net: avoid potential UAF in default_operstate()

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

net: avoid potential UAF in default_operstate()

syzbot reported an UAF in default_operstate() [1]

Issue is a race between device and netns dismantles.

After calling __rtnl_unlock() from netdev_run_todo(),
we can not assume the netns of each device is still alive.

Make sure the device is not in NETREG_UNREGISTERED state,
and add an ASSERT_RTNL() before the call to
__dev_get_by_index().

We might move this ASSERT_RTNL() in __dev_get_by_index()
in the future.

[1]

BUG: KASAN: slab-use-after-free in __dev_get_by_index+0x5d/0x110 net/core/dev.c:852
Read of size 8 at addr ffff888043eba1b0 by task syz.0.0/5339

CPU: 0 UID: 0 PID: 5339 Comm: syz.0.0 Not tainted 6.12.0-syzkaller-10296-gaaf20f870da0 #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0x169/0x550 mm/kasan/report.c:489
kasan_report+0x143/0x180 mm/kasan/report.c:602
__dev_get_by_index+0x5d/0x110 net/core/dev.c:852
default_operstate net/core/link_watch.c:51 [inline]
rfc2863_policy+0x224/0x300 net/core/link_watch.c:67
linkwatch_do_dev+0x3e/0x170 net/core/link_watch.c:170
netdev_run_todo+0x461/0x1000 net/core/dev.c:10894
rtnl_unlock net/core/rtnetlink.c:152 [inline]
rtnl_net_unlock include/linux/rtnetlink.h:133 [inline]
rtnl_dellink+0x760/0x8d0 net/core/rtnetlink.c:3520
rtnetlink_rcv_msg+0x791/0xcf0 net/core/rtnetlink.c:6911
netlink_rcv_skb+0x1e3/0x430 net/netlink/af_netlink.c:2541
netlink_unicast_kernel net/netlink/af_netlink.c:1321 [inline]
netlink_unicast+0x7f6/0x990 net/netlink/af_netlink.c:1347
netlink_sendmsg+0x8e4/0xcb0 net/netlink/af_netlink.c:1891
sock_sendmsg_nosec net/socket.c:711 [inline]
__sock_sendmsg+0x221/0x270 net/socket.c:726
____sys_sendmsg+0x52a/0x7e0 net/socket.c:2583
___sys_sendmsg net/socket.c:2637 [inline]
__sys_sendmsg+0x269/0x350 net/socket.c:2669
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f2a3cb80809
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f2a3d9cd058 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007f2a3cd45fa0 RCX: 00007f2a3cb80809
RDX: 0000000000000000 RSI: 0000000020000000 RDI: 0000000000000008
RBP: 00007f2a3cbf393e R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00007f2a3cd45fa0 R15: 00007ffd03bc65c8
</TASK>

Allocated by task 5339:
kasan_save_stack mm/kasan/common.c:47 [inline]
kasan_save_track+0x3f/0x80 mm/kasan/common.c:68
poison_kmalloc_redzone mm/kasan/common.c:377 [inline]
__kasan_kmalloc+0x98/0xb0 mm/kasan/common.c:394
kasan_kmalloc include/linux/kasan.h:260 [inline]
__kmalloc_cache_noprof+0x243/0x390 mm/slub.c:4314
kmalloc_noprof include/linux/slab.h:901 [inline]
kmalloc_array_noprof include/linux/slab.h:945 [inline]
netdev_create_hash net/core/dev.c:11870 [inline]
netdev_init+0x10c/0x250 net/core/dev.c:11890
ops_init+0x31e/0x590 net/core/net_namespace.c:138
setup_net+0x287/0x9e0 net/core/net_namespace.c:362
copy_net_ns+0x33f/0x570 net/core/net_namespace.c:500
create_new_namespaces+0x425/0x7b0 kernel/nsproxy.c:110
unshare_nsproxy_namespaces+0x124/0x180 kernel/nsproxy.c:228
ksys_unshare+0x57d/0xa70 kernel/fork.c:3314
__do_sys_unshare kernel/fork.c:3385 [inline]
__se_sys_unshare kernel/fork.c:3383 [inline]
__x64_sys_unshare+0x38/0x40 kernel/fork.c:3383
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x8
---truncated---