macvlan: fix null dereference in macvlan_device_event()

In the macvlan_device_event(), the list_first_entry_or_null() is used.
This function could return null pointer if there is no node.
But, the macvlan module doesn't check the null pointer.
So, null-ptr-deref would occur.

      bond0
        |
   +----+-----+
   |          |
macvlan0   macvlan1
   |          |
 dummy0     dummy1

The problem scenario.
If dummy1 is removed,
1. ->dellink() of dummy1 is called.
2. NETDEV_UNREGISTER of dummy1 notification is sent to macvlan module.
3. ->dellink() of macvlan1 is called.
4. NETDEV_UNREGISTER of macvlan1 notification is sent to bond module.
5. __bond_release_one() is called and it internally calls
   dev_set_mac_address().
6. dev_set_mac_address() calls the ->ndo_set_mac_address() of macvlan1,
   which is macvlan_set_mac_address().
7. macvlan_set_mac_address() calls the dev_set_mac_address() with dummy1.
8. NETDEV_CHANGEADDR of dummy1 is sent to macvlan module.
9. In the macvlan_device_event(), it calls list_first_entry_or_null().
At this point, dummy1 and macvlan1 were removed.
So, list_first_entry_or_null() will return NULL.

Test commands:
    ip netns add nst
    ip netns exec nst ip link add bond0 type bond
    for i in {0..10}
    do
        ip netns exec nst ip link add dummy$i type dummy
	ip netns exec nst ip link add macvlan$i link dummy$i \
		type macvlan mode passthru
	ip netns exec nst ip link set macvlan$i master bond0
    done
    ip netns del nst

Splat looks like:
[   40.585687][  T146] general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP DEI
[   40.587249][  T146] KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
[   40.588342][  T146] CPU: 1 PID: 146 Comm: kworker/u8:2 Not tainted 5.7.0-rc1+ #532
[   40.589299][  T146] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[   40.590469][  T146] Workqueue: netns cleanup_net
[   40.591045][  T146] RIP: 0010:macvlan_device_event+0x4e2/0x900 [macvlan]
[   40.591905][  T146] Code: 00 00 00 00 00 fc ff df 80 3c 06 00 0f 85 45 02 00 00 48 89 da 48 b8 00 00 00 00 00 fc ff d2
[   40.594126][  T146] RSP: 0018:ffff88806116f4a0 EFLAGS: 00010246
[   40.594783][  T146] RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000
[   40.595653][  T146] RDX: 0000000000000000 RSI: ffff88806547ddd8 RDI: ffff8880540f1360
[   40.596495][  T146] RBP: ffff88804011a808 R08: fffffbfff4fb8421 R09: fffffbfff4fb8421
[   40.597377][  T146] R10: ffffffffa7dc2107 R11: 0000000000000000 R12: 0000000000000008
[   40.598186][  T146] R13: ffff88804011a000 R14: ffff8880540f1000 R15: 1ffff1100c22de9a
[   40.599012][  T146] FS:  0000000000000000(0000) GS:ffff888067800000(0000) knlGS:0000000000000000
[   40.600004][  T146] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   40.600665][  T146] CR2: 00005572d3a807b8 CR3: 000000005fcf4003 CR4: 00000000000606e0
[   40.601485][  T146] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   40.602461][  T146] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   40.603443][  T146] Call Trace:
[   40.603871][  T146]  ? nf_tables_dump_setelem+0xa0/0xa0 [nf_tables]
[   40.604587][  T146]  ? macvlan_uninit+0x100/0x100 [macvlan]
[   40.605212][  T146]  ? __module_text_address+0x13/0x140
[   40.605842][  T146]  notifier_call_chain+0x90/0x160
[   40.606477][  T146]  dev_set_mac_address+0x28e/0x3f0
[   40.607117][  T146]  ? netdev_notify_peers+0xc0/0xc0
[   40.607762][  T146]  ? __module_text_address+0x13/0x140
[   40.608440][  T146]  ? notifier_call_chain+0x90/0x160
[   40.609097][  T146]  ? dev_set_mac_address+0x1f0/0x3f0
[   40.609758][  T146]  dev_set_mac_address+0x1f0/0x3f0
[   40.610402][  T146]  ? __local_bh_enable_ip+0xe9/0x1b0
[   40.611071][  T146]  ? bond_hw_addr_flush+0x77/0x100 [bonding]
[   40.611823][  T146]  ? netdev_notify_peers+0xc0/0xc0
[   40.612461][  T146]  ? bond_hw_addr_flush+0x77/0x100 [bonding]
[   40.613213][  T146]  ? bond_hw_addr_flush+0x77/0x100 [bonding]
[   40.613963][  T146]  ? __local_bh_enable_ip+0xe9/0x1b0
[   40.614631][  T146]  ? bond_time_in_interval.isra.31+0x90/0x90 [bonding]
[   40.615484][  T146]  ? __bond_release_one+0x9f0/0x12c0 [bonding]
[   40.616230][  T146]  __bond_release_one+0x9f0/0x12c0 [bonding]
[   40.616949][  T146]  ? bond_enslave+0x47c0/0x47c0 [bonding]
[   40.617642][  T146]  ? lock_downgrade+0x730/0x730
[   40.618218][  T146]  ? check_flags.part.42+0x450/0x450
[   40.618850][  T146]  ? __mutex_unlock_slowpath+0xd0/0x670
[   40.619519][  T146]  ? trace_hardirqs_on+0x30/0x180
[   40.620117][  T146]  ? wait_for_completion+0x250/0x250
[   40.620754][  T146]  bond_netdev_event+0x822/0x970 [bonding]
[   40.621460][  T146]  ? __module_text_address+0x13/0x140
[   40.622097][  T146]  notifier_call_chain+0x90/0x160
[   40.622806][  T146]  rollback_registered_many+0x660/0xcf0
[   40.623522][  T146]  ? netif_set_real_num_tx_queues+0x780/0x780
[   40.624290][  T146]  ? notifier_call_chain+0x90/0x160
[   40.624957][  T146]  ? netdev_upper_dev_unlink+0x114/0x180
[   40.625686][  T146]  ? __netdev_adjacent_dev_unlink_neighbour+0x30/0x30
[   40.626421][  T146]  ? mutex_is_locked+0x13/0x50
[   40.627016][  T146]  ? unregister_netdevice_queue+0xf2/0x240
[   40.627663][  T146]  unregister_netdevice_many.part.134+0x13/0x1b0
[   40.628362][  T146]  default_device_exit_batch+0x2d9/0x390
[   40.628987][  T146]  ? unregister_netdevice_many+0x40/0x40
[   40.629615][  T146]  ? dev_change_net_namespace+0xcb0/0xcb0
[   40.630279][  T146]  ? prepare_to_wait_exclusive+0x2e0/0x2e0
[   40.630943][  T146]  ? ops_exit_list.isra.9+0x97/0x140
[   40.631554][  T146]  cleanup_net+0x441/0x890
[ ... ]

Fixes: e289fd28 ("macvlan: fix the problem when mac address changes for passthru mode")
Reported-by:  <syzbot+5035b1f9dc7ea4558d5a@syzkaller.appspotmail.com>
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>