Re: [PATCH 02/13] drm/i915: Implement command buffer parsing logic

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Jan 30, 2014 at 09:53:28AM +0100, Daniel Vetter wrote:
> On Wed, Jan 29, 2014 at 10:28:36PM +0000, Chris Wilson wrote:
> > On Wed, Jan 29, 2014 at 01:55:03PM -0800, bradley.d.volkin@intel.com wrote:
> > > +/*
> > > + * Returns a pointer to a descriptor for the command specified by cmd_header.
> > > + *
> > > + * The caller must supply space for a default descriptor via the default_desc
> > > + * parameter. If no descriptor for the specified command exists in the ring's
> > > + * command parser tables, this function fills in default_desc based on the
> > > + * ring's default length encoding and returns default_desc.
> > > + */
> > > +static const struct drm_i915_cmd_descriptor*
> > > +find_cmd(struct intel_ring_buffer *ring,
> > > +	 u32 cmd_header,
> > > +	 struct drm_i915_cmd_descriptor *default_desc)
> > > +{
> > > +	u32 mask;
> > > +	int i;
> > > +
> > > +	for (i = 0; i < ring->cmd_table_count; i++) {
> > > +		const struct drm_i915_cmd_descriptor *desc;
> > > +
> > > +		desc = find_cmd_in_table(&ring->cmd_tables[i], cmd_header);
> > > +		if (desc)
> > > +			return desc;
> > > +	}
> > > +
> > > +	mask = ring->get_cmd_length_mask(cmd_header);
> > > +	if (!mask)
> > > +		return NULL;
> > > +
> > > +	BUG_ON(!default_desc);
> > > +	default_desc->flags = CMD_DESC_SKIP;
> > > +	default_desc->length.mask = mask;
> > 
> > If we turn off all hw validation (through use of the secure bit) should
> > we not default to a whitelist of commands? Otherwise it just seems to be
> > a case of running a fuzzer until we kill the machine.
> 
> Preventing hangs and dos is imo not the attack model, gpus are too fickle
> for that. The attach model here is to prevent priveledge escalation and
> information leaks. I.e. we want just containement of all read/write access
> to the gtt space.
> 
> I think for that purpose an explicit whitelist of commands which target
> things outside of the (pp)gtt is sufficient. radeon's checker design is
> completely different, but pretty much the only command they have is
> to load register values. Intel gpus otoh have a big set of special-purpose
> commands to load (most) of the rendering pipeline state. So we have
> hw built-in register whitelists for all that stuff since you just can't
> load arbitrary registers and state with those commands.
> 
> Also note that for raw register access Bradley's scanner _is_ whitelist
> based. And for general reads/writes gpu designers confirmed that those are
> all MI_ commands (with very few specific exceptions like PIPE_CONTROL), so
> as long as we check for the exceptions and otherwise only whitelist MI_
> commands we know about we should be covered.
> 
> So I think this is sound.

Hm, but while scrolling through the checker I haven't spotted a "reject
everything unknown" for MI_CLIENT commands. Bradley, have I missed that?

I think submitting an invented MI_CLIENT command would also be a good
testcase.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch