Re: [PATCH 13/13] drm: allow render capable master with DRM_AUTH ioctls

[Emil Velikov <emil.l.velikov@gmail.com>]

On 2019/05/27, Christian König wrote:
> Am 27.05.19 um 10:17 schrieb Emil Velikov:
> > From: Emil Velikov <emil.velikov@collabora.com>
> > 
> > There are cases (in mesa and applications) where one would open the
> > primary node without properly authenticating the client.
> > 
> > Sometimes we don't check if the authentication succeeds, but there's
> > also cases we simply forget to do it.
> > 
> > The former was a case for Mesa where it did not not check the return
> > value of drmGetMagic() [1]. That was fixed recently although, there's
> > the question of older drivers or other apps that exbibit this behaviour.
> > 
> > While omitting the call results in issues as seen in [2] and [3].
> > 
> > In the libva case, libva itself doesn't authenticate the DRM client and
> > the vaGetDisplayDRM documentation doesn't mention if the app should
> > either.
> > 
> > As of today, the official vainfo utility doesn't authenticate.
> > 
> > To workaround issues like these, some users resort to running their apps
> > under sudo. Which admittedly isn't always a good idea.
> > 
> > Since any DRIVER_RENDER driver has sufficient isolation between clients,
> > we can use that, for unauthenticated [primary node] ioctls that require
> > DRM_AUTH. But only if the respective ioctl is tagged as DRM_RENDER_ALLOW.
> > 
> > v2:
> > - Rework/simplify if check (Daniel V)
> > - Add examples to commit messages, elaborate. (Daniel V)
> > 
> > v3:
> > - Use single unlikely (Daniel V)
> > 
> > v4:
> > - Patch was reverted because it broke AMDGPU, apply again. The AMDGPU
> > issue is fixed with earlier patch.
> 
> As far as I can see this only affects the following two IOCTLs after
> removing DRM_AUTH from the DRM_RENDER_ALLOW IOCTLs:
> > DRM_IOCTL_DEF(DRM_IOCTL_PRIME_HANDLE_TO_FD,
> > drm_prime_handle_to_fd_ioctl, DRM_AUTH|DRM_UNLOCKED|DRM_RENDER_ALLOW),
> >         DRM_IOCTL_DEF(DRM_IOCTL_PRIME_FD_TO_HANDLE,
> > drm_prime_fd_to_handle_ioctl, DRM_AUTH|DRM_UNLOCKED|DRM_RENDER_ALLOW)
> 
> So I think it would be simpler to just remove DRM_AUTH from those two
> instead of allowing it for everybody.
> 
If I understand you correctly this will remove DRM_AUTH also for drivers
which expose only a primary node. I'm not sure if that is a good idea.

That said, if others are OK with the idea I will prepare a patch.

Thanks
Emil
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