WARNING: refcount bug in nsim_fib6_rt_nh_del

Hi,

We found a bug in Linux 6.10. It is probably a use-after-free bug.
When fib6_info_release(line 341 of include/net/ip6_fib.h) executes
"refcount_dec_and_test(&f6i->fib6_ref)", it is possible that f6i is
already free previously.

The bug report is as follows, but unfortunately there is no generated
syzkaller reproducer.

Bug report:

netdevsim netdevsim0 netdevsim1 (unregistering): unset [1, 0] type 2
family 0 port 6081 - 0
netdevsim netdevsim0 netdevsim0 (unregistering): unset [1, 0] type 2
family 0 port 6081 - 0
------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 0 PID: 8850 at lib/refcount.c:28
refcount_warn_saturate+0x13f/0x1a0 lib/refcount.c:28
Modules linked in:
CPU: 0 PID: 8850 Comm: kworker/u4:11 Not tainted 6.10.0 #13
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
Workqueue: netns cleanup_net
RIP: 0010:refcount_warn_saturate+0x13f/0x1a0 lib/refcount.c:28
Code: 0a 01 48 c7 c7 40 fb a8 8b e8 5d b2 fe fc 0f 0b eb a9 e8 a4 63
3a fd c6 05 d5 3e b8 0a 01 48 c7 c7 a0 fb a8 8b e8 41 b2 fe fc <0f> 0b
eb 8d e8 88 63 3a fd c6 05 b6 3e b8 0a 01 48 c7 c7 e0 fa a8
RSP: 0018:ffffc90002bff668 EFLAGS: 00010246
RAX: de8053514f502200 RBX: ffff8880320bcc2c RCX: ffff8880231b0000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000003 R08: ffffffff8155a25a R09: 1ffff1100c74519a
R10: dffffc0000000000 R11: ffffed100c74519b R12: 0000000000000000
R13: dead000000000122 R14: ffff8880320bcc00 R15: ffff8880320bcc2c
FS:  0000000000000000(0000) GS:ffff888063a00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f13fe745f40 CR3: 00000000413e6000 CR4: 0000000000350ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 __refcount_sub_and_test include/linux/refcount.h:275 [inline]
 __refcount_dec_and_test include/linux/refcount.h:307 [inline]
 refcount_dec_and_test include/linux/refcount.h:325 [inline]
 fib6_info_release include/net/ip6_fib.h:341 [inline]
 nsim_rt6_release drivers/net/netdevsim/fib.c:515 [inline]
 nsim_fib6_rt_nh_del+0x277/0x2a0 drivers/net/netdevsim/fib.c:534
 nsim_fib6_rt_destroy drivers/net/netdevsim/fib.c:583 [inline]
 nsim_fib6_rt_free drivers/net/netdevsim/fib.c:1069 [inline]
 nsim_fib_rt_free+0x4a4/0x7a0 drivers/net/netdevsim/fib.c:1082
 rhashtable_free_one lib/rhashtable.c:1113 [inline]
 rhashtable_free_and_destroy+0x5ab/0x910 lib/rhashtable.c:1164
 nsim_fib_destroy+0xb0/0x180 drivers/net/netdevsim/fib.c:1659
 nsim_dev_reload_destroy+0x2db/0x480 drivers/net/netdevsim/dev.c:1662
 nsim_dev_reload_down+0x93/0xc0 drivers/net/netdevsim/dev.c:965
 devlink_reload+0x188/0x840 net/devlink/dev.c:461
 devlink_pernet_pre_exit+0x1ee/0x440 net/devlink/core.c:509
 ops_pre_exit_list net/core/net_namespace.c:163 [inline]
 cleanup_net+0x61e/0xcd0 net/core/net_namespace.c:620
 process_one_work kernel/workqueue.c:3248 [inline]
 process_scheduled_works+0x977/0x1410 kernel/workqueue.c:3329
 worker_thread+0xaa0/0x1020 kernel/workqueue.c:3409
 kthread+0x2eb/0x380 kernel/kthread.c:389
 ret_from_fork+0x49/0x80 arch/x86/kernel/process.c:147
 ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:244
 </TASK>

-- 
Yours sincerely,
Xingyu