[syzbot] [mptcp?] WARNING: refcount bug in mptcp_schedule_work (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 24172e0d7990 Merge tag 'arm64-fixes' of git://git.kernel.o..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11da1c12580000
kernel config: https://syzkaller.appspot.com/x/.config?x=41ad820f608cb833
dashboard link: https://syzkaller.appspot.com/bug?extid=355158e7e301548a1424
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/38a395e6659a/disk-24172e0d.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/23093acc54e6/vmlinux-24172e0d.xz
kernel image: https://storage.googleapis.com/syzbot-assets/ef12edc574bd/bzImage-24172e0d.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+355158...@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: addition on 0; use-after-free.
WARNING: CPU: 0 PID: 16 at lib/refcount.c:25 refcount_warn_saturate+0xfa/0x1d0 lib/refcount.c:25
Modules linked in:
CPU: 0 UID: 0 PID: 16 Comm: ktimers/0 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+0xfa/0x1d0 lib/refcount.c:25
Code: 00 00 e8 79 5a 45 fd 5b 41 5e c3 cc cc cc cc cc e8 6b 5a 45 fd c6 05 3f 24 47 0a 01 90 48 c7 c7 80 2e 3d 8b e8 57 a1 09 fd 90 <0f> 0b 90 90 eb d7 e8 4b 5a 45 fd c6 05 20 24 47 0a 01 90 48 c7 c7
RSP: 0018:ffffc90000157830 EFLAGS: 00010246
RAX: 8853f029dd577500 RBX: 0000000000000002 RCX: ffff88801b2dda00
RDX: 0000000000000100 RSI: 0000000000000000 RDI: 0000000000000100
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000100
R10: dffffc0000000000 R11: ffffed101710487b R12: 1ffff9200002af18
R13: ffff88805cbc5e60 R14: ffff88805cbc5a80 R15: dffffc0000000000
FS: 0000000000000000(0000) GS:ffff888126df7000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b2ee13ff8 CR3: 0000000056192000 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]
sock_hold include/net/sock.h:816 [inline]
mptcp_schedule_work+0x164/0x1a0 net/mptcp/protocol.c:943
mptcp_tout_timer+0x21/0xa0 net/mptcp/protocol.c:2316
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>


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup