[PATCH net v4 3/3] netconsole: recheck device registration after waiting for carrier

[Gustavo Luiz Duarte]

netcons_netpoll_setup() drops RTNL while it waits for carrier when the
device is not yet up. If the device is unregistered in that window, the
NETDEV_UNREGISTER notifier runs before np->dev is set and has nothing
to match.

We might then end up setting up netpoll on a device that is being
unregistered while holding a reference to it, stalling
unregister_netdevice() in netdev_wait_allrefs_any().

Add a check to confirm the device is still registered once we retake
RTNL. Release the device reference and return -ENODEV otherwise.

Closes: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261012-netcons-fixes-v3-1-1aac85ed7665@xxxxxxxxx
Signed-off-by: Gustavo Luiz Duarte <gustavold@xxxxxxxxx>
---
 drivers/net/netconsole.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index be7eb632ee42..bd36ebd334aa 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -534,6 +534,16 @@ static int netcons_netpoll_setup(struct netconsole_target *nt)
 		rtnl_unlock();
 		netcons_wait_carrier(np, ndev);
 		rtnl_lock();
+
+		/* Check that ndev hasn't been unregistered when we released
+		 * the RTNL above.
+		 */
+		if (ndev->reg_state != NETREG_REGISTERED) {
+			np_err(np, "%s was unregistered while waiting for carrier, aborting\n",
+			       netcons_egress_dev(np, buf, sizeof(buf)));
+			err = -ENODEV;
+			goto put;
+		}
 	}
 
 	if (netcons_local_ip_unset(nt)) {

-- 
2.56.0