[syzbot] [net?] [trace?] WARNING: refcount bug in call_timer_fn (4)

[syzbot <syzbot+07dcf509f4c013e25dc5@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    8b690556d8fe Merge tag 'for-linus' of git://git.kernel.org..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11715658580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=38a0c4cddc846161
dashboard link: https://syzkaller.appspot.com/bug?extid=07dcf509f4c013e25dc5
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/3ca2379f6d75/disk-8b690556.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/ce7e9a97f8b4/vmlinux-8b690556.xz
kernel image: https://storage.googleapis.com/syzbot-assets/0936d2c0b069/bzImage-8b690556.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+07dcf509f4c013e25dc5@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 1 PID: 29 at lib/refcount.c:28 refcount_warn_saturate+0x11a/0x1d0 lib/refcount.c:28
Modules linked in:
CPU: 1 UID: 0 PID: 29 Comm: ktimers/1 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
RIP: 0010:refcount_warn_saturate+0x11a/0x1d0 lib/refcount.c:28
Code: c0 30 3d 8b e8 c7 91 09 fd 90 0f 0b 90 90 eb d7 e8 ab 4a 45 fd c6 05 10 1a 47 0a 01 90 48 c7 c7 20 31 3d 8b e8 a7 91 09 fd 90 <0f> 0b 90 90 eb b7 e8 8b 4a 45 fd c6 05 ed 19 47 0a 01 90 48 c7 c7
RSP: 0018:ffffc90000a3f888 EFLAGS: 00010246
RAX: 74da3aaac3757600 RBX: 0000000000000003 RCX: ffff88801be99e00
RDX: 0000000000000100 RSI: 0000000000000000 RDI: 0000000000000100
RBP: ffffc90000a3f990 R08: 0000000000000000 R09: 0000000000000100
R10: dffffc0000000000 R11: ffffed101712487b R12: 1ffff92000147f18
R13: ffff88805d5311b8 R14: ffff88805d531020 R15: 0000000000000001
FS:  0000000000000000(0000) GS:ffff888126ef7000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000404030 CR3: 000000000d3a6000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 call_timer_fn+0x17e/0x5f0 kernel/time/timer.c:1747
 expire_timers kernel/time/timer.c:1798 [inline]
 __run_timers kernel/time/timer.c:2372 [inline]
 __run_timer_base+0x648/0x970 kernel/time/timer.c:2384
 run_timer_base kernel/time/timer.c:2393 [inline]
 run_timer_softirq+0xb7/0x180 kernel/time/timer.c:2403
 handle_softirqs+0x22f/0x710 kernel/softirq.c:622
 __do_softirq kernel/softirq.c:656 [inline]
 run_ktimerd+0xcf/0x190 kernel/softirq.c:1138
 smpboot_thread_fn+0x542/0xa60 kernel/smpboot.c:160
 kthread+0x711/0x8a0 kernel/kthread.c:463
 ret_from_fork+0x4bc/0x870 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>


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