openvswitch: fix lockup on tx to unregistering netdev with carrier

In the Linux kernel, the following vulnerability has been resolved:

openvswitch: fix lockup on tx to unregistering netdev with carrier

Commit in a fixes tag attempted to fix the issue in the following

sequence of calls:

do_output

-> ovs_vport_send

-> dev_queue_xmit

-> __dev_queue_xmit

-> netdev_core_pick_tx

-> skb_tx_hash

When device is unregistering, the 'dev->real_num_tx_queues' goes to

zero and the 'while (unlikely(hash >= qcount))' loop inside the

'skb_tx_hash' becomes infinite, locking up the core forever.

But unfortunately, checking just the carrier status is not enough to

fix the issue, because some devices may still be in unregistering

state while reporting carrier status OK.

One example of such device is a net/dummy. It sets carrier ON

on start, but it doesn't implement .ndo_stop to set the carrier off.

And it makes sense, because dummy doesn't really have a carrier.

Therefore, while this device is unregistering, it's still easy to hit

the infinite loop in the skb_tx_hash() from the OVS datapath. There

might be other drivers that do the same, but dummy by itself is

important for the OVS ecosystem, because it is frequently used as a

packet sink for tcpdump while debugging OVS deployments. And when the

issue is hit, the only way to recover is to reboot.

Fix that by also checking if the device is running. The running

state is handled by the net core during unregistering, so it covers

unregistering case better, and we don't really need to send packets

to devices that are not running anyway.

While only checking the running state might be enough, the carrier

check is preserved. The running and the carrier states seem disjoined

throughout the code and different drivers. And other core functions

like __dev_direct_xmit() check both before attempting to transmit

a packet. So, it seems safer to check both flags in OVS as well.