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---