Describe CVE-2021-47139.
What is CVE-2021-47139?
In the Linux kernel, the following vulnerability has been resolved:

net: hns3: put off calling register_netdev() until client initialize complete

Currently, the netdevice is registered before client initializing
complete. So there is a timewindow between netdevice available
and usable. In this case, if user try to change the channel number
or ring param, it may cause the hns3_set_rx_cpu_rmap() being called
twice, and report bug.

[47199.416502] hns3 0000:35:00.0 eth1: set channels: tqp_num=1, rxfh=0
[47199.430340] hns3 0000:35:00.0 eth1: already uninitialized
[47199.438554] hns3 0000:35:00.0: rss changes from 4 to 1
[47199.511854] hns3 0000:35:00.0: Channels changed, rss_size from 4 to 1, tqps from 4 to 1
[47200.163524] ------------[ cut here ]------------
[47200.171674] kernel BUG at lib/cpu_rmap.c:142!
[47200.177847] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
[47200.185259] Modules linked in: hclge(+) hns3(-) hns3_cae(O) hns_roce_hw_v2 hnae3 vfio_iommu_type1 vfio_pci vfio_virqfd vfio pv680_mii(O) [last unloaded: hclge]
[47200.205912] CPU: 1 PID: 8260 Comm: ethtool Tainted: G           O      5.11.0-rc3+ #1
[47200.215601] Hardware name:  , xxxxxx 02/04/2021
[47200.223052] pstate: 60400009 (nZCv daif +PAN -UAO -TCO BTYPE=--)
[47200.230188] pc : cpu_rmap_add+0x38/0x40
[47200.237472] lr : irq_cpu_rmap_add+0x84/0x140
[47200.243291] sp : ffff800010e93a30
[47200.247295] x29: ffff800010e93a30 x28: ffff082100584880
[47200.254155] x27: 0000000000000000 x26: 0000000000000000
[47200.260712] x25: 0000000000000000 x24: 0000000000000004
[47200.267241] x23: ffff08209ba03000 x22: ffff08209ba038c0
[47200.273789] x21: 000000000000003f x20: ffff0820e2bc1680
[47200.280400] x19: ffff0820c970ec80 x18: 00000000000000c0
[47200.286944] x17: 0000000000000000 x16: ffffb43debe4a0d0
[47200.293456] x15: fffffc2082990600 x14: dead000000000122
[47200.300059] x13: ffffffffffffffff x12: 000000000000003e
[47200.306606] x11: ffff0820815b8080 x10: ffff53e411988000
[47200.313171] x9 : 0000000000000000 x8 : ffff0820e2bc1700
[47200.319682] x7 : 0000000000000000 x6 : 000000000000003f
[47200.326170] x5 : 0000000000000040 x4 : ffff800010e93a20
[47200.332656] x3 : 0000000000000004 x2 : ffff0820c970ec80
[47200.339168] x1 : ffff0820e2bc1680 x0 : 0000000000000004
[47200.346058] Call trace:
[47200.349324]  cpu_rmap_add+0x38/0x40
[47200.354300]  hns3_set_rx_cpu_rmap+0x6c/0xe0 [hns3]
[47200.362294]  hns3_reset_notify_init_enet+0x1cc/0x340 [hns3]
[47200.370049]  hns3_change_channels+0x40/0xb0 [hns3]
[47200.376770]  hns3_set_channels+0x12c/0x2a0 [hns3]
[47200.383353]  ethtool_set_channels+0x140/0x250
[47200.389772]  dev_ethtool+0x714/0x23d0
[47200.394440]  dev_ioctl+0x4cc/0x640
[47200.399277]  sock_do_ioctl+0x100/0x2a0
[47200.404574]  sock_ioctl+0x28c/0x470
[47200.409079]  __arm64_sys_ioctl+0xb4/0x100
[47200.415217]  el0_svc_common.constprop.0+0x84/0x210
[47200.422088]  do_el0_svc+0x28/0x34
[47200.426387]  el0_svc+0x28/0x70
[47200.431308]  el0_sync_handler+0x1a4/0x1b0
[47200.436477]  el0_sync+0x174/0x180
[47200.441562] Code: 11000405 79000c45 f8247861 d65f03c0 (d4210000)
[47200.448869] ---[ end trace a01efe4ce42e5f34 ]---

The process is like below:
excuting hns3_client_init
|
register_netdev()
|                           hns3_set_channels()
|                           |
hns3_set_rx_cpu_rmap()      hns3_reset_notify_uninit_enet()
|                               |
|                            quit without calling function
|                            hns3_free_rx_cpu_rmap for flag
|                            HNS3_NIC_STATE_INITED is unset.
|                           |
|                           hns3_reset_notify_init_enet()
|                               |
set HNS3_NIC_STATE_INITED    call hns3_set_rx_cpu_rmap()-- crash

Fix it by calling register_netdev() at the end of function
hns3_client_init()..