Re: [syzbot] [net?] KASAN: slab-use-after-free Read in __ethtool_get_link_ksettings

[Hillf Danton <hdanton@sina.com>]

> syzbot has found a reproducer for the following issue on:
> 
> HEAD commit:    d505d3593b52 net: wwan: qcom_bam_dmux: Fix missing pm_runt..
> git tree:       net
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=150d959f980000

#syz test https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git  61387b8dcf1d

--- l/drivers/infiniband/core/verbs.c
+++ v/drivers/infiniband/core/verbs.c
@@ -1976,15 +1976,14 @@ int ib_get_eth_speed(struct ib_device *d
 	if (rdma_port_get_link_layer(dev, port_num) != IB_LINK_LAYER_ETHERNET)
 		return -EINVAL;
 
-	netdev = ib_device_get_netdev(dev, port_num);
-	if (!netdev)
-		return -ENODEV;
-
 	rtnl_lock();
-	rc = __ethtool_get_link_ksettings(netdev, &lksettings);
+	netdev = ib_device_get_netdev(dev, port_num);
+	if (netdev)
+		rc = __ethtool_get_link_ksettings(netdev, &lksettings);
 	rtnl_unlock();
 
-	dev_put(netdev);
+	if (!netdev)
+		return -ENODEV;
 
 	if (!rc && lksettings.base.speed != (u32)SPEED_UNKNOWN) {
 		netdev_speed = lksettings.base.speed;
@@ -1995,6 +1994,7 @@ int ib_get_eth_speed(struct ib_device *d
 				netdev->name, netdev_speed);
 	}
 
+	dev_put(netdev);
 	ib_get_width_and_speed(netdev_speed, lksettings.lanes,
 			       speed, width);
 
--