syzbot

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refcount_t: underflow; use-after-free.
WARNING: lib/refcount.c:28 at refcount_warn_saturate+0xf4/0x130 lib/refcount.c:28, CPU#3: swapper/3/0
Modules linked in:
CPU: 3 UID: 0 PID: 0 Comm: swapper/3 Tainted: G             L      syzkaller #0 PREEMPT(full) 
Tainted: [L]=SOFTLOCKUP
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:refcount_warn_saturate+0xf4/0x130 lib/refcount.c:28
Code: 06 e8 50 8e 1f fd 48 8d 3d 69 21 da 0b 67 48 0f b9 3a e8 3f 8e 1f fd 5b 5d c3 cc cc cc cc e8 33 8e 1f fd 48 8d 3d 5c 21 da 0b <67> 48 0f b9 3a e8 22 8e 1f fd 5b 5d e9 9b 82 92 06 e8 16 8e 1f fd
RSP: 0018:ffffc900006f8d90 EFLAGS: 00010046
RAX: 0000000080010002 RBX: ffff88802ea59548 RCX: ffffffff84e76cdb
RDX: ffff88801e698000 RSI: ffffffff84e76d6d RDI: ffffffff90c18ed0
RBP: 0000000000000003 R08: 0000000000000005 R09: 0000000000000004
R10: 0000000000000003 R11: 0000000000000001 R12: ffff88802ea59548
R13: ffff88805cdca000 R14: 000000000000000b R15: 1ffff11004e8f00c
FS:  0000000000000000(0000) GS:ffff8880d68db000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fe9684fbff8 CR3: 00000000551c9000 CR4: 0000000000352ef0
Call Trace:
 <IRQ>
 __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]
 p9_req_put+0x1ec/0x250 net/9p/client.c:281
 req_done+0x1dc/0x2e0 net/9p/trans_virtio.c:147
 vring_interrupt+0x33b/0x430 drivers/virtio/virtio_ring.c:2718
 __handle_irq_event_percpu+0x232/0x800 kernel/irq/handle.c:211
 handle_irq_event_percpu kernel/irq/handle.c:248 [inline]
 handle_irq_event+0xab/0x1e0 kernel/irq/handle.c:265
 handle_edge_irq+0x375/0x970 kernel/irq/chip.c:855
 generic_handle_irq_desc include/linux/irqdesc.h:172 [inline]
 handle_irq arch/x86/kernel/irq.c:255 [inline]
 call_irq_handler arch/x86/kernel/irq.c:311 [inline]
 __common_interrupt+0xd8/0x2f0 arch/x86/kernel/irq.c:326
 common_interrupt+0xb9/0xe0 arch/x86/kernel/irq.c:319
 </IRQ>
 <TASK>
 asm_common_interrupt+0x26/0x40 arch/x86/include/asm/idtentry.h:688
RIP: 0010:pv_native_safe_halt+0xf/0x20 arch/x86/kernel/paravirt.c:82
Code: 56 78 02 c3 cc cc cc cc 0f 1f 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa eb 07 0f 00 2d 83 ac 14 00 fb f4 <e9> fc 31 03 00 66 2e 0f 1f 84 00 00 00 00 00 66 90 90 90 90 90 90
RSP: 0018:ffffc90000197df0 EFLAGS: 00000202
RAX: 0000000000c1e889 RBX: ffff88801e698000 RCX: ffffffff8b76d4b5
RDX: 0000000000000000 RSI: ffffffff8dc45b4f RDI: ffffffff8bfa3620
RBP: 0000000000000003 R08: 0000000000000001 R09: ffffed100d4e673d
R10: ffff88806a7339eb R11: 0000000000000000 R12: ffffed1003cd3000
R13: 0000000000000003 R14: ffffffff90b749d0 R15: 0000000000000000
 arch_safe_halt arch/x86/include/asm/paravirt.h:107 [inline]
 default_idle+0x9/0x10 arch/x86/kernel/process.c:767
 default_idle_call+0x6c/0xb0 kernel/sched/idle.c:122
 cpuidle_idle_call kernel/sched/idle.c:191 [inline]
 do_idle+0x35b/0x4b0 kernel/sched/idle.c:332
 cpu_startup_entry+0x4f/0x60 kernel/sched/idle.c:430
 start_secondary+0x21d/0x2d0 arch/x86/kernel/smpboot.c:312
 common_startup_64+0x13e/0x148
 </TASK>
----------------
Code disassembly (best guess), 1 bytes skipped:
   0:	e8 50 8e 1f fd       	call   0xfd1f8e55
   5:	48 8d 3d 69 21 da 0b 	lea    0xbda2169(%rip),%rdi        # 0xbda2175
   c:	67 48 0f b9 3a       	ud1    (%edx),%rdi
  11:	e8 3f 8e 1f fd       	call   0xfd1f8e55
  16:	5b                   	pop    %rbx
  17:	5d                   	pop    %rbp
  18:	c3                   	ret
  19:	cc                   	int3
  1a:	cc                   	int3
  1b:	cc                   	int3
  1c:	cc                   	int3
  1d:	e8 33 8e 1f fd       	call   0xfd1f8e55
  22:	48 8d 3d 5c 21 da 0b 	lea    0xbda215c(%rip),%rdi        # 0xbda2185
* 29:	67 48 0f b9 3a       	ud1    (%edx),%rdi <-- trapping instruction
  2e:	e8 22 8e 1f fd       	call   0xfd1f8e55
  33:	5b                   	pop    %rbx
  34:	5d                   	pop    %rbp
  35:	e9 9b 82 92 06       	jmp    0x69282d5
  3a:	e8 16 8e 1f fd       	call   0xfd1f8e55