drm/virtio: Fix GEM handle creation UAF

Userspace can guess the handle value and try to race GEM object creation
with handle close, resulting in a use-after-free if we dereference the
object after dropping the handle's reference.  For that reason, dropping
the handle's reference must be done *after* we are done dereferencing
the object.
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Fixes: 62fb7a5e ("virtio-gpu: add 3d/virgl support")
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221216233355.542197-2-robdclark@gmail.com

drivers/gpu/drm/virtio/virtgpu_ioctl.c

@@ -358,10 +358,18 @@ static int virtio_gpu_resource_create_ioctl(struct drm_device *dev, void *data,
 		drm_gem_object_release(obj);
 		return ret;
 	}
-	drm_gem_object_put(obj);
 
 	rc->res_handle = qobj->hw_res_handle; /* similiar to a VM address */
 	rc->bo_handle = handle;
+
+	/*
+	 * The handle owns the reference now.  But we must drop our
+	 * remaining reference *after* we no longer need to dereference
+	 * the obj.  Otherwise userspace could guess the handle and
+	 * race closing it from another thread.
+	 */
+	drm_gem_object_put(obj);
+
 	return 0;
 }
 
@@ -723,11 +731,18 @@ static int virtio_gpu_resource_create_blob_ioctl(struct drm_device *dev,
 		drm_gem_object_release(obj);
 		return ret;
 	}
-	drm_gem_object_put(obj);
 
 	rc_blob->res_handle = bo->hw_res_handle;
 	rc_blob->bo_handle = handle;
 
+	/*
+	 * The handle owns the reference now.  But we must drop our
+	 * remaining reference *after* we no longer need to dereference
+	 * the obj.  Otherwise userspace could guess the handle and
+	 * race closing it from another thread.
+	 */
+	drm_gem_object_put(obj);
+
 	return 0;
 }