[PATCH net] netdev-genl: ignore NETDEV_UNREGISTER rebroadcasts

[Naman Gulati <namangulati@google.com>]

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:
   <TASK>
   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 <kuniyu@google.com>
Signed-off-by: Naman Gulati <namangulati@google.com>
---
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