[v5.15] WARNING in rt6_multipath_rebalance

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 7b232985052f Linux 5.15.199
git tree: linux-5.15.y
console output: https://syzkaller.appspot.com/x/log.txt?x=176bf8aa580000
kernel config: https://syzkaller.appspot.com/x/.config?x=353ae28c40b35af5
dashboard link: https://syzkaller.appspot.com/bug?extid=797eb03bd68dbed8d216
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/97a23052e207/disk-7b232985.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/e9871fb1bda3/vmlinux-7b232985.xz
kernel image: https://storage.googleapis.com/syzbot-assets/b08e48f5be44/bzImage-7b232985.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+797eb0...@syzkaller.appspotmail.com

device veth1_macvtap left promiscuous mode
device veth0_macvtap left promiscuous mode
device veth1_vlan left promiscuous mode
device veth0_vlan left promiscuous mode
------------[ cut here ]------------
WARNING: CPU: 1 PID: 155 at net/ipv6/route.c:4767 rt6_multipath_first_sibling net/ipv6/route.c:4689 [inline]
WARNING: CPU: 1 PID: 155 at net/ipv6/route.c:4767 rt6_multipath_rebalance+0x449/0x8a0 net/ipv6/route.c:4766
Modules linked in:
CPU: 1 PID: 155 Comm: kworker/u4:3 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026
Workqueue: netns cleanup_net
RIP: 0010:rt6_multipath_rebalance+0x449/0x8a0 net/ipv6/route.c:4767
Code: e8 8c 6c 1e f9 e9 1c fe ff ff 44 89 e1 80 e1 07 38 c1 0f 8c 85 fe ff ff 4c 89 e7 e8 51 6c 1e f9 e9 78 fe ff ff e8 b7 60 d9 f8 <0f> 0b eb 07 e8 ae 60 d9 f8 0f 0b 48 83 c4 10 5b 41 5c 41 5d 41 5e
RSP: 0018:ffffc90001ff70c0 EFLAGS: 00010293
RAX: ffffffff889fc019 RBX: ffff88805f70dc00 RCX: ffff88801d619dc0
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000000
RBP: dffffc0000000000 R08: ffff88807ba80ba7 R09: 1ffff1100f750174
R10: dffffc0000000000 R11: ffffed100f750175 R12: ffff88805f70dcb6
R13: ffff88805f70dc80 R14: 0000000000000000 R15: 1ffff1100bee1b90
FS: 0000000000000000(0000) GS:ffff8880b9100000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000110c41751e CR3: 000000005f58e000 CR4: 00000000003506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
fib6_ifdown+0x3ef/0x4a0 net/ipv6/route.c:4887
fib6_clean_node+0x249/0x540 net/ipv6/ip6_fib.c:2190
fib6_walk_continue+0x640/0x8c0 net/ipv6/ip6_fib.c:2115
fib6_walk+0x149/0x290 net/ipv6/ip6_fib.c:2163
fib6_clean_tree net/ipv6/ip6_fib.c:2243 [inline]
__fib6_clean_all+0x25f/0x3c0 net/ipv6/ip6_fib.c:2259
rt6_sync_down_dev net/ipv6/route.c:4915 [inline]
rt6_disable_ip+0x127/0x6b0 net/ipv6/route.c:4920
addrconf_ifdown+0x155/0x19c0 net/ipv6/addrconf.c:3775
addrconf_notify+0x445/0xf00 net/ipv6/addrconf.c:-1
notifier_call_chain kernel/notifier.c:83 [inline]
raw_notifier_call_chain+0xcb/0x160 kernel/notifier.c:391
call_netdevice_notifiers_extack net/core/dev.c:2074 [inline]
call_netdevice_notifiers net/core/dev.c:2088 [inline]
dev_close_many+0x29f/0x400 net/core/dev.c:1663
unregister_netdevice_many+0x481/0x19f0 net/core/dev.c:11123
default_device_exit_batch+0x364/0x3c0 net/core/dev.c:11680
ops_exit_list net/core/net_namespace.c:177 [inline]
cleanup_net+0x791/0xba0 net/core/net_namespace.c:635
process_one_work+0x85f/0x1010 kernel/workqueue.c:2310
worker_thread+0xaa6/0x1290 kernel/workqueue.c:2457
kthread+0x436/0x520 kernel/kthread.c:334
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:287
</TASK>


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