drm/virtio: Fix GEM handle creation UAF

[ Upstream commit 5253125 ]

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 <[email protected]>
Reviewed-by: Chia-I Wu <[email protected]>
Fixes: 62fb7a5 ("virtio-gpu: add 3d/virgl support")
Cc: [email protected]
Signed-off-by: Dmitry Osipenko <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Signed-off-by: Sasha Levin <[email protected]>

drivers/gpu/drm/virtio/virtgpu_ioctl.c

@@ -309,7 +309,6 @@ static int virtio_gpu_resource_create_ioctl(struct drm_device *dev, void *data,
 		}
 		return ret;
 	}
-	drm_gem_object_put_unlocked(obj);
 
 	rc->res_handle = res_id; /* similiar to a VM address */
 	rc->bo_handle = handle;
@@ -318,6 +317,15 @@ static int virtio_gpu_resource_create_ioctl(struct drm_device *dev, void *data,
 		virtio_gpu_unref_list(&validate_list);
 		dma_fence_put(&fence->f);
 	}
+
+	/*
+	 * 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_unlocked(obj);
+
 	return 0;
 fail_unref:
 	if (vgdev->has_virgl_3d) {