[v5.15] WARNING: suspicious RCU usage in rt6_fill_node_nexthop

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 674f684f7c61 Linux 5.15.212
git tree: linux-5.15.y
console output: https://syzkaller.appspot.com/x/log.txt?x=148632ea580000
kernel config: https://syzkaller.appspot.com/x/.config?x=f161cbc9aef65db0
dashboard link: https://syzkaller.appspot.com/bug?extid=c105998c0a370624eb05
compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/f55be2937b89/disk-674f684f.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/a19fb0ffc049/vmlinux-674f684f.xz
kernel image: https://storage.googleapis.com/syzbot-assets/365d4529a007/bzImage-674f684f.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+c10599...@syzkaller.appspotmail.com

=============================
WARNING: suspicious RCU usage
syzkaller #0 Not tainted
-----------------------------
include/net/nexthop.h:319 suspicious rcu_dereference_protected() usage!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 1
3 locks held by syz.6.1384/8672:
#0: ffff888044434690 (nlk_cb_mutex-ROUTE){+.+.}-{3:3}, at: __netlink_dump_start+0xf5/0x720 net/netlink/af_netlink.c:2363
#1: ffffffff8c31ef60 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:313
#2: ffff88805fb73c30 (&tb->tb6_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:369 [inline]
#2: ffff88805fb73c30 (&tb->tb6_lock){+...}-{2:2}, at: fib6_dump_table+0x1ce/0x5b0 net/ipv6/ip6_fib.c:587

stack backtrace:
CPU: 0 PID: 8672 Comm: syz.6.1384 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/16/2026
Call Trace:
<TASK>
dump_stack_lvl+0x188/0x250 lib/dump_stack.c:106
nexthop_mpath_fill_node include/net/nexthop.h:319 [inline]
rt6_fill_node_nexthop+0x2cc/0x600 net/ipv6/route.c:5594
rt6_fill_node+0x109e/0x1a40 net/ipv6/route.c:5756
rt6_dump_route+0x592/0xa70 net/ipv6/route.c:5926
fib6_dump_node+0xa2/0x250 net/ipv6/ip6_fib.c:521
fib6_walk_continue+0x662/0x8b0 net/ipv6/ip6_fib.c:2118
fib6_walk+0x149/0x270 net/ipv6/ip6_fib.c:2166
fib6_dump_table+0x1da/0x5b0 net/ipv6/ip6_fib.c:588
inet6_dump_fib+0x6bc/0xa60 net/ipv6/ip6_fib.c:702
netlink_dump+0x6d2/0xdd0 net/netlink/af_netlink.c:2291
__netlink_dump_start+0x4e4/0x720 net/netlink/af_netlink.c:2398
netlink_dump_start include/linux/netlink.h:260 [inline]
rtnetlink_rcv_msg+0xd91/0xf10 net/core/rtnetlink.c:5654
netlink_rcv_skb+0x1e9/0x430 net/netlink/af_netlink.c:2519
netlink_unicast_kernel net/netlink/af_netlink.c:1314 [inline]
netlink_unicast+0x78f/0x940 net/netlink/af_netlink.c:1340
netlink_sendmsg+0x8ba/0xbe0 net/netlink/af_netlink.c:1919
sock_sendmsg_nosec net/socket.c:706 [inline]
__sock_sendmsg net/socket.c:718 [inline]
____sys_sendmsg+0x5be/0x8f0 net/socket.c:2446
___sys_sendmsg+0x236/0x2e0 net/socket.c:2500
__sys_sendmsg net/socket.c:2529 [inline]
__do_sys_sendmsg net/socket.c:2538 [inline]
__se_sys_sendmsg+0x1b7/0x2a0 net/socket.c:2536
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x66/0xd0
RIP: 0033:0x7f193810ce99
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f1936365028 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007f1938394fa0 RCX: 00007f193810ce99
RDX: 0000000000000000 RSI: 0000200000000100 RDI: 0000000000000005
RBP: 00007f19381a2eaf R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f1938395038 R14: 00007f1938394fa0 R15: 00007ffced9b0878
</TASK>


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup