[PATCH] net: hsr: avoid synchronize_net() in hsr_del_port() under rtnl_mutex

[Shardul Bankar <shardulsb08@gmail.com>]

hsr_del_port() calls netdev_rx_handler_unregister(), which calls
synchronize_net() while rtnl_mutex is held.  During netns teardown,
cleanup_net() walks pernet_list and reaches default_device_exit_batch(),
which takes rtnl_mutex and iterates devices calling
rtnl_link_ops->dellink() for each.  For HSR this resolves to
hsr_dellink() -> hsr_del_ports() -> hsr_del_port(), so the
synchronize_net() call runs under rtnl_mutex.  Under contention this
stalls other rtnl_mutex waiters long enough to trip the hung-task
detector:

  kworker cleanup_net -> default_device_exit_batch -> hsr_del_port
    -> netdev_rx_handler_unregister -> synchronize_rcu_expedited
    [slow, holds rtnl_mutex]
  syz-executor -> ksys_unshare -> setup_net -> ip_tunnel_init_net
    -> rtnl_lock  [blocked on rtnl_mutex]

Open-code netdev_rx_handler_unregister() in hsr_del_port() without
the synchronize_net() step.  This is safe because hsr_del_port()
already defers the port free via kfree_rcu(port, rcu), so the
rx_handler_data memory remains valid until an RCU grace period
elapses naturally.  hsr_handle_frame() additionally re-validates
rx_handler via hsr_port_get_rcu() before dereferencing
rx_handler_data, so a reader that observes rx_handler_data == NULL
in the brief window between the two clears takes the NULL path and
returns RX_HANDLER_PASS without touching the port.

Reported-by: syzbot+f2fbf7478a35a94c8b7c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?id=cb64c22a492202ca929e18262fdb8cb89e635c70
Signed-off-by: Shardul Bankar <shardul.b@mpiricsoftware.com>
---
Testing status:

The hang was originally observed on 7.0-rc6 (commit 7ca6d1cfec80) with
the syzkaller reproducer and a KASAN+LOCKDEP config.  Subsequent
reproduction attempts with the same reproducer on current mainline did
not retrigger this specific signature, so I could not verify the fix
against a live reproducer.  The patch is submitted on the basis of
code analysis.

 net/hsr/hsr_slave.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/net/hsr/hsr_slave.c b/net/hsr/hsr_slave.c
index d9af9e65f72f..5e92f23fa1a5 100644
--- a/net/hsr/hsr_slave.c
+++ b/net/hsr/hsr_slave.c
@@ -239,7 +239,18 @@ void hsr_del_port(struct hsr_port *port)
 	if (port != master) {
 		netdev_update_features(master->dev);
 		dev_set_mtu(master->dev, hsr_get_max_mtu(hsr));
-		netdev_rx_handler_unregister(port->dev);
+		/* Open-code netdev_rx_handler_unregister() without the
+		 * synchronize_net() step: holding rtnl_mutex across the
+		 * grace-period wait stalls other rtnl_mutex waiters long
+		 * enough to trip the hung-task detector under load.
+		 * Skipping synchronize_net() is safe because the port is
+		 * freed via kfree_rcu() below (so rx_handler_data memory
+		 * outlives any in-flight reader), and hsr_handle_frame()
+		 * re-validates rx_handler via hsr_port_get_rcu() before
+		 * dereferencing rx_handler_data.
+		 */
+		RCU_INIT_POINTER(port->dev->rx_handler, NULL);
+		RCU_INIT_POINTER(port->dev->rx_handler_data, NULL);
 		if (!port->hsr->fwd_offloaded)
 			dev_set_promiscuity(port->dev, -1);
 		netdev_upper_dev_unlink(port->dev, master->dev);
-- 
2.34.1