[PATCH AUTOSEL 6.18] net: lapbether: handle NETDEV_PRE_TYPE_CHANGE

[Sasha Levin]

From: Eric Dumazet <edumazet@xxxxxxxxxx>

[ Upstream commit b120e4432f9f56c7103133d6a11245e617695adb ]

lapbeth_data_transmit() expects the underlying device type
to be ARPHRD_ETHER.

Returning NOTIFY_BAD from lapbeth_device_event() makes sure
bonding driver can not break this expectation.

Fixes: 872254dd6b1f ("net/bonding: Enable bonding to enslave non ARPHRD_ETHER")
Reported-by: syzbot+d8c285748fa7292580a9@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/netdev/69cd22a1.050a0220.70c3a.0002.GAE@xxxxxxxxxx/T/#u
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Martin Schiller <ms@xxxxxxxxxx>
Reviewed-by: Simon Horman <horms@xxxxxxxxxx>
Link: https://patch.msgid.link/20260402103519.1201565-1-edumazet@xxxxxxxxxx
Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---

LLM Generated explanations, may be completely bogus:

Error: Failed to generate final synthesis

 drivers/net/wan/lapbether.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/drivers/net/wan/lapbether.c b/drivers/net/wan/lapbether.c
index f357a7ac70ac4..9861c99ea56c4 100644
--- a/drivers/net/wan/lapbether.c
+++ b/drivers/net/wan/lapbether.c
@@ -446,33 +446,36 @@ static void lapbeth_free_device(struct lapbethdev *lapbeth)
 static int lapbeth_device_event(struct notifier_block *this,
 				unsigned long event, void *ptr)
 {
-	struct lapbethdev *lapbeth;
 	struct net_device *dev = netdev_notifier_info_to_dev(ptr);
+	struct lapbethdev *lapbeth;
 
 	if (dev_net(dev) != &init_net)
 		return NOTIFY_DONE;
 
-	if (!dev_is_ethdev(dev) && !lapbeth_get_x25_dev(dev))
+	lapbeth = lapbeth_get_x25_dev(dev);
+	if (!dev_is_ethdev(dev) && !lapbeth)
 		return NOTIFY_DONE;
 
 	switch (event) {
 	case NETDEV_UP:
 		/* New ethernet device -> new LAPB interface	 */
-		if (!lapbeth_get_x25_dev(dev))
+		if (!lapbeth)
 			lapbeth_new_device(dev);
 		break;
 	case NETDEV_GOING_DOWN:
 		/* ethernet device closes -> close LAPB interface */
-		lapbeth = lapbeth_get_x25_dev(dev);
 		if (lapbeth)
 			dev_close(lapbeth->axdev);
 		break;
 	case NETDEV_UNREGISTER:
 		/* ethernet device disappears -> remove LAPB interface */
-		lapbeth = lapbeth_get_x25_dev(dev);
 		if (lapbeth)
 			lapbeth_free_device(lapbeth);
 		break;
+	case NETDEV_PRE_TYPE_CHANGE:
+		/* Our underlying device type must not change. */
+		if (lapbeth)
+			return NOTIFY_BAD;
 	}
 
 	return NOTIFY_DONE;
-- 
2.53.0