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

Daniel Vetter
2014-01-30T09:12:13Z

On Thu, Jan 30, 2014 at 10:05:28AM +0100, Daniel Vetter wrote:
> 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.

One more: I think it would be good to have an overview comment at the top
of i915_cmd_parser.c which details the security attack model and the
overall blacklist/whitelist design of the checker. We don't (yet) have
autogenerated documentation for i915, but that's something I'm working on.
And the kerneldoc system can also pull in multi-paragraph overview
comments besides the usual api documentation, so it's good to have things
ready.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch