Describe CVE-2021-47266.
What is CVE-2021-47266?
In the Linux kernel, the following vulnerability has been resolved:

RDMA/ipoib: Fix warning caused by destroying non-initial netns

After the commit 5ce2dced8e95 ("RDMA/ipoib: Set rtnl_link_ops for ipoib
interfaces"), if the IPoIB device is moved to non-initial netns,
destroying that netns lets the device vanish instead of moving it back to
the initial netns, This is happening because default_device_exit() skips
the interfaces due to having rtnl_link_ops set.

Steps to reporoduce:
  ip netns add foo
  ip link set mlx5_ib0 netns foo
  ip netns delete foo

WARNING: CPU: 1 PID: 704 at net/core/dev.c:11435 netdev_exit+0x3f/0x50
Modules linked in: xt_CHECKSUM xt_MASQUERADE xt_conntrack ipt_REJECT
nf_reject_ipv4 nft_compat nft_counter nft_chain_nat nf_nat nf_conntrack
nf_defrag_ipv6 nf_defrag_ipv4 nf_tables nfnetlink tun d
 fuse
CPU: 1 PID: 704 Comm: kworker/u64:3 Tainted: G S      W  5.13.0-rc1+ #1
Hardware name: Dell Inc. PowerEdge R630/02C2CP, BIOS 2.1.5 04/11/2016
Workqueue: netns cleanup_net
RIP: 0010:netdev_exit+0x3f/0x50
Code: 48 8b bb 30 01 00 00 e8 ef 81 b1 ff 48 81 fb c0 3a 54 a1 74 13 48
8b 83 90 00 00 00 48 81 c3 90 00 00 00 48 39 d8 75 02 5b c3 <0f> 0b 5b
c3 66 66 2e 0f 1f 84 00 00 00 00 00 66 90 0f 1f 44 00
RSP: 0018:ffffb297079d7e08 EFLAGS: 00010206
RAX: ffff8eb542c00040 RBX: ffff8eb541333150 RCX: 000000008010000d
RDX: 000000008010000e RSI: 000000008010000d RDI: ffff8eb440042c00
RBP: ffffb297079d7e48 R08: 0000000000000001 R09: ffffffff9fdeac00
R10: ffff8eb5003be000 R11: 0000000000000001 R12: ffffffffa1545620
R13: ffffffffa1545628 R14: 0000000000000000 R15: ffffffffa1543b20
FS:  0000000000000000(0000) GS:ffff8ed37fa00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00005601b5f4c2e8 CR3: 0000001fc8c10002 CR4: 00000000003706e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 ops_exit_list.isra.9+0x36/0x70
 cleanup_net+0x234/0x390
 process_one_work+0x1cb/0x360
 ? process_one_work+0x360/0x360
 worker_thread+0x30/0x370
 ? process_one_work+0x360/0x360
 kthread+0x116/0x130
 ? kthread_park+0x80/0x80
 ret_from_fork+0x22/0x30

To avoid the above warning and later on the kernel panic that could happen
on shutdown due to a NULL pointer dereference, make sure to set the
netns_refund flag that was introduced by commit 3a5ca857079e ("can: dev:
Move device back to init netns on owning netns delete") to properly
restore the IPoIB interfaces to the initial netns..