[syzbot] [net?] WARNING: refcount bug in nsim_fib_event_nb (3)

[syzbot <syzbot+cb2aa2390ac024e25f5c@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    6f3ed7fec72f Merge tag 'for-7.1/dm-fixes-3' of git://git.k..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1144db7e580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=bd38685893011045
dashboard link: https://syzkaller.appspot.com/bug?extid=cb2aa2390ac024e25f5c
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/625fc484e456/disk-6f3ed7fe.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/fd23de5678d1/vmlinux-6f3ed7fe.xz
kernel image: https://storage.googleapis.com/syzbot-assets/5395fddce64e/bzImage-6f3ed7fe.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+cb2aa2390ac024e25f5c@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: addition on 0; use-after-free.
WARNING: lib/refcount.c:25 at refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25, CPU#1: kworker/u8:8/1044
Modules linked in:
CPU: 1 UID: 0 PID: 1044 Comm: kworker/u8:8 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
Workqueue: netns cleanup_net
RIP: 0010:refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25
Code: eb 66 85 db 74 3e 83 fb 01 75 4c e8 4b 36 23 fd 48 8d 3d 14 85 f1 0a 67 48 0f b9 3a eb 4a e8 38 36 23 fd 48 8d 3d 11 85 f1 0a <67> 48 0f b9 3a eb 37 e8 25 36 23 fd 48 8d 3d 0e 85 f1 0a 67 48 0f
RSP: 0018:ffffc90005e4f270 EFLAGS: 00010293
RAX: ffffffff84a135d8 RBX: 0000000000000002 RCX: ffff888027863d80
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff8f92baf0
RBP: 0000000000000000 R08: ffff888027863d80 R09: 0000000000000005
R10: 0000000000000100 R11: 0000000000000004 R12: ffff8880117bd000
R13: dffffc0000000000 R14: ffff88803392903c R15: ffff8880117bd000
FS:  0000000000000000(0000) GS:ffff888126283000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000557b57a3d220 CR3: 00000000352b4000 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+0x1055/0x1240 drivers/net/netdevsim/fib.c:1043
 call_fib_notifier+0x45/0x80 net/core/fib_notifier.c:25
 call_fib_entry_notifier net/ipv4/fib_trie.c:90 [inline]
 fib_leaf_notify net/ipv4/fib_trie.c:2176 [inline]
 fib_table_notify net/ipv4/fib_trie.c:2194 [inline]
 fib_notify+0x36b/0x5e0 net/ipv4/fib_trie.c:2217
 fib_net_dump net/core/fib_notifier.c:70 [inline]
 register_fib_notifier+0x184/0x360 net/core/fib_notifier.c:108
 nsim_fib_create+0x85d/0x9f0 drivers/net/netdevsim/fib.c:1596
 nsim_dev_reload_create drivers/net/netdevsim/dev.c:1604 [inline]
 nsim_dev_reload_up+0x374/0x7c0 drivers/net/netdevsim/dev.c:1058
 devlink_reload+0x501/0x8d0 net/devlink/dev.c:475
 devlink_pernet_pre_exit+0x1ff/0x420 net/devlink/core.c:558
 ops_pre_exit_list net/core/net_namespace.c:161 [inline]
 ops_undo_list+0x187/0x940 net/core/net_namespace.c:234
 cleanup_net+0x56e/0x800 net/core/net_namespace.c:702
 process_one_work kernel/workqueue.c:3314 [inline]
 process_scheduled_works+0xb5d/0x1860 kernel/workqueue.c:3397
 worker_thread+0xa53/0xfc0 kernel/workqueue.c:3478
 kthread+0x388/0x470 kernel/kthread.c:436
 ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>
----------------
Code disassembly (best guess):
   0:	eb 66                	jmp    0x68
   2:	85 db                	test   %ebx,%ebx
   4:	74 3e                	je     0x44
   6:	83 fb 01             	cmp    $0x1,%ebx
   9:	75 4c                	jne    0x57
   b:	e8 4b 36 23 fd       	call   0xfd23365b
  10:	48 8d 3d 14 85 f1 0a 	lea    0xaf18514(%rip),%rdi        # 0xaf1852b
  17:	67 48 0f b9 3a       	ud1    (%edx),%rdi
  1c:	eb 4a                	jmp    0x68
  1e:	e8 38 36 23 fd       	call   0xfd23365b
  23:	48 8d 3d 11 85 f1 0a 	lea    0xaf18511(%rip),%rdi        # 0xaf1853b
* 2a:	67 48 0f b9 3a       	ud1    (%edx),%rdi <-- trapping instruction
  2f:	eb 37                	jmp    0x68
  31:	e8 25 36 23 fd       	call   0xfd23365b
  36:	48 8d 3d 0e 85 f1 0a 	lea    0xaf1850e(%rip),%rdi        # 0xaf1854b
  3d:	67                   	addr32
  3e:	48                   	rex.W
  3f:	0f                   	.byte 0xf


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