Re: [PATCH] drm/i915: Add OACONTROL to the command parser register whitelist.

["Volkin, Bradley D" <bradley.d.volkin@intel.com>]

On Fri, May 16, 2014 at 12:53:30PM -0700, Jesse Barnes wrote:
> On Fri, 16 May 2014 12:34:08 -0700
> Jesse Barnes <jbarnes@virtuousgeek.org> wrote:
> 
> > On Fri, 16 May 2014 20:20:50 +0100
> > Chris Wilson <chris@chris-wilson.co.uk> wrote:
> > > Yes, X only sets the secure bit when it pokes the display registers, and
> > > those registers should be privileged even with a cmd parser in place
> > > (which they are).
> > > 
> > > Daniel's argument presumes that we haven't been patching out the
> > > cmd parser all this time anyway.
> > 
> > Yeah I know we have some perf issues as it is; it would be nice if the
> > overhead were so minimal that it didn't matter.  But just on principle,
> > scanning secure buffers seems wrong, and I'm trying to understand why
> > Daniel would want it.
> 
> Ok Daniel explained on IRC that we actually have a special whitelist
> for the secure batch case.  The idea is to allow a DRM_MASTER to submit
> secure batches, but still prevent a local root exploit.  I suppose that
> means preventing access to most commands and registers, but allowing a
> few extra things like wait events and display register updates.

Just to clarify further: the additional register whitelist and commands
are only based on DRM_MASTER. Setting I915_EXEC_SECURE is not required. So
I suppose we could stop scanning batches that have I915_EXEC_SECURE and
userspace could stop sending such batches when the parser is fully enabled.

Brad

> 
> I suppose it's not entirely unreasonable, but it does add complexity to
> the scanner and overhead to all users; not sure it's worth it.
> 
> -- 
> Jesse Barnes, Intel Open Source Technology Center
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