drm/gem: completely close gem_open vs. gem_close races

The gem flink name holds a reference onto the object itself, and this
self-reference would prevent an flink'ed object from every being
freed. To break that loop we remove the flink name when the last
userspace handle disappears, i.e. when obj->handle_count reaches 0.

Now in gem_open we drop the dev->object_name_lock between the flink
name lookup and actually adding the handle. This means a concurrent
gem_close of the last handle could result in the flink name getting
reaped right inbetween, i.e.

Thread 1		Thread 2
gem_open		gem_close

flink -> obj lookup
			handle_count drops to 0
			remove flink name
create_handle
handle_count++

If someone now flinks this object again, we'll get a new flink name.

We can close this race by removing the lock dropping and making the
entire lookup+handle_create sequence atomic. Unfortunately to still be
able to share the handle_create logic this requires a
handle_create_tail function which drops the lock - we can't hold the
object_name_lock while calling into a driver's ->gem_open callback.

Note that for flink fixing this race isn't really important, since
racing gem_open against gem_close is clearly a userspace bug. And no
matter how the race ends, we won't leak any references.

But with dma-buf where the userspace dma-buf fd itself is refcounted
this is a valid sequence and hence we should fix it. Therefore this
patch here is just a warm-up exercise (and for consistency between
flink buffer sharing and dma-buf buffer sharing with self-imports).

Also note that this extension of the critical section in gem_open
protected by dev->object_name_lock only works because it's now a
mutex: A spinlock would conflict with the potential memory allocation
in idr_preload().

This is exercises by igt/gem_flink_race/flink_name.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_gem.c

@@ -308,23 +308,26 @@ int drm_gem_dumb_destroy(struct drm_file *file,
 EXPORT_SYMBOL(drm_gem_dumb_destroy);
 
 /**
- * Create a handle for this object. This adds a handle reference
- * to the object, which includes a regular reference count. Callers
- * will likely want to dereference the object afterwards.
+ * drm_gem_handle_create_tail - internal functions to create a handle
+ * 
+ * This expects the dev->object_name_lock to be held already and will drop it
+ * before returning. Used to avoid races in establishing new handles when
+ * importing an object from either an flink name or a dma-buf.
  */
 int
-drm_gem_handle_create(struct drm_file *file_priv,
+drm_gem_handle_create_tail(struct drm_file *file_priv,
 			   struct drm_gem_object *obj,
 			   u32 *handlep)
 {
 	struct drm_device *dev = obj->dev;
 	int ret;
 
+	WARN_ON(!mutex_is_locked(&dev->object_name_lock));
+
 	/*
 	 * Get the user-visible handle using idr.  Preload and perform
 	 * allocation under our spinlock.
 	 */
-	mutex_lock(&dev->object_name_lock);
 	idr_preload(GFP_KERNEL);
 	spin_lock(&file_priv->table_lock);
 
@@ -351,6 +354,21 @@ drm_gem_handle_create(struct drm_file *file_priv,
 
 	return 0;
 }
+
+/**
+ * Create a handle for this object. This adds a handle reference
+ * to the object, which includes a regular reference count. Callers
+ * will likely want to dereference the object afterwards.
+ */
+int
+drm_gem_handle_create(struct drm_file *file_priv,
+		       struct drm_gem_object *obj,
+		       u32 *handlep)
+{
+	mutex_lock(&obj->dev->object_name_lock);
+
+	return drm_gem_handle_create_tail(file_priv, obj, handlep);
+}
 EXPORT_SYMBOL(drm_gem_handle_create);
 
 
@@ -627,13 +645,15 @@ drm_gem_open_ioctl(struct drm_device *dev, void *data,
 
 	mutex_lock(&dev->object_name_lock);
 	obj = idr_find(&dev->object_name_idr, (int) args->name);
-	if (obj)
+	if (obj) {
 		drm_gem_object_reference(obj);
+	} else {
 		mutex_unlock(&dev->object_name_lock);
-	if (!obj)
 		return -ENOENT;
+	}
 
-	ret = drm_gem_handle_create(file_priv, obj, &handle);
+	/* drm_gem_handle_create_tail unlocks dev->object_name_lock. */
+	ret = drm_gem_handle_create_tail(file_priv, obj, &handle);
 	drm_gem_object_unreference_unlocked(obj);
 	if (ret)
 		return ret;

include/drm/drmP.h

@@ -1575,6 +1575,9 @@ drm_gem_object_unreference_unlocked(struct drm_gem_object *obj)
 	}
 }
 
+int drm_gem_handle_create_tail(struct drm_file *file_priv,
+			       struct drm_gem_object *obj,
+			       u32 *handlep);
 int drm_gem_handle_create(struct drm_file *file_priv,
 			  struct drm_gem_object *obj,
 			  u32 *handlep);