syzbot

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refcount_t: addition on 0; use-after-free.
WARNING: CPU: 1 PID: 29 at lib/refcount.c:25 refcount_warn_saturate+0xfa/0x1d0 lib/refcount.c:25
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+0xfa/0x1d0 lib/refcount.c:25
Code: 00 00 e8 99 56 45 fd 5b 41 5e e9 c1 d8 4a 06 cc e8 8b 56 45 fd c6 05 4f 22 47 0a 01 90 48 c7 c7 c0 2e 3d 8b e8 a7 9d 09 fd 90 <0f> 0b 90 90 eb d7 e8 6b 56 45 fd c6 05 30 22 47 0a 01 90 48 c7 c7
RSP: 0018:ffffc90000a3f830 EFLAGS: 00010246
RAX: 8f01efff24585c00 RBX: 0000000000000002 RCX: ffff88801be99e00
RDX: 0000000000000100 RSI: 0000000000000000 RDI: 0000000000000100
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000100
R10: dffffc0000000000 R11: ffffed101712487b R12: 1ffff92000147f18
R13: ffff88803c7b8460 R14: ffff88803c7b8080 R15: dffffc0000000000
FS:  0000000000000000(0000) GS:ffff888126ef7000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000404030 CR3: 0000000034ca4000 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>