drm/i915: Prevent double unref following alloc failure during execbuffer

Whilst looking up the objects required for an execbuffer, an untimely
allocation failure in creating the vma results in the object being
unreferenced from two lists. The ownership during the lookup is meant to
be moved from the list of objects being looked to the vma, and this
double unreference upon error results in a use-after-free.

Fixes regression from
commit 27173f1f
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Wed Aug 14 11:38:36 2013 +0200

    drm/i915: Convert execbuf code to use vmas

Based on the fix by Ben Widawsky.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: stable@vger.kernel.org
[danvet: Bikeshed the crucial comment above the ownership transfer as
discussed on irc.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -93,7 +93,7 @@ eb_lookup_vmas(struct eb_vmas *eb,
 {
 	struct drm_i915_gem_object *obj;
 	struct list_head objects;
-	int i, ret = 0;
+	int i, ret;
 
 	INIT_LIST_HEAD(&objects);
 	spin_lock(&file->table_lock);
@@ -106,7 +106,7 @@ eb_lookup_vmas(struct eb_vmas *eb,
 			DRM_DEBUG("Invalid object handle %d at index %d\n",
 				   exec[i].handle, i);
 			ret = -ENOENT;
-			goto out;
+			goto err;
 		}
 
 		if (!list_empty(&obj->obj_exec_link)) {
@@ -114,7 +114,7 @@ eb_lookup_vmas(struct eb_vmas *eb,
 			DRM_DEBUG("Object %p [handle %d, index %d] appears more than once in object list\n",
 				   obj, exec[i].handle, i);
 			ret = -EINVAL;
-			goto out;
+			goto err;
 		}
 
 		drm_gem_object_reference(&obj->base);
@@ -123,9 +123,13 @@ eb_lookup_vmas(struct eb_vmas *eb,
 	spin_unlock(&file->table_lock);
 
 	i = 0;
-	list_for_each_entry(obj, &objects, obj_exec_link) {
+	while (!list_empty(&objects)) {
 		struct i915_vma *vma;
 
+		obj = list_first_entry(&objects,
+				       struct drm_i915_gem_object,
+				       obj_exec_link);
+
 		/*
 		 * NOTE: We can leak any vmas created here when something fails
 		 * later on. But that's no issue since vma_unbind can deal with
@@ -138,10 +142,12 @@ eb_lookup_vmas(struct eb_vmas *eb,
 		if (IS_ERR(vma)) {
 			DRM_DEBUG("Failed to lookup VMA\n");
 			ret = PTR_ERR(vma);
-			goto out;
+			goto err;
 		}
 
+		/* Transfer ownership from the objects list to the vmas list. */
 		list_add_tail(&vma->exec_list, &eb->vmas);
+		list_del_init(&obj->obj_exec_link);
 
 		vma->exec_entry = &exec[i];
 		if (eb->and < 0) {
@@ -155,16 +161,22 @@ eb_lookup_vmas(struct eb_vmas *eb,
 		++i;
 	}
 
+	return 0;
+
 
-out:
+err:
 	while (!list_empty(&objects)) {
 		obj = list_first_entry(&objects,
 				       struct drm_i915_gem_object,
 				       obj_exec_link);
 		list_del_init(&obj->obj_exec_link);
-		if (ret)
-			drm_gem_object_unreference(&obj->base);
+		drm_gem_object_unreference(&obj->base);
 	}
+	/*
+	 * Objects already transfered to the vmas list will be unreferenced by
+	 * eb_destroy.
+	 */
+
 	return ret;
 }