linux is vulnerable to denial of service. The root cause of this vulnerability is that the ioctl$DRM_IOCTL_MODE_DESTROY_DUMB can decrease refcount of *drm_vgem_gem_object *(created in vgem_gem_dumb_create) concurrently, and *vgem_gem_dumb_create *will access the freed drm_vgem_gem_object.