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

[syzbot <syzbot+43060000c399ac5661c1@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    feacb1774bd5 Merge tag 'sched_ext-for-6.16' of git://git.k..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=138bfbf4580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=fab6c0cdf4e7c7d8
dashboard link: https://syzkaller.appspot.com/bug?extid=43060000c399ac5661c1
compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/13170f121f0c/disk-feacb177.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/9c38ba3fb72d/vmlinux-feacb177.xz
kernel image: https://storage.googleapis.com/syzbot-assets/4109204e310f/bzImage-feacb177.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+43060000c399ac5661c1@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 1 PID: 6377 at lib/refcount.c:28 refcount_warn_saturate+0x14a/0x210 lib/refcount.c:28
Modules linked in:
CPU: 1 UID: 0 PID: 6377 Comm: kworker/u8:14 Not tainted 6.15.0-syzkaller-03589-gfeacb1774bd5 #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+0x14a/0x210 lib/refcount.c:28
Code: ff 89 de e8 08 b5 e6 fc 84 db 0f 85 66 ff ff ff e8 1b ba e6 fc c6 05 3c 2f 9a 0b 01 90 48 c7 c7 e0 ab f4 8b e8 97 ed a5 fc 90 <0f> 0b 90 90 e9 43 ff ff ff e8 f8 b9 e6 fc 0f b6 1d 17 2f 9a 0b 31
RSP: 0018:ffffc90003e8f798 EFLAGS: 00010286
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff817aa8e8
RDX: ffff888026190000 RSI: ffffffff817aa8f5 RDI: 0000000000000001
RBP: ffffe8ffffc5f3ee R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000001 R12: 0000000000000002
R13: 0000000000000000 R14: ffffe8ffffc5f3ee R15: ffffe8ffffc5f30a
FS:  0000000000000000(0000) GS:ffff888124aa5000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f4b6de1ed58 CR3: 0000000030c1b000 CR4: 0000000000350ef0
Call Trace:
 <TASK>
 __refcount_sub_and_test include/linux/refcount.h:400 [inline]
 __refcount_dec_and_test include/linux/refcount.h:432 [inline]
 refcount_dec_and_test include/linux/refcount.h:450 [inline]
 skb_unref include/linux/skbuff.h:1249 [inline]
 __sk_skb_reason_drop net/core/skbuff.c:1185 [inline]
 sk_skb_reason_drop+0x183/0x1a0 net/core/skbuff.c:1213
 kfree_skb_reason include/linux/skbuff.h:1279 [inline]
 kfree_skb include/linux/skbuff.h:1288 [inline]
 gro_cleanup+0x8c/0x330 net/core/gro.c:815
 __netif_napi_del_locked+0x2f5/0x490 net/core/dev.c:7301
 __netif_napi_del include/linux/netdevice.h:2876 [inline]
 gro_cells_destroy net/core/gro_cells.c:117 [inline]
 gro_cells_destroy+0x132/0x540 net/core/gro_cells.c:106
 unregister_netdevice_many_notify+0x1821/0x26f0 net/core/dev.c:11994
 unregister_netdevice_many net/core/dev.c:12036 [inline]
 default_device_exit_batch+0x853/0xaf0 net/core/dev.c:12530
 ops_exit_list+0x12b/0x180 net/core/net_namespace.c:177
 cleanup_net+0x5c1/0xb30 net/core/net_namespace.c:654
 process_one_work+0x9cf/0x1b70 kernel/workqueue.c:3238
 process_scheduled_works kernel/workqueue.c:3321 [inline]
 worker_thread+0x6c8/0xf10 kernel/workqueue.c:3402
 kthread+0x3c5/0x780 kernel/kthread.c:464
 ret_from_fork+0x5d7/0x6f0 arch/x86/kernel/process.c:148
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>


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