KASAN: null-ptr-deref Write in fib6_purge_rt (2)

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 241e3900 nfsd: replace Jeff by Chuck as nfsd co-maintainer
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15a806b1a00000
kernel config: https://syzkaller.appspot.com/x/.config?x=e5c77f8090a3b96b
dashboard link: https://syzkaller.appspot.com/bug?extid=f855c2ccf4c4bb9b7e5d
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
CC: [da...@davemloft.net kuz...@ms2.inr.ac.ru
linux-...@vger.kernel.org net...@vger.kernel.org yosh...@linux-ipv6.org]

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+f855c2...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: null-ptr-deref in fib6_drop_pcpu_from net/ipv6/ip6_fib.c:921
[inline]
BUG: KASAN: null-ptr-deref in fib6_purge_rt+0x1b3/0x5e0
net/ipv6/ip6_fib.c:933
Write of size 8 at addr 00000000000000e7 by task kworker/u4:4/2546

CPU: 0 PID: 2546 Comm: kworker/u4:4 Not tainted 5.2.0-rc5+ #32
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Workqueue: netns cleanup_net
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x172/0x1f0 lib/dump_stack.c:113
__kasan_report.cold+0x5/0x40 mm/kasan/report.c:321
kasan_report+0x12/0x20 mm/kasan/common.c:614
check_memory_region_inline mm/kasan/generic.c:185 [inline]
check_memory_region+0x123/0x190 mm/kasan/generic.c:191
kasan_check_write+0x14/0x20 mm/kasan/common.c:100
fib6_drop_pcpu_from net/ipv6/ip6_fib.c:921 [inline]
fib6_purge_rt+0x1b3/0x5e0 net/ipv6/ip6_fib.c:933
fib6_del_route net/ipv6/ip6_fib.c:1808 [inline]
fib6_del+0xac2/0x10a0 net/ipv6/ip6_fib.c:1839
fib6_clean_node+0x3a5/0x590 net/ipv6/ip6_fib.c:2001
fib6_walk_continue+0x4a9/0x8e0 net/ipv6/ip6_fib.c:1923
fib6_walk+0x9d/0x100 net/ipv6/ip6_fib.c:1971
fib6_clean_tree+0xe0/0x120 net/ipv6/ip6_fib.c:2050
__fib6_clean_all+0x118/0x2a0 net/ipv6/ip6_fib.c:2066
fib6_clean_all+0x2b/0x40 net/ipv6/ip6_fib.c:2077
rt6_sync_down_dev+0x134/0x150 net/ipv6/route.c:4165
rt6_disable_ip+0x27/0x5f0 net/ipv6/route.c:4170
addrconf_ifdown+0xa2/0x1220 net/ipv6/addrconf.c:3705
addrconf_notify+0x5db/0x2270 net/ipv6/addrconf.c:3630
notifier_call_chain+0xc2/0x230 kernel/notifier.c:95
__raw_notifier_call_chain kernel/notifier.c:396 [inline]
raw_notifier_call_chain+0x2e/0x40 kernel/notifier.c:403
call_netdevice_notifiers_info+0x3f/0x90 net/core/dev.c:1749
call_netdevice_notifiers_extack net/core/dev.c:1761 [inline]
call_netdevice_notifiers net/core/dev.c:1775 [inline]
dev_close_many+0x33f/0x6f0 net/core/dev.c:1518
rollback_registered_many+0x43b/0xfc0 net/core/dev.c:8183
unregister_netdevice_many.part.0+0x1b/0x1f0 net/core/dev.c:9314
unregister_netdevice_many net/core/dev.c:9313 [inline]
default_device_exit_batch+0x34e/0x410 net/core/dev.c:9783
ops_exit_list.isra.0+0xfc/0x150 net/core/net_namespace.c:157
cleanup_net+0x3fb/0x960 net/core/net_namespace.c:553
process_one_work+0x989/0x1790 kernel/workqueue.c:2269
worker_thread+0x98/0xe40 kernel/workqueue.c:2415
kthread+0x354/0x420 kernel/kthread.c:255
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
==================================================================


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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.