Wayland content-protection extension

[Ramalingam C]

Pekka,

Thanks a lot for the responses.

On Friday 29 June 2018 04:28 PM, Pekka Paalanen wrote:
> On Mon, 18 Jun 2018 15:56:40 +0530
> Ramalingam C <ramalingam.c at intel.com> wrote:
>
>> On Monday 18 June 2018 03:52 PM, Pekka Paalanen wrote:
>>> On Mon, 18 Jun 2018 14:32:32 +0530
>>> Ramalingam C <ramalingam.c at intel.com> wrote:
>>>   
>>>> On Monday 18 June 2018 02:23 PM, Pekka Paalanen wrote:
>>>>> On Mon, 18 Jun 2018 13:38:09 +0530
>>>>> Ramalingam C <ramalingam.c at intel.com> wrote:
>>>>>      
>>>>>> On Monday 18 June 2018 01:34 PM, Pekka Paalanen wrote:
>>>>>>> On Sat, 16 Jun 2018 12:50:52 +0530
>>>>>>> Ramalingam C <ramalingam.c at intel.com> wrote:
>>>>>>> The SRM table smells very much like compositor configuration,
>>>>>>> especially because a) it is global state: you cannot program two
>>>>>>> different tables to the same connector, and b) the compositor is
>>>>>>> required to save it and use it later for all clients(?). One can also
>>>>>>> envision a security issue, if a system allows third party apps: an app
>>>>>>> could install a fake SRM table with a fake date.
>>>>>> Compositor is expected to store the latest SRM in the non-volatile and
>>>>>> update with only newest versions.
>>>>>> And it will supply the latest version to kernel(irrespective of what
>>>>>> version is provided by app). This caching is not per connector.
>>>>>> SRM table itself provides the version of it. and The validity of an SRM
>>>>>> is established by verifying the integrity of its
>>>>>> signature with the Digital Content Protection LLC public key, which is
>>>>>> specified by the Digital
>>>>>> Content Protection LLC. So no fake SRM will be accepted.
>>>>> Right, so I would propose to make that completely separate.
>>>> Ok. So how that should be implemented? As another protocol extension?
>>> I don't know. Is there a reason to do it by Wayland?
>> Yes. As we need to supply the SRM table through drm connector properties
>> to each ports HDCP authentication.
>> At the least we need to receive the signature validated latest SRM from
>> client and program into drm connector property.
>> Means storing SRM could be kept within the wayland client but only
>> drm_master can set the blob for the connectors.
>>
>> And AFAIK signature verification also a crypto calculation with DCP LLC
>> public Key.
> But nothing there requires the use of Wayland for it. Sure, the
> compositor will have to program the SRM table via KMS, but it doesn't
> mean the compositor needs to get it by Wayland.
True. thats what I meant. I used the word "wayland" loosely. :(
>
> The compositor does not care which process provides a SRM table,
> because:
> - the SRM table is global and cross-vendor
> - the SRM table is signed, so no fakes can be provided
> - the SRM table is versioned/dated, so always the most recent will be
>    in use, regardless of which version a client might propose
> - clients are happy to use a SRM table that is more recent than what
>    they have themselves
>
> Did I misunderstand something?
You got it completely.
>
> Let me put it in another way: we don't need to standardise a Wayland
> interface, it could be DBus or something else since there is no
> connection to window management.
>
> I'm also concerned about the maintenance of an authenticated SRM table.
> Would we have Weston link to a new library to validate signatures? How
> does Weston load a guaranteed good DCP public key? Where will Weston
> store and load the SRM tables?
DCP public key is static value. Available in the HDCP specs.
Storing place for the SRM from weston is not thought about.
>
> I would assume the SRM table support to be both uninteresting to
> upstream in general, and possibly quite specific to the system where
> weston is running. Therefore, I would propose the following:
>
> Have the SRM table verification, maintenance, and IPC in an external
> Weston plugin.
Actually I am afraid that I am not understanding this part.
By this are you referring a library which provides the function pointers
for weston compositor for verify the signature, update the latest SRM
in Non volatile memory and to get the latest SRM etc?

If not please elaborate on this.

> In upstream Weston, only add an API through the
> plugin-registry to set up an already validated, single SRM table for
> all connectors that support it. In other words:
>
> int
> set_srm_table(something);
>
> libweston makes an internal copy of the data and programs the same data
> to any connector that gets HDCP enabled, or it will fail to enable HDCP
> at all for the connector. Calling set_srm_table() again will
> unconditionally overwrite the earlier data.
SRM need not be available with all players. Even if he is not providing
we could use the latest SRM from the non-volatile memory.

Agreed. Here set_srm_table() will make a copy and write into Kernel
to overwrite the SRM exist in kernel if any.
>
> This way upstream does not need to design for a use case they have no
> idea about, a vendor can implement the SRM table management any way
> they want, and once there is an actual working implementation suitable
> for serious use, we can have a look at what it involves and whether it
> would be suitable for Weston upstream.
>
> I actually doubt whether vendors would even want the SRM table
> management code open sourced.
>
> For testing the API, we would probably need a dummy SRM table plugin,
> which just feeds in a hardcoded table or from a file, without any
> authentication or runtime updates, or even better, allows the user to
> set the list of revoked devices so one can easily test revoking ones
> own HDCP display.
>
> How's that?
Reference SRM tables are provided in HDCP spec for testing purpose.
>
>
> Btw. I would like to see (lib)Weston used in media devices and for that
> at least IMHO I would be ready to take related features into upstream
> Weston, so in general I would be quite positive about new features like
> this. It just needs to happen at upstream terms.
We would be pleased to follow the upstream terms to enable this feature 
in weston.

Thanks and regards,
Ram
>
>
> Thanks,
> pq
>
>
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