[PATCH 1/2] drm: vc4: set permissions for ioctls

[Eric Anholt]

Emil Velikov <emil.l.velikov at gmail.com> writes:

> On 10 June 2016 at 21:08, Eric Anholt <eric at anholt.net> wrote:
>> Emil Velikov <emil.l.velikov at gmail.com> writes:
>>
>>> On 10 June 2016 at 00:42, Eric Anholt <eric at anholt.net> wrote:
>>>> Rob Herring <robh at kernel.org> writes:
>>>>
>>>>> Ioctls generally have DRM_AUTH and DRM_RENDER_ALLOW set to restrict them
>>>>> to authorized clients and render nodes. Without this, access from render
>>>>> nodes fails.
>>>>
>>>> We've already got a fix to add RENDER_ALLOW submitted in the latest
>>>> drm-vc4-fixes.  There's no reason to require auth on this
>>>> implementation, though.
>>>>
>>> Not 100% sure but I think you do. At least every other driver does...
>>>
>>> Why: I'm thinking that without DRM_AUTH one will be able to open the
>>> card# node and issue the said IOCTLs even if the client is not
>>> authenticated. Which, obviously isn't a huge deal, but doesn't sound
>>> right.
>>>
>>> Then again, my knowledge of vc4 is virtually non-existent, so there
>>> might be something special happening here ?
>>
>> Let's flip this around: What is the problem you see with calling any of
>> the ioctls without having gone through the auth dance?  I don't believe
>> there's any reason to require auth, since you only have access to the
>> buffers you create or import.
>>
>> Basically, auth was created a stopgap solution for "but if anyone had
>> access to the DRM device, they could scrape the X frontbuffer!"
>
> Personally I don't see any serious issues* with keeping DRM_AUTH out
> of these. Although one could argue that the lack of it up-to recently
> one was using non-auth access to the card node. The latter of which
> lead to the DRM_RENDER_ALLOW going unnoticed.
>
> That aside, I would urge that we have consistency on the topic.
> Whether adding DRM_AUTH to the said VC4 ioctls, dropping DRM_AUTH
> everywhere (if DRM_RENDER_ALLOW is present on the said ioclt) or
> something else.

DRM_AUTH is not safe to remove from other drivers, unless they enforce
access control to their buffers.
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