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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, syzbot+1269fe6cc2016008124e@syzkaller.appspotmail.com Content-Type: text/plain; charset="us-ascii" On Mon, 26 Oct 2026 15:28:44 -0700, Stanislav Fomichev wrote: > On 10/26, Naman Gulati wrote: > > 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; > > Same as > https://lore.kernel.org/netdev/20261026063743.1797-1-lege.wang@jaguarmicro.com/ > ? > > reg_state is protected by netdev, why not do this under netdev_lock? > Also seems like we are fixing a symptom, not the problem? fwiw, I suggested not to take the lock since we are only interested in NETREG_UNREGISTERED here and there is a clear ordering in the case. The root cause is netdevsim this time, and I have a fix, https://syzkaller.appspot.com/text?tag=Patch&x=15923c21580000 but rebroadcast itself is not-ideal-but-valid behaviour, so each NETDEV_UNREGISTER must be idempotent unless we can completley remove the rebroadcast mechanism itself in the future. > > > netdev_lock(netdev); > > netdev_genl_dev_notify(netdev, NETDEV_CMD_DEV_DEL_NTF); > > netdev_unlock(netdev);