Subject: [syzbot] [hams?] WARNING: refcount bug in nr_del_neigh (2)

From: syzbot <[email-masked]>
Date: Fri, 21 Nov 2025 01:20:31 -0800

Hello,

syzbot found the following issue on:

HEAD commit:    407a06507c23 mlxsw: spectrum: Fix memory leak in mlxsw_sp_..
git tree:       net
console output: https://syzkaller.appspot.com/x/log.txt?x=10c11b42580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=e46b8a1c645465a9
dashboard link: https://syzkaller.appspot.com/bug?extid=3f991c449d23d41216a2
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/62d11ae9564e/disk-407a0650.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/997cf054bfb9/vmlinux-407a0650.xz
kernel image: https://storage.googleapis.com/syzbot-assets/827a9f03aefe/bzImage-407a0650.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: [email-masked]

------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 1 PID: 13218 at lib/refcount.c:28 refcount_warn_saturate+0x11a/0x1d0 lib/refcount.c:28
Modules linked in:
CPU: 1 UID: 0 PID: 13218 Comm: syz.3.2115 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
RIP: 0010:refcount_warn_saturate+0x11a/0x1d0 lib/refcount.c:28
Code: 40 58 be 8b e8 07 a4 f9 fc 90 0f 0b 90 90 eb d7 e8 1b 88 36 fd c6 05 b9 84 dd 0a 01 90 48 c7 c7 a0 58 be 8b e8 e7 a3 f9 fc 90 <0f> 0b 90 90 eb b7 e8 fb 87 36 fd c6 05 96 84 dd 0a 01 90 48 c7 c7
RSP: 0018:ffffc9001a9a7a58 EFLAGS: 00010246
RAX: df5324e680d95e00 RBX: 0000000000000003 RCX: ffff88802f279e40
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000002
RBP: 0000000000000000 R08: 0000000000000003 R09: 0000000000000004
R10: dffffc0000000000 R11: fffffbfff1bba684 R12: dffffc0000000000
R13: ffffffff8c7f8848 R14: ffff888056783c3c R15: 0000000000000000
FS:  00007f267354c6c0(0000) GS:ffff88812623b000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000007e2f0000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 nr_del_neigh+0x16c/0x1d0 net/netrom/nr_route.c:440
 nr_rt_ioctl+0xb1b/0xd50 net/netrom/nr_route.c:682
 sock_do_ioctl+0xdc/0x300 net/socket.c:1254
 sock_ioctl+0x576/0x790 net/socket.c:1375
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f267278f6c9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f267354c038 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f26729e5fa0 RCX: 00007f267278f6c9
RDX: 00002000000000c0 RSI: 000000000000890c RDI: 0000000000000013
RBP: 00007f2672811f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f26729e6038 R14: 00007f26729e5fa0 R15: 00007fff2eed0978
 </TASK>

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