ipvlan: Fix use-after-free in ipvlan_get_iflink().

In the Linux kernel, the following vulnerability has been resolved:

ipvlan: Fix use-after-free in ipvlan_get_iflink().

syzbot presented an use-after-free report [0] regarding ipvlan and
linkwatch.

ipvlan does not hold a refcnt of the lower device unlike vlan and
macvlan.

If the linkwatch work is triggered for the ipvlan dev, the lower dev
might have already been freed, resulting in UAF of ipvlan->phy_dev in
ipvlan_get_iflink().

We can delay the lower dev unregistration like vlan and macvlan by
holding the lower dev's refcnt in dev->netdev_ops->ndo_init() and
releasing it in dev->priv_destructor().

Jakub pointed out calling .ndo_XXX after unregister_netdevice() has
returned is error prone and suggested [1] addressing this UAF in the
core by taking commit 750e51603395 ("net: avoid potential UAF in
default_operstate()") further.

Let's assume unregistering devices DOWN and use RCU protection in
default_operstate() not to race with the device unregistration.

[0]:
BUG: KASAN: slab-use-after-free in ipvlan_get_iflink+0x84/0x88 drivers/net/ipvlan/ipvlan_main.c:353
Read of size 4 at addr ffff0000d768c0e0 by task kworker/u8:35/6944

CPU: 0 UID: 0 PID: 6944 Comm: kworker/u8:35 Not tainted 6.13.0-rc2-g9bc5c9515b48 #12 4c3cb9e8b4565456f6a355f312ff91f4f29b3c47
Hardware name: linux,dummy-virt (DT)
Workqueue: events_unbound linkwatch_event
Call trace:
show_stack+0x38/0x50 arch/arm64/kernel/stacktrace.c:484 (C)
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0xbc/0x108 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0x16c/0x6f0 mm/kasan/report.c:489
kasan_report+0xc0/0x120 mm/kasan/report.c:602
__asan_report_load4_noabort+0x20/0x30 mm/kasan/report_generic.c:380
ipvlan_get_iflink+0x84/0x88 drivers/net/ipvlan/ipvlan_main.c:353
dev_get_iflink+0x7c/0xd8 net/core/dev.c:674
default_operstate net/core/link_watch.c:45 [inline]
rfc2863_policy+0x144/0x360 net/core/link_watch.c:72
linkwatch_do_dev+0x60/0x228 net/core/link_watch.c:175
__linkwatch_run_queue+0x2f4/0x5b8 net/core/link_watch.c:239
linkwatch_event+0x64/0xa8 net/core/link_watch.c:282
process_one_work+0x700/0x1398 kernel/workqueue.c:3229
process_scheduled_works kernel/workqueue.c:3310 [inline]
worker_thread+0x8c4/0xe10 kernel/workqueue.c:3391
kthread+0x2b0/0x360 kernel/kthread.c:389
ret_from_fork+0x10/0x20 arch/arm64/kernel/entry.S:862

Allocated by task 9303:
kasan_save_stack mm/kasan/common.c:47 [inline]
kasan_save_track+0x30/0x68 mm/kasan/common.c:68
kasan_save_alloc_info+0x44/0x58 mm/kasan/generic.c:568
poison_kmalloc_redzone mm/kasan/common.c:377 [inline]
__kasan_kmalloc+0x84/0xa0 mm/kasan/common.c:394
kasan_kmalloc include/linux/kasan.h:260 [inline]
__do_kmalloc_node mm/slub.c:4283 [inline]
__kmalloc_node_noprof+0x2a0/0x560 mm/slub.c:4289
__kvmalloc_node_noprof+0x9c/0x230 mm/util.c:650
alloc_netdev_mqs+0xb4/0x1118 net/core/dev.c:11209
rtnl_create_link+0x2b8/0xb60 net/core/rtnetlink.c:3595
rtnl_newlink_create+0x19c/0x868 net/core/rtnetlink.c:3771
__rtnl_newlink net/core/rtnetlink.c:3896 [inline]
rtnl_newlink+0x122c/0x15c0 net/core/rtnetlink.c:4011
rtnetlink_rcv_msg+0x61c/0x918 net/core/rtnetlink.c:6901
netlink_rcv_skb+0x1dc/0x398 net/netlink/af_netlink.c:2542
rtnetlink_rcv+0x34/0x50 net/core/rtnetlink.c:6928
netlink_unicast_kernel net/netlink/af_netlink.c:1321 [inline]
netlink_unicast+0x618/0x838 net/netlink/af_netlink.c:1347
netlink_sendmsg+0x5fc/0x8b0 net/netlink/af_netlink.c:1891
sock_sendmsg_nosec net/socket.c:711 [inline]
__sock_sendmsg net/socket.c:726 [inline]
__sys_sendto+0x2ec/0x438 net/socket.c:2197
__do_sys_sendto net/socket.c:2204 [inline]
__se_sys_sendto net/socket.c:2200 [inline]
__arm64_sys_sendto+0xe4/0x110 net/socket.c:2200
__invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
invoke_syscall+0x90/0x278 arch/arm64/kernel/syscall.c:49
el0_svc_common+0x13c/0x250 arch/arm64/kernel/syscall.c:132
do_el0_svc+0x54/0x70 arch/arm64/kernel/syscall.c:151
el
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