CVE-2021-47606 net: netlink: af_netlink: Prevent empty skb by adding a check on len.
In the Linux kernel, the following vulnerability has been resolved:

net: netlink: af_netlink: Prevent empty skb by adding a check on len.

Adding a check on len parameter to avoid empty skb. This prevents a
division error in netem_enqueue function which is caused when skb->len=0
and skb->data_len=0 in the randomized corruption step as shown below.

skb->data[prandom_u32() % skb_headlen(skb)] ^= 1<<(prandom_u32() % 8);

Crash Report:
[  343.170349] netdevsim netdevsim0 netdevsim3: set [1, 0] type 2 family
0 port 6081 - 0
[  343.216110] netem: version 1.3
[  343.235841] divide error: 0000 [#1] PREEMPT SMP KASAN NOPTI
[  343.236680] CPU: 3 PID: 4288 Comm: reproducer Not tainted 5.16.0-rc1+
[  343.237569] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS 1.11.0-2.el7 04/01/2014
[  343.238707] RIP: 0010:netem_enqueue+0x1590/0x33c0 [sch_netem]
[  343.239499] Code: 89 85 58 ff ff ff e8 5f 5d e9 d3 48 8b b5 48 ff ff
ff 8b 8d 50 ff ff ff 8b 85 58 ff ff ff 48 8b bd 70 ff ff ff 31 d2 2b 4f
74 <f7> f1 48 b8 00 00 00 00 00 fc ff df 49 01 d5 4c 89 e9 48 c1 e9 03
[  343.241883] RSP: 0018:ffff88800bcd7368 EFLAGS: 00010246
[  343.242589] RAX: 00000000ba7c0a9c RBX: 0000000000000001 RCX:
0000000000000000
[  343.243542] RDX: 0000000000000000 RSI: ffff88800f8edb10 RDI:
ffff88800f8eda40
[  343.244474] RBP: ffff88800bcd7458 R08: 0000000000000000 R09:
ffffffff94fb8445
[  343.245403] R10: ffffffff94fb8336 R11: ffffffff94fb8445 R12:
0000000000000000
[  343.246355] R13: ffff88800a5a7000 R14: ffff88800a5b5800 R15:
0000000000000020
[  343.247291] FS:  00007fdde2bd7700(0000) GS:ffff888109780000(0000)
knlGS:0000000000000000
[  343.248350] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  343.249120] CR2: 00000000200000c0 CR3: 000000000ef4c000 CR4:
00000000000006e0
[  343.250076] Call Trace:
[  343.250423]  <TASK>
[  343.250713]  ? memcpy+0x4d/0x60
[  343.251162]  ? netem_init+0xa0/0xa0 [sch_netem]
[  343.251795]  ? __sanitizer_cov_trace_pc+0x21/0x60
[  343.252443]  netem_enqueue+0xe28/0x33c0 [sch_netem]
[  343.253102]  ? stack_trace_save+0x87/0xb0
[  343.253655]  ? filter_irq_stacks+0xb0/0xb0
[  343.254220]  ? netem_init+0xa0/0xa0 [sch_netem]
[  343.254837]  ? __kasan_check_write+0x14/0x20
[  343.255418]  ? _raw_spin_lock+0x88/0xd6
[  343.255953]  dev_qdisc_enqueue+0x50/0x180
[  343.256508]  __dev_queue_xmit+0x1a7e/0x3090
[  343.257083]  ? netdev_core_pick_tx+0x300/0x300
[  343.257690]  ? check_kcov_mode+0x10/0x40
[  343.258219]  ? _raw_spin_unlock_irqrestore+0x29/0x40
[  343.258899]  ? __kasan_init_slab_obj+0x24/0x30
[  343.259529]  ? setup_object.isra.71+0x23/0x90
[  343.260121]  ? new_slab+0x26e/0x4b0
[  343.260609]  ? kasan_poison+0x3a/0x50
[  343.261118]  ? kasan_unpoison+0x28/0x50
[  343.261637]  ? __kasan_slab_alloc+0x71/0x90
[  343.262214]  ? memcpy+0x4d/0x60
[  343.262674]  ? write_comp_data+0x2f/0x90
[  343.263209]  ? __kasan_check_write+0x14/0x20
[  343.263802]  ? __skb_clone+0x5d6/0x840
[  343.264329]  ? __sanitizer_cov_trace_pc+0x21/0x60
[  343.264958]  dev_queue_xmit+0x1c/0x20
[  343.265470]  netlink_deliver_tap+0x652/0x9c0
[  343.266067]  netlink_unicast+0x5a0/0x7f0
[  343.266608]  ? netlink_attachskb+0x860/0x860
[  343.267183]  ? __sanitizer_cov_trace_pc+0x21/0x60
[  343.267820]  ? write_comp_data+0x2f/0x90
[  343.268367]  netlink_sendmsg+0x922/0xe80
[  343.268899]  ? netlink_unicast+0x7f0/0x7f0
[  343.269472]  ? __sanitizer_cov_trace_pc+0x21/0x60
[  343.270099]  ? write_comp_data+0x2f/0x90
[  343.270644]  ? netlink_unicast+0x7f0/0x7f0
[  343.271210]  sock_sendmsg+0x155/0x190
[  343.271721]  ____sys_sendmsg+0x75f/0x8f0
[  343.272262]  ? kernel_sendmsg+0x60/0x60
[  343.272788]  ? write_comp_data+0x2f/0x90
[  343.273332]  ? write_comp_data+0x2f/0x90
[  343.273869]  ___sys_sendmsg+0x10f/0x190
[  343.274405]  ? sendmsg_copy_msghdr+0x80/0x80
[  343.274984]  ? slab_post_alloc_hook+0x70/0x230
[  343.275597]  ? futex_wait_setup+0x240/0x240
[  343.276175]  ? security_file_alloc+0x3e/0x170
[  343.276779]  ? write_comp_d
---truncated---
CVSS v3.1 score: 5.5