[syzbot] [net?] WARNING: refcount bug in nsim_fib_event_nb (2)

[syzbot <syzbot+ea02c8daa5dc1c52b364@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    6832a9317eee Merge tag 'net-6.16-rc7' of git://git.kernel...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1004038c580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=8bff2df4655c5f44
dashboard link: https://syzkaller.appspot.com/bug?extid=ea02c8daa5dc1c52b364
compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/818bd549a31a/disk-6832a931.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/f971d7bc88e2/vmlinux-6832a931.xz
kernel image: https://storage.googleapis.com/syzbot-assets/f637a5f4829c/bzImage-6832a931.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+ea02c8daa5dc1c52b364@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: addition on 0; use-after-free.
WARNING: CPU: 1 PID: 6193 at lib/refcount.c:25 refcount_warn_saturate+0xfa/0x1d0 lib/refcount.c:25
Modules linked in:
CPU: 1 UID: 0 PID: 6193 Comm: kworker/u8:17 Not tainted 6.16.0-rc6-syzkaller-00121-g6832a9317eee #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
Workqueue: netns cleanup_net
RIP: 0010:refcount_warn_saturate+0xfa/0x1d0 lib/refcount.c:25
Code: 00 00 e8 49 ea 06 fd 5b 41 5e e9 d1 05 b0 06 cc e8 3b ea 06 fd c6 05 b6 66 d1 0a 01 90 48 c7 c7 40 6e e1 8b e8 b7 d7 ca fc 90 <0f> 0b 90 90 eb d7 e8 1b ea 06 fd c6 05 97 66 d1 0a 01 90 48 c7 c7
RSP: 0000:ffffc9000b5b7248 EFLAGS: 00010246
RAX: 6b82cdd65d8ecc00 RBX: 0000000000000002 RCX: ffff88807b493c00
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000002
RBP: 0000000000000000 R08: 0000000000000003 R09: 0000000000000004
R10: dffffc0000000000 R11: fffffbfff1bfaa6c R12: dffffc0000000000
R13: ffff888061720020 R14: ffff88802422823c R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff888125d59000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000020000000e000 CR3: 00000000400d2000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 __refcount_add include/linux/refcount.h:-1 [inline]
 __refcount_inc include/linux/refcount.h:366 [inline]
 refcount_inc include/linux/refcount.h:383 [inline]
 fib_info_hold include/net/ip_fib.h:629 [inline]
 nsim_fib4_prepare_event drivers/net/netdevsim/fib.c:930 [inline]
 nsim_fib_event_schedule_work drivers/net/netdevsim/fib.c:1000 [inline]
 nsim_fib_event_nb+0xe9a/0x1080 drivers/net/netdevsim/fib.c:1043
 call_fib_notifier+0x42/0x80 net/core/fib_notifier.c:24
 call_fib_entry_notifier net/ipv4/fib_trie.c:90 [inline]
 fib_leaf_notify net/ipv4/fib_trie.c:2175 [inline]
 fib_table_notify net/ipv4/fib_trie.c:2193 [inline]
 fib_notify+0x359/0x5d0 net/ipv4/fib_trie.c:2216
 fib_net_dump net/core/fib_notifier.c:69 [inline]
 register_fib_notifier+0x184/0x360 net/core/fib_notifier.c:107
 nsim_fib_create+0x847/0x9d0 drivers/net/netdevsim/fib.c:1596
 nsim_dev_reload_create drivers/net/netdevsim/dev.c:1483 [inline]
 nsim_dev_reload_up+0x36b/0x780 drivers/net/netdevsim/dev.c:988
 devlink_reload+0x4ec/0x8d0 net/devlink/dev.c:474
 devlink_pernet_pre_exit+0x1d9/0x3d0 net/devlink/core.c:509
 ops_pre_exit_list net/core/net_namespace.c:162 [inline]
 ops_undo_list+0x184/0x990 net/core/net_namespace.c:235
 cleanup_net+0x4c5/0x800 net/core/net_namespace.c:686
 process_one_work kernel/workqueue.c:3238 [inline]
 process_scheduled_works+0xade/0x17b0 kernel/workqueue.c:3321
 worker_thread+0x8a0/0xda0 kernel/workqueue.c:3402
 kthread+0x711/0x8a0 kernel/kthread.c:464
 ret_from_fork+0x3fc/0x770 arch/x86/kernel/process.c:148
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>


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