[PATCH] drm: Create support for Write-Only property blob

[Jani Nikula]

On Wed, 29 Jun 2022, Mark Yacoub <markyacoub at chromium.org> wrote:
> Hi Jani, let me know if you need more info or more changes are needed. Thanks!

Sorry, I really haven't had the time to think this through.

At a glance, I'm unable to find the hdcp key property that this is being
used with. Am I missing something? Where's that created and used in the
kernel?


BR,
Jani.



>
> On Wed, May 25, 2022 at 3:31 PM Mark Yacoub <markyacoub at chromium.org> wrote:
>>
>> Hi Jani, thanks for your review. I got all the user space
>> implementation ready to see it in context.
>>
>> libdrm patch to wrap this functionality:
>> https://www.spinics.net/lists/dri-devel/msg347318.html
>>
>> Chromium user space implementation making direct use of the new prop flag:
>> crrev.com/c/3655850
>> The first call made to such functionality is in
>> https://chromium-review.googlesource.com/c/chromium/src/+/3655850/2/ui/display/manager/content_protection_key_manager.cc#137
>> where the call stack flows to the libdrm wrapper call at
>> https://chromium-review.googlesource.com/c/chromium/src/+/3655850/2/ui/ozone/platform/drm/gpu/drm_display.cc#203
>>
>> I also wrote an IGT test to verify the intended behavior:
>> https://patchwork.freedesktop.org/patch/487331/?series=104373&rev=1
>>
>> Let me know if I would need to update the commit message with any of
>> the aforementioned context.
>>
>> Thanks!
>> -Mark Yacoub
>>
>> On Tue, May 17, 2022 at 3:53 AM Jani Nikula <jani.nikula at linux.intel.com> wrote:
>> >
>> > On Tue, 10 May 2022, Mark Yacoub <markyacoub at chromium.org> wrote:
>> > > [Why]
>> > > User space might need to inject data into the kernel without allowing it
>> > > to be read again by any user space.
>> > > An example of where this is particularly useful is secret keys fetched
>> > > by user space and injected into the kernel to enable content protection.
>> >
>> > I think we're going to need more than an example in the commit
>> > message. See Documentation/gpu/drm-uapi.rst.
>> >
>> > BR,
>> > Jani.
>> >
>> >
>> > >
>> > > [How]
>> > > Create a DRM_MODE_CREATE_BLOB_WRITE_ONLY flag used by user space to
>> > > create a blob and mark the blob as write only.
>> > > On reading back the blob, data will be not be copied if it's a write
>> > > only blob
>> > >
>> > > Signed-off-by: Mark Yacoub <markyacoub at chromium.org>
>> > >
>> > > ---
>> > >  drivers/gpu/drm/drm_property.c | 3 ++-
>> > >  include/drm/drm_property.h     | 2 ++
>> > >  include/uapi/drm/drm_mode.h    | 6 ++++++
>> > >  3 files changed, 10 insertions(+), 1 deletion(-)
>> > >
>> > > diff --git a/drivers/gpu/drm/drm_property.c b/drivers/gpu/drm/drm_property.c
>> > > index dfec479830e4..afedf7109d00 100644
>> > > --- a/drivers/gpu/drm/drm_property.c
>> > > +++ b/drivers/gpu/drm/drm_property.c
>> > > @@ -765,7 +765,7 @@ int drm_mode_getblob_ioctl(struct drm_device *dev,
>> > >       if (!blob)
>> > >               return -ENOENT;
>> > >
>> > > -     if (out_resp->length == blob->length) {
>> > > +     if (out_resp->length == blob->length && !blob->is_write_only) {
>> > >               if (copy_to_user(u64_to_user_ptr(out_resp->data),
>> > >                                blob->data,
>> > >                                blob->length)) {
>> > > @@ -800,6 +800,7 @@ int drm_mode_createblob_ioctl(struct drm_device *dev,
>> > >               ret = -EFAULT;
>> > >               goto out_blob;
>> > >       }
>> > > +     blob->is_write_only = out_resp->flags & DRM_MODE_CREATE_BLOB_WRITE_ONLY;
>> > >
>> > >       /* Dropping the lock between create_blob and our access here is safe
>> > >        * as only the same file_priv can remove the blob; at this point, it is
>> > > diff --git a/include/drm/drm_property.h b/include/drm/drm_property.h
>> > > index 65bc9710a470..700782f021b9 100644
>> > > --- a/include/drm/drm_property.h
>> > > +++ b/include/drm/drm_property.h
>> > > @@ -205,6 +205,7 @@ struct drm_property {
>> > >   *   &drm_mode_config.property_blob_list.
>> > >   * @head_file: entry on the per-file blob list in &drm_file.blobs list.
>> > >   * @length: size of the blob in bytes, invariant over the lifetime of the object
>> > > + * @is_write_only: user space can't read the blob data.
>> > >   * @data: actual data, embedded at the end of this structure
>> > >   *
>> > >   * Blobs are used to store bigger values than what fits directly into the 64
>> > > @@ -219,6 +220,7 @@ struct drm_property_blob {
>> > >       struct list_head head_global;
>> > >       struct list_head head_file;
>> > >       size_t length;
>> > > +     bool is_write_only;
>> > >       void *data;
>> > >  };
>> > >
>> > > diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
>> > > index 0a0d56a6158e..de192d3813e9 100644
>> > > --- a/include/uapi/drm/drm_mode.h
>> > > +++ b/include/uapi/drm/drm_mode.h
>> > > @@ -1107,6 +1107,9 @@ struct drm_format_modifier {
>> > >       __u64 modifier;
>> > >  };
>> > >
>> > > +#define DRM_MODE_CREATE_BLOB_WRITE_ONLY                                        \
>> > > +     (1 << 0) /* data of the blob can't be read by user space */
>> > > +
>> > >  /**
>> > >   * struct drm_mode_create_blob - Create New blob property
>> > >   *
>> > > @@ -1120,6 +1123,9 @@ struct drm_mode_create_blob {
>> > >       __u32 length;
>> > >       /** @blob_id: Return: new property ID. */
>> > >       __u32 blob_id;
>> > > +     /** Flags for special handling. */
>> > > +     __u32 flags;
>> > > +     __u32 pad;
>> > >  };
>> > >
>> > >  /**
>> >
>> > --
>> > Jani Nikula, Intel Open Source Graphics Center

-- 
Jani Nikula, Intel Open Source Graphics Center