CVE-2021-47517

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

ethtool: do not perform operations on net devices being unregistered

There is a short period between a net device starts to be unregistered

and when it is actually gone. In that time frame ethtool operations

could still be performed, which might end up in unwanted or undefined

behaviours[1].

Do not allow ethtool operations after a net device starts its

unregistration. This patch targets the netlink part as the ioctl one

isn't affected: the reference to the net device is taken and the

operation is executed within an rtnl lock section and the net device

won't be found after unregister.

[1] For example adding Tx queues after unregister ends up in NULL

pointer exceptions and UaFs, such as:

BUG: KASAN: use-after-free in kobject_get+0x14/0x90

Read of size 1 at addr ffff88801961248c by task ethtool/755

CPU: 0 PID: 755 Comm: ethtool Not tainted 5.15.0-rc6+ #778

Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-4.fc34 04/014

Call Trace:

dump_stack_lvl+0x57/0x72

print_address_description.constprop.0+0x1f/0x140

kasan_report.cold+0x7f/0x11b

kobject_get+0x14/0x90

kobject_add_internal+0x3d1/0x450

kobject_init_and_add+0xba/0xf0

netdev_queue_update_kobjects+0xcf/0x200

netif_set_real_num_tx_queues+0xb4/0x310

veth_set_channels+0x1c3/0x550

ethnl_set_channels+0x524/0x610