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

net: vlan: fix VLAN 0 refcount imbalance of toggling filtering during runtime

Assuming the "rx-vlan-filter" feature is enabled on a net device, the 8021q module will automatically add or remove VLAN 0 when the net device is put administratively up or down, respectively. There are a couple of problems with the above scheme.

The first problem is a memory leak that can happen if the "rx-vlan-filter" feature is disabled while the device is running:

ip link add bond1 up type bond mode 0

ethtool -K bond1 rx-vlan-filter off

ip link del dev bond1

When the device is put administratively down the "rx-vlan-filter" feature is disabled, so the 8021q module will not remove VLAN 0 and the memory will be leaked [1].

Another problem that can happen is that the kernel can automatically delete VLAN 0 when the device is put administratively down despite not adding it when the device was put administratively up since during that time the "rx-vlan-filter" feature was disabled. null-ptr-unref or bug_on[2] will be triggered by unregister_vlan_dev() for refcount imbalance if toggling filtering during runtime:

$ ip link add bond0 type bond mode 0 $ ip link add link bond0 name vlan0 type vlan id 0 protocol 802.1q $ ethtool -K bond0 rx-vlan-filter off $ ifconfig bond0 up $ ethtool -K bond0 rx-vlan-filter on $ ifconfig bond0 down $ ip link del vlan0

Root cause is as below: step1: add vlan0 for real_dev, such as bond, team. register_vlan_dev vlan_vid_add(real_dev,htons(ETH_P_8021Q),0) //refcnt=1 step2: disable vlan filter feature and enable real_dev step3: change filter from 0 to 1 vlan_device_event vlan_filter_push_vids ndo_vlan_rx_add_vid //No refcnt added to real_dev vlan0 step4: real_dev down vlan_device_event vlan_vid_del(dev, htons(ETH_P_8021Q), 0); //refcnt=0 vlan_info_rcu_free //free vlan0 step5: delete vlan0 unregister_vlan_dev BUG_ON(!vlan_info); //vlan_info is null...