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

In the rmnet_changelink(), it uses IFLA_LINK without checking
NULL pointer.
tb[IFLA_LINK] could be NULL pointer.
So, NULL-ptr-deref could occur.

rmnet already has a lower interface (real_dev).
So, after this patch, rmnet_changelink() does not use IFLA_LINK anymore.

Test commands:
    modprobe rmnet
    ip link add dummy0 type dummy
    ip link add rmnet0 link dummy0 type rmnet mux_id 1
    ip link set rmnet0 type rmnet mux_id 2

Splat looks like:
[   73.784702][  T944] general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 I
[   73.786128][  T944] KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
[   73.786894][  T944] CPU: 0 PID: 944 Comm: ip Not tainted 5.5.0+ torvalds#406
[   73.787573][  T944] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[   73.788865][  T944] RIP: 0010:rmnet_changelink+0x5a/0x8a0 [rmnet]
[   73.789895][  T944] Code: 83 ec 20 48 c1 ea 03 80 3c 02 00 0f 85 6f 07 00 00 48 8b 5e 28 48 b8 00 00 00 00 00 fc ff d0
[   73.792187][  T944] RSP: 0018:ffff88804ca571b8 EFLAGS: 00010247
[   73.792939][  T944] RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffff88804ca578b0
[   73.793880][  T944] RDX: 0000000000000000 RSI: ffff88804ca574a0 RDI: 0000000000000004
[   73.794904][  T944] RBP: ffff88806241f400 R08: 0000000000000002 R09: 0000000000000002
[   73.796720][  T944] R10: ffffffffc03f2a80 R11: 0000000000000000 R12: ffff88804d094000
[   73.797721][  T944] R13: ffff88804d094000 R14: ffff88806241e800 R15: 0000000000000000
[   73.798770][  T944] FS:  00007fc9d81650c0(0000) GS:ffff88806c000000(0000) knlGS:0000000000000000
[   73.800184][  T944] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   73.801265][  T944] CR2: 0000557c57167458 CR3: 0000000066eea003 CR4: 00000000000606f0
[   73.802297][  T944] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   73.803290][  T944] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   73.804433][  T944] Call Trace:
[   73.804855][  T944]  __rtnl_newlink+0x922/0x1270
[   73.805459][  T944]  ? lock_downgrade+0x6e0/0x6e0
[   73.806068][  T944]  ? rtnl_link_unregister+0x220/0x220
[   73.806737][  T944]  ? lock_acquire+0x164/0x3b0
[   73.807328][  T944]  ? is_bpf_image_address+0xff/0x1d0
[   73.807983][  T944]  ? rtnl_newlink+0x4c/0x90
[   73.808540][  T944]  ? kernel_text_address+0x111/0x140
[   73.809196][  T944]  ? __kernel_text_address+0xe/0x30
[   73.809859][  T944]  ? unwind_get_return_address+0x5f/0xa0
[   73.810560][  T944]  ? create_prof_cpu_mask+0x20/0x20
[   73.811614][  T944]  ? arch_stack_walk+0x83/0xb0
[   73.812350][  T944]  ? stack_trace_save+0x82/0xb0
[   73.812976][  T944]  ? stack_trace_consume_entry+0x160/0x160
[   73.814174][  T944]  ? deactivate_slab.isra.78+0x2c5/0x800
[   73.814920][  T944]  ? kasan_unpoison_shadow+0x30/0x40
[   73.815594][  T944]  ? kmem_cache_alloc_trace+0x135/0x350
[   73.816789][  T944]  ? rtnl_newlink+0x4c/0x90
[   73.817357][  T944]  rtnl_newlink+0x65/0x90
[ ... ]

Fixes: 23790ef ("net: qualcomm: rmnet: Allow to configure flags for existing devices")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>

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

@@ -300,10 +300,8 @@ static int rmnet_changelink(struct net_device *dev, struct nlattr *tb[],
 	if (!dev)
 		return -ENODEV;
 
-	real_dev = __dev_get_by_index(dev_net(dev),
-				      nla_get_u32(tb[IFLA_LINK]));
-
-	if (!real_dev || !rmnet_is_real_dev_registered(real_dev))
+	real_dev = priv->real_dev;
+	if (!rmnet_is_real_dev_registered(real_dev))
 		return -ENODEV;
 
 	port = rmnet_get_port_rtnl(real_dev);