ipv6: fix possible race in __fib6_drop_pcpu_from()

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

ipv6: fix possible race in __fib6_drop_pcpu_from()

syzbot found a race in __fib6_drop_pcpu_from() [1]

If compiler reads more than once (*ppcpu_rt),
second read could read NULL, if another cpu clears
the value in rt6_get_pcpu_route().

Add a READ_ONCE() to prevent this race.

Also add rcu_read_lock()/rcu_read_unlock() because
we rely on RCU protection while dereferencing pcpu_rt.

[1]

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000012: 0000 [#1] PREEMPT SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000090-0x0000000000000097]
CPU: 0 PID: 7543 Comm: kworker/u8:17 Not tainted 6.10.0-rc1-syzkaller-00013-g2bfcfd584ff5 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/02/2024
Workqueue: netns cleanup_net
RIP: 0010:__fib6_drop_pcpu_from.part.0+0x10a/0x370 net/ipv6/ip6_fib.c:984
Code: f8 48 c1 e8 03 80 3c 28 00 0f 85 16 02 00 00 4d 8b 3f 4d 85 ff 74 31 e8 74 a7 fa f7 49 8d bf 90 00 00 00 48 89 f8 48 c1 e8 03 <80> 3c 28 00 0f 85 1e 02 00 00 49 8b 87 90 00 00 00 48 8b 0c 24 48
RSP: 0018:ffffc900040df070 EFLAGS: 00010206
RAX: 0000000000000012 RBX: 0000000000000001 RCX: ffffffff89932e16
RDX: ffff888049dd1e00 RSI: ffffffff89932d7c RDI: 0000000000000091
RBP: dffffc0000000000 R08: 0000000000000005 R09: 0000000000000007
R10: 0000000000000001 R11: 0000000000000006 R12: ffff88807fa080b8
R13: fffffbfff1a9a07d R14: ffffed100ff41022 R15: 0000000000000001
FS: 0000000000000000(0000) GS:ffff8880b9200000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b32c26000 CR3: 000000005d56e000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
__fib6_drop_pcpu_from net/ipv6/ip6_fib.c:966 [inline]
fib6_drop_pcpu_from net/ipv6/ip6_fib.c:1027 [inline]
fib6_purge_rt+0x7f2/0x9f0 net/ipv6/ip6_fib.c:1038
fib6_del_route net/ipv6/ip6_fib.c:1998 [inline]
fib6_del+0xa70/0x17b0 net/ipv6/ip6_fib.c:2043
fib6_clean_node+0x426/0x5b0 net/ipv6/ip6_fib.c:2205
fib6_walk_continue+0x44f/0x8d0 net/ipv6/ip6_fib.c:2127
fib6_walk+0x182/0x370 net/ipv6/ip6_fib.c:2175
fib6_clean_tree+0xd7/0x120 net/ipv6/ip6_fib.c:2255
__fib6_clean_all+0x100/0x2d0 net/ipv6/ip6_fib.c:2271
rt6_sync_down_dev net/ipv6/route.c:4906 [inline]
rt6_disable_ip+0x7ed/0xa00 net/ipv6/route.c:4911
addrconf_ifdown.isra.0+0x117/0x1b40 net/ipv6/addrconf.c:3855
addrconf_notify+0x223/0x19e0 net/ipv6/addrconf.c:3778
notifier_call_chain+0xb9/0x410 kernel/notifier.c:93
call_netdevice_notifiers_info+0xbe/0x140 net/core/dev.c:1992
call_netdevice_notifiers_extack net/core/dev.c:2030 [inline]
call_netdevice_notifiers net/core/dev.c:2044 [inline]
dev_close_many+0x333/0x6a0 net/core/dev.c:1585
unregister_netdevice_many_notify+0x46d/0x19f0 net/core/dev.c:11193
unregister_netdevice_many net/core/dev.c:11276 [inline]
default_device_exit_batch+0x85b/0xae0 net/core/dev.c:11759
ops_exit_list+0x128/0x180 net/core/net_namespace.c:178
cleanup_net+0x5b7/0xbf0 net/core/net_namespace.c:640
process_one_work+0x9fb/0x1b60 kernel/workqueue.c:3231
process_scheduled_works kernel/workqueue.c:3312 [inline]
worker_thread+0x6c8/0xf70 kernel/workqueue.c:3393
kthread+0x2c1/0x3a0 kernel/kthread.c:389
ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:147
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244