[v5.15] WARNING: suspicious RCU usage in __ipv4_neigh_lookup_noref

[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=151005c1580000
kernel config: https://syzkaller.appspot.com/x/.config?x=f161cbc9aef65db0
dashboard link: https://syzkaller.appspot.com/bug?extid=047faacac7feb67b6c6c
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+047faa...@syzkaller.appspotmail.com

=============================
WARNING: suspicious RCU usage
syzkaller #0 Not tainted
-----------------------------
include/net/neighbour.h:286 suspicious rcu_dereference_check() usage!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 1
1 lock held by syz.4.249/5174:
#0: ffffffff8c31ef60 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:313

stack backtrace:
CPU: 1 PID: 5174 Comm: syz.4.249 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
___neigh_lookup_noref include/net/neighbour.h:286 [inline]
__ipv4_neigh_lookup_noref+0x376/0x420 include/net/arp.h:27
bpf_ipv4_fib_lookup+0xc8a/0x1310 net/core/filter.c:5703
____bpf_xdp_fib_lookup net/core/filter.c:5873 [inline]
bpf_xdp_fib_lookup+0x17c/0x1c0 net/core/filter.c:5861
bpf_prog_744adbbd36285891+0x3e/0xa48
__bpf_prog_run include/linux/filter.h:621 [inline]
bpf_prog_run_xdp include/linux/filter.h:830 [inline]
bpf_test_run+0x567/0x800 net/bpf/test_run.c:117
bpf_prog_test_run_xdp+0x3e8/0x910 net/bpf/test_run.c:821
bpf_prog_test_run+0x31c/0x390 kernel/bpf/syscall.c:3374
__sys_bpf+0x5a5/0x6f0 kernel/bpf/syscall.c:4675
__do_sys_bpf kernel/bpf/syscall.c:4761 [inline]
__se_sys_bpf kernel/bpf/syscall.c:4759 [inline]
__x64_sys_bpf+0x78/0x90 kernel/bpf/syscall.c:4759
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: 0033:0x7f57c0301e99
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:00007f57be55a028 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
RAX: ffffffffffffffda RBX: 00007f57c0589fa0 RCX: 00007f57c0301e99
RDX: 0000000000000050 RSI: 0000200000000700 RDI: 000000000000000a
RBP: 00007f57c0397eaf R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f57c058a038 R14: 00007f57c0589fa0 R15: 00007ffe051e4f68
</TASK>

=============================
WARNING: suspicious RCU usage
syzkaller #0 Not tainted
-----------------------------
include/net/neighbour.h:291 suspicious rcu_dereference_check() usage!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 1
1 lock held by syz.4.249/5174:
#0: ffffffff8c31ef60 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:313

stack backtrace:
CPU: 1 PID: 5174 Comm: syz.4.249 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
___neigh_lookup_noref include/net/neighbour.h:291 [inline]
__ipv4_neigh_lookup_noref+0x39f/0x420 include/net/arp.h:27
bpf_ipv4_fib_lookup+0xc8a/0x1310 net/core/filter.c:5703
____bpf_xdp_fib_lookup net/core/filter.c:5873 [inline]
bpf_xdp_fib_lookup+0x17c/0x1c0 net/core/filter.c:5861
bpf_prog_744adbbd36285891+0x3e/0xa48
__bpf_prog_run include/linux/filter.h:621 [inline]
bpf_prog_run_xdp include/linux/filter.h:830 [inline]
bpf_test_run+0x567/0x800 net/bpf/test_run.c:117
bpf_prog_test_run_xdp+0x3e8/0x910 net/bpf/test_run.c:821
bpf_prog_test_run+0x31c/0x390 kernel/bpf/syscall.c:3374
__sys_bpf+0x5a5/0x6f0 kernel/bpf/syscall.c:4675
__do_sys_bpf kernel/bpf/syscall.c:4761 [inline]
__se_sys_bpf kernel/bpf/syscall.c:4759 [inline]
__x64_sys_bpf+0x78/0x90 kernel/bpf/syscall.c:4759
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: 0033:0x7f57c0301e99
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:00007f57be55a028 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
RAX: ffffffffffffffda RBX: 00007f57c0589fa0 RCX: 00007f57c0301e99
RDX: 0000000000000050 RSI: 0000200000000700 RDI: 000000000000000a
RBP: 00007f57c0397eaf R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f57c058a038 R14: 00007f57c0589fa0 R15: 00007ffe051e4f68
</TASK>


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