can: netlink: can_changelink(): fix NULL pointer deref of struct can_priv::do_set_mode

Andrei Lalaev reported a NULL pointer deref when a CAN device is
restarted from Bus Off and the driver does not implement the struct
can_priv::do_set_mode callback.

There are 2 code path that call struct can_priv::do_set_mode:
- directly by a manual restart from the user space, via
  can_changelink()
- delayed automatic restart after bus off (deactivated by default)

To prevent the NULL pointer deference, refuse a manual restart or
configure the automatic restart delay in can_changelink() and report
the error via extack to user space.

As an additional safety measure let can_restart() return an error if
can_priv::do_set_mode is not set instead of dereferencing it
unchecked.

Reported-by: Andrei Lalaev <andrey.lalaev@gmail.com>
Closes: https://lore.kernel.org/all/20250714175520.307467-1-andrey.lalaev@gmail.com
Fixes: 39549eef35 ("can: CAN Network device driver and Netlink interface")
Link: https://patch.msgid.link/20250718-fix-nullptr-deref-do_set_mode-v1-1-0b520097bb96@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/dev/dev.c

@@ -145,13 +145,16 @@ void can_change_state(struct net_device *dev, struct can_frame *cf,
 EXPORT_SYMBOL_GPL(can_change_state);
 
 /* CAN device restart for bus-off recovery */
-static void can_restart(struct net_device *dev)
+static int can_restart(struct net_device *dev)
 {
 	struct can_priv *priv = netdev_priv(dev);
 	struct sk_buff *skb;
 	struct can_frame *cf;
 	int err;
 
+	if (!priv->do_set_mode)
+		return -EOPNOTSUPP;
+
 	if (netif_carrier_ok(dev))
 		netdev_err(dev, "Attempt to restart for bus-off recovery, but carrier is OK?\n");
 
@@ -173,10 +176,14 @@ static void can_restart(struct net_device *dev)
 	if (err) {
 		netdev_err(dev, "Restart failed, error %pe\n", ERR_PTR(err));
 		netif_carrier_off(dev);
+
+		return err;
 	} else {
 		netdev_dbg(dev, "Restarted\n");
 		priv->can_stats.restarts++;
 	}
+
+	return 0;
 }
 
 static void can_restart_work(struct work_struct *work)
@@ -201,9 +208,8 @@ int can_restart_now(struct net_device *dev)
 		return -EBUSY;
 
 	cancel_delayed_work_sync(&priv->restart_work);
-	can_restart(dev);
 
-	return 0;
+	return can_restart(dev);
 }
 
 /* CAN bus-off

drivers/net/can/dev/netlink.c

@@ -285,6 +285,12 @@ static int can_changelink(struct net_device *dev, struct nlattr *tb[],
 	}
 
 	if (data[IFLA_CAN_RESTART_MS]) {
+		if (!priv->do_set_mode) {
+			NL_SET_ERR_MSG(extack,
+				       "Device doesn't support restart from Bus Off");
+			return -EOPNOTSUPP;
+		}
+
 		/* Do not allow changing restart delay while running */
 		if (dev->flags & IFF_UP)
 			return -EBUSY;
@@ -292,6 +298,12 @@ static int can_changelink(struct net_device *dev, struct nlattr *tb[],
 	}
 
 	if (data[IFLA_CAN_RESTART]) {
+		if (!priv->do_set_mode) {
+			NL_SET_ERR_MSG(extack,
+				       "Device doesn't support restart from Bus Off");
+			return -EOPNOTSUPP;
+		}
+
 		/* Do not allow a restart while not running */
 		if (!(dev->flags & IFF_UP))
 			return -EINVAL;