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Mon, 26 Oct 2026 15:28:49 -0700 (PDT) Received: from localhost ([2a03:2880:9ff:43::]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-36a01fa4c58sm27343359eec.21.2026.10.26.15.28.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 26 Oct 2026 15:28:49 -0700 (PDT) Date: Mon, 26 Oct 2026 15:28:44 -0700 From: Stanislav Fomichev To: Naman Gulati Cc: "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Kuniyuki Iwashima , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, syzbot+1269fe6cc2016008124e@syzkaller.appspotmail.com Subject: Re: [PATCH net] netdev-genl: ignore NETDEV_UNREGISTER rebroadcasts Message-ID: References: <20261026221306.3151025-1-namangulati@google.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20261026221306.3151025-1-namangulati@google.com> 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? > netdev_lock(netdev); > netdev_genl_dev_notify(netdev, NETDEV_CMD_DEV_DEL_NTF); > netdev_unlock(netdev);