netdev_wait_allrefs_any() re-sends NETDEV_UNREGISTER every second while it waits for the last references on a device to be dropped. By then netdev_run_todo() has already marked the device NETREG_UNREGISTERED and RTNL has been dropped at least once since the initial notification sent by unregister_netdevice_many_notify(). netdev_genl_netdevice_event() does not tell these rebroadcasts apart from the initial notification and sends a NETDEV_CMD_DEV_DEL_NTF multicast for each of them. At best this spams NETDEV_NLGRP_MGMT listeners with duplicate notifications; at worst the pernet state of the netns the device belongs to is already gone and the handler dereferences a NULL pointer. default_device_exit_batch() only unregisters the devices still listed in the exiting netns; it does not wait for in-flight unregistrations of devices that were already unlisted (the barrier doing that was removed by commit 8a4fc54b07d7 ("net: get rid of rtnl_lock_unregistering()")). So when a device is unregistered by something other than the cleanup_net() of its own netns while references on it are still held, e.g. a veth whose peer in another netns gets deleted, and that netns exits while the device is stuck in netdev_wait_allrefs_any(), the remaining pernet exit handlers run and genl_pernet_exit() sets net->genl_sock to NULL. The next rebroadcast then crashes in genl_has_listeners(): BUG: KASAN: null-ptr-deref in netlink_has_listeners+0x3d/0x3f0 net/netlink/af_netlink.c:1388 Read of size 8 at addr 0000000000000578 by task syz.0.0/5325 Call Trace: netlink_has_listeners+0x3d/0x3f0 net/netlink/af_netlink.c:1388 genl_has_listeners include/net/genetlink.h:644 [inline] netdev_genl_dev_notify+0x161/0x450 net/core/netdev-genl.c:93 netdev_genl_netdevice_event+0x98/0x3c0 net/core/netdev-genl.c:1491 notifier_call_chain+0x1a5/0x3d0 kernel/notifier.c:85 netdev_wait_allrefs_any net/core/dev.c:11690 [inline] netdev_run_todo+0x731/0x1060 net/core/dev.c:11808 rtnl_unlock net/core/rtnetlink.c:157 [inline] rtnl_dellink+0x68f/0x810 net/core/rtnetlink.c:3707 syzbot hit this by moving one end of a veth pair into a short-lived netns while netdevsim's deferred FIB event processing still held a reference on it, and then deleting the other end (repro [1]). While the source of this bug is netdevsim deferring FIB processing for and holding onto the device reference for O(seconds) which is being fixed separately [2], any driver may introduce such a delay and trigger the null-ptr-deref. Skip the rebroadcasts, which are the only NETDEV_UNREGISTER events delivered with reg_state == NETREG_UNREGISTERED. The notifications sent by unregister_netdevice_many_notify() and, for a device leaving a netns, by __dev_change_net_namespace() are issued under RTNL in the same critical section that unlists the device, before its netns can be torn down, and keep being handled. This mirrors what ip6_route_dev_notify() already does to cope with the rebroadcasts. This change should not break the UAPI spec as it defines dev-del-ntf as a "Notification about device disappearing." which is a single event. The rebroadcast in netdev_wait_allrefs_any() exists to re-kick in-kernel subsystems holding references; netdev-genl happens to be on the notifier chain and forwards it. [1] https://gist.github.com/NamanGulati/2d1b49b8757f9a88b0bd6e55018c02ae [2] https://lore.kernel.org/r/20261023191517.607834-1-kuniyu@google.com Fixes: d3d854fd6a1d ("netdev-genl: create a simple family for netdev stuff") Reported-by: syzbot+1269fe6cc2016008124e@syzkaller.appspotmail.com Closes: https://lore.kernel.org/netdev/6adf9f64.6200f071.51519.0ed1.GAE@google.com/ Reviewed-by: Kuniyuki Iwashima Signed-off-by: Naman Gulati --- Note that the reproducer attached to the syzbot bug targets a different bug (via devlink_port_notify() on netns teardown) that happens to share the same crash title, not the one in the original reported crash report. --- net/core/netdev-genl.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/net/core/netdev-genl.c b/net/core/netdev-genl.c index 33b9f4eb9565a..2032ab9a864c2 100644 --- a/net/core/netdev-genl.c +++ b/net/core/netdev-genl.c @@ -1487,6 +1487,12 @@ static int netdev_genl_netdevice_event(struct notifier_block *nb, netdev_unlock_full_to_ops(netdev); break; case NETDEV_UNREGISTER: + /* netdev_wait_allrefs_any() rebroadcasts NETDEV_UNREGISTER + * while waiting for the last references to be dropped; the + * netns may already be torn down by then. Notify only once. + */ + if (READ_ONCE(netdev->reg_state) == NETREG_UNREGISTERED) + break; netdev_lock(netdev); netdev_genl_dev_notify(netdev, NETDEV_CMD_DEV_DEL_NTF); netdev_unlock(netdev); -- 2.56.0.307.g5fe7bd680c-goog