net: rmnet: fix NULL pointer dereference in rmnet_newlink()

rmnet registers IFLA_LINK interface as a lower interface.
But, IFLA_LINK could be NULL.
In the current code, rmnet doesn't check IFLA_LINK.
So, panic would occur.

Test commands:
    modprobe rmnet
    ip link add rmnet0 type rmnet mux_id 1

Splat looks like:
[   79.718433][  T923] general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 I
[   79.721234][  T923] KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
[   79.722852][  T923] CPU: 0 PID: 923 Comm: ip Not tainted 5.5.0+ torvalds#394
[   79.723897][  T923] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[   79.725395][  T923] RIP: 0010:rmnet_newlink+0x54/0x510 [rmnet]
[   79.726349][  T923] Code: 83 ec 18 48 c1 e9 03 80 3c 01 00 0f 85 d4 03 00 00 48 8b 6a 28 48 b8 00 00 00 00 00 fc ff dc
[   79.729114][  T923] RSP: 0018:ffff88804cc9f1c0 EFLAGS: 00010247
[   79.729970][  T923] RAX: dffffc0000000000 RBX: ffff8880620dfa00 RCX: 1ffff11009993e99
[   79.731122][  T923] RDX: 0000000000000000 RSI: ffff888064f34000 RDI: 0000000000000004
[   79.732389][  T923] RBP: 0000000000000000 R08: ffff88804cc9f8b0 R09: ffff8880644f0990
[   79.733533][  T923] R10: ffffffffc04cfa40 R11: ffffed100c89e137 R12: ffffffff96ceacc0
[   79.734810][  T923] R13: ffff888064f34000 R14: ffff88804cc9f8b0 R15: ffff888064f34000
[   79.736102][  T923] FS:  00007f11d83b60c0(0000) GS:ffff88806c000000(0000) knlGS:0000000000000000
[   79.737509][  T923] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   79.738430][  T923] CR2: 00005614e7c6bb00 CR3: 0000000064d58006 CR4: 00000000000606f0
[   79.739545][  T923] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   79.740663][  T923] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   79.741786][  T923] Call Trace:
[   79.742244][  T923]  __rtnl_newlink+0xbdb/0x1270
[   79.742846][  T923]  ? lock_downgrade+0x6e0/0x6e0
[   79.743450][  T923]  ? rtnl_link_unregister+0x220/0x220
[   79.744122][  T923]  ? lock_acquire+0x164/0x3b0
[   79.744724][  T923]  ? is_bpf_image_address+0xff/0x1d0
[   79.745397][  T923]  ? rtnl_newlink+0x4c/0x90
[   79.745958][  T923]  ? kernel_text_address+0x111/0x140
[   79.746621][  T923]  ? __kernel_text_address+0xe/0x30
[   79.747270][  T923]  ? unwind_get_return_address+0x5f/0xa0
[   79.747974][  T923]  ? create_prof_cpu_mask+0x20/0x20
[   79.748635][  T923]  ? arch_stack_walk+0x83/0xb0
[   79.749238][  T923]  ? stack_trace_save+0x82/0xb0
[   79.749850][  T923]  ? stack_trace_consume_entry+0x160/0x160
[   79.750574][  T923]  ? deactivate_slab.isra.78+0x2c5/0x800
[   79.751306][  T923]  ? kasan_unpoison_shadow+0x30/0x40
[   79.751971][  T923]  ? kmem_cache_alloc_trace+0x135/0x350
[   79.752671][  T923]  ? rtnl_newlink+0x4c/0x90
[   79.753239][  T923]  rtnl_newlink+0x65/0x90
[ ... ]

Fixes: ceed73a ("drivers: net: ethernet: qualcomm: rmnet: Initial implementation")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>

Author: TaeheeYoo

drivers/net/ethernet/qualcomm/rmnet/rmnet_config.c

@@ -135,6 +135,11 @@ static int rmnet_newlink(struct net *src_net, struct net_device *dev,
 	int err = 0;
 	u16 mux_id;
 
+	if (!tb[IFLA_LINK]) {
+		NL_SET_ERR_MSG_MOD(extack, "link not specified");
+		return -EINVAL;
+	}
+
 	real_dev = __dev_get_by_index(src_net, nla_get_u32(tb[IFLA_LINK]));
 	if (!real_dev || !dev)
 		return -ENODEV;