drm/i915: Fix possible security hole in command parsing

i915_parse_cmds returns -EACCES on chained batches, which "tells the
caller to abort and dispatch the workload as a non-secure batch",
but the mechanism implementing that was broken when
flags |= I915_DISPATCH_SECURE was moved from i915_gem_execbuffer_parse
to i915_gem_do_execbuffer (17cabf57):
i915_gem_execbuffer_parse returns the original batch_obj in this case,
and i915_gem_do_execbuffer doesn't check for that.

Don't set the secure bit in this case to make sure such batches don't
run with elevated priviledges.
Signed-off-by: Rebecca Palmer <rebecca_palmer@zoho.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
[danvet: Stitch together commit message. Also remove a comment as
suggested by Mika. And style-align the comment while at it.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -1540,28 +1540,39 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
 	}
 
 	if (i915_needs_cmd_parser(ring) && args->batch_len) {
-		batch_obj = i915_gem_execbuffer_parse(ring,
+		struct drm_i915_gem_object *parsed_batch_obj;
+
+		parsed_batch_obj = i915_gem_execbuffer_parse(ring,
 						      &shadow_exec_entry,
 						      eb,
 						      batch_obj,
 						      args->batch_start_offset,
 						      args->batch_len,
 						      file->is_master);
-		if (IS_ERR(batch_obj)) {
-			ret = PTR_ERR(batch_obj);
+		if (IS_ERR(parsed_batch_obj)) {
+			ret = PTR_ERR(parsed_batch_obj);
 			goto err;
 		}
 
 		/*
+		 * parsed_batch_obj == batch_obj means batch not fully parsed:
+		 * Accept, but don't promote to secure.
+		 */
+
+		if (parsed_batch_obj != batch_obj) {
+			/*
+			 * Batch parsed and accepted:
+			 *
 			 * Set the DISPATCH_SECURE bit to remove the NON_SECURE
-		 * bit from MI_BATCH_BUFFER_START commands issued in the
-		 * dispatch_execbuffer implementations. We specifically
-		 * don't want that set when the command parser is
-		 * enabled.
+			 * bit from MI_BATCH_BUFFER_START commands issued in
+			 * the dispatch_execbuffer implementations. We
+			 * specifically don't want that set on batches the
+			 * command parser has accepted.
 			 */
 			dispatch_flags |= I915_DISPATCH_SECURE;
-
 			exec_start = 0;
+			batch_obj = parsed_batch_obj;
+		}
 	}
 
 	batch_obj->base.pending_read_domains |= I915_GEM_DOMAIN_COMMAND;