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.
