nbd: defer config put in recv_work

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: nbd: defer config put in recv_work There is one uaf issue in recv_work when running NBD_CLEAR_SOCK and NBD_CMD_RECONFIGURE: nbd_genl_connect // conf_ref=2 (connect and recv_work A) nbd_open // conf_ref=3 recv_work A done // conf_ref=2 NBD_CLEAR_SOCK // conf_ref=1 nbd_genl_reconfigure // conf_ref=2 (trigger recv_work B) close nbd // conf_ref=1 recv_work B config_put // conf_ref=0 atomic_dec(&config->recv_threads); -> UAF Or only running NBD_CLEAR_SOCK: nbd_genl_connect // conf_ref=2 nbd_open // conf_ref=3 NBD_CLEAR_SOCK // conf_ref=2 close nbd nbd_release config_put // conf_ref=1 recv_work config_put // conf_ref=0 atomic_dec(&config->recv_threads); -> UAF Commit 87aac3a80af5 ("nbd: call nbd_config_put() before notifying the waiter") moved nbd_config_put() to run before waking up the waiter in recv_work, in order to ensure that nbd_start_device_ioctl() would not be woken up while nbd->task_recv was still uncleared. However, in nbd_start_device_ioctl(), after being woken up it explicitly calls flush_workqueue() to make sure all current works are finished. Therefore, there is no need to move the config put ahead of the wakeup. Move nbd_config_put() to the end of recv_work, so that the reference is held for the whole lifetime of the worker thread. This makes sure the config cannot be freed while recv_work is still running, even if clear + reconfigure interleave. In addition, we don't need to worry about recv_work dropping the last nbd_put (which causes deadlock): path A (netlink with NBD_CFLAG_DESTROY_ON_DISCONNECT): connect // nbd_refs=1 (trigger recv_work) open nbd // nbd_refs=2 NBD_CLEAR_SOCK close nbd nbd_release nbd_disconnect_and_put flush_workqueue // recv_work done nbd_config_put nbd_put // nbd_refs=1 nbd_put // nbd_refs=0 queue_work path B (netlink without NBD_CFLAG_DESTROY_ON_DISCONNECT): connect // nbd_refs=2 (trigger recv_work) open nbd // nbd_refs=3 NBD_CLEAR_SOCK // conf_refs=2 close nbd nbd_release nbd_config_put // conf_refs=1 nbd_put // nbd_refs=2 recv_work done // conf_refs=0, nbd_refs=1 rmmod // nbd_refs=0 Depends-on: e2daec488c57 ("nbd: Fix hungtask when nbd_config_put")

References

• https://git.kernel.org/stable/c/198aa230a6f8c1f6af7ed26b29180749c3e79e4d
• https://git.kernel.org/stable/c/d3ba312675911ff9e3fefefd551751e153a9f0a9
• https://git.kernel.org/stable/c/3692884bd6187d89d41eef81e5a9724519fd01c1
• https://git.kernel.org/stable/c/1ba2ced2bbdf7e64a30c3e88c70ea8bc208d1509
• https://git.kernel.org/stable/c/6b69593f72e1bfba6ca47ca8d9b619341fded7d6
• https://git.kernel.org/stable/c/443a1721806b6ff6303b5229e9811d68172d622f
• https://git.kernel.org/stable/c/742012f6bf29553fdc460bf646a58df3a7b43d01
• https://git.kernel.org/stable/c/9517b82d8d422d426a988b213fdd45c6b417b86d