/gpu/drm/drm_atomic_uapi.c | 9 +++++++++
> >   include/drm/drm_connector.h       | 6 ++++++
> >   include/drm/drm_mode_config.h     | 6 ++++++
> >   3 files changed, 21 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/drm_atomic_uapi.c b/drivers/gpu/drm/drm_atomic_uapi.c
> > index d867e7f9f2cd5..e20bc57cdb05c 100644
> > --- a/drivers/gpu/drm/drm_atomic_uapi.c
> > +++ b/drivers/gpu/drm/drm_atomic_uapi.c
> > @@ -749,6 +749,11 @@ static int drm_atomic_connector_set_property(struct drm_connector *connector,
> >               state->content_protection = val;
> >       } else if (property == config->hdcp_content_type_property) {
> >               state->hdcp_content_type = val;
> > +     } else if (property == config->content_protection_key_property) {
> > +             ret = drm_atomic_replace_property_blob_from_id(
> > +                     dev, &state->content_protection_key, val, -1, -1,
> > +                     &replaced);
> > +             return ret;
> >       } else if (property == connector->colorspace_property) {
> >               state->colorspace = val;
> >       } else if (property == config->writeback_fb_id_property) {
> > @@ -843,6 +848,10 @@ drm_atomic_connector_get_property(struct drm_connector *connector,
> >               *val = state->content_protection;
> >       } else if (property == config->hdcp_content_type_property) {
> >               *val = state->hdcp_content_type;
> > +     } else if (property == config->content_protection_key_property) {
> > +             *val = state->content_protection_key ?
> > +                            state->content_protection_key->base.id :
> > +                            0;
> >       } else if (property == config->writeback_fb_id_property) {
> >               /* Writeback framebuffer is one-shot, write and forget */
> >               *val = 0;
> > diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
> > index 7b5048516185c..2fbe51272bfeb 100644
> > --- a/include/drm/drm_connector.h
> > +++ b/include/drm/drm_connector.h
> > @@ -896,6 +896,12 @@ struct drm_connector_state {
> >        */
> >       unsigned int content_protection;
> >
> > +     /**
> > +      * @content_protection_key: DRM blob property for holding the Content
> > +      * Protection Key injected from user space.
> > +      */
> > +     struct drm_property_blob *content_protection_key;
> > +
> >       /**
> >        * @colorspace: State variable for Connector property to request
> >        * colorspace change on Sink. This is most commonly used to switch
> > diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
> > index e5b053001d22e..615d1e5f57562 100644
> > --- a/include/drm/drm_mode_config.h
> > +++ b/include/drm/drm_mode_config.h
> > @@ -887,6 +887,12 @@ struct drm_mode_config {
> >        */
> >       struct drm_property *hdcp_content_type_property;
> >
> > +     /**
> > +      * @content_protection_key_property: DRM blob property that receives the
> > +      * content protection key from user space to be injected into the kernel.
> > +      */
> > +     struct drm_property *content_protection_key_property;
> > +
> >       /* dumb ioctl parameters */
> >       uint32_t preferred_depth, prefer_shadow;
> >
>
> --
> With best wishes
> Dmitry
>