WARNING: refcount bug in kobject_add_internal

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    6fbc7275 Linux 5.2-rc7
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=120e0fc5a00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=bff6583efcfaed3f
dashboard link: https://syzkaller.appspot.com/bug?extid=32259bb9bc1a487ad206
compiler:       clang version 9.0.0 (/home/glider/llvm/clang  
80fee25776c2fb61e74c1ecb1a523375c2500b69)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=115bad39a00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1241bdd5a00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+32259bb9bc1a487ad206@xxxxxxxxxxxxxxxxxxxxxxxxx

------------[ cut here ]------------
refcount_t: increment on 0; use-after-free.
WARNING: CPU: 1 PID: 8294 at lib/refcount.c:156  
refcount_inc_checked+0x4b/0x50 lib/refcount.c:156
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 8294 Comm: syz-executor086 Not tainted 5.2.0-rc7 #12
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1d8/0x2f8 lib/dump_stack.c:113
 panic+0x28a/0x7c9 kernel/panic.c:219
 __warn+0x216/0x220 kernel/panic.c:576
 report_bug+0x190/0x290 lib/bug.c:186
 fixup_bug arch/x86/kernel/traps.c:179 [inline]
 do_error_trap+0xd7/0x450 arch/x86/kernel/traps.c:272
 do_invalid_op+0x36/0x40 arch/x86/kernel/traps.c:291
 invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:986
RIP: 0010:refcount_inc_checked+0x4b/0x50 lib/refcount.c:156
Code: 3d a7 68 7a 05 01 75 08 e8 42 47 16 fe 5b 5d c3 e8 3a 47 16 fe c6 05  
91 68 7a 05 01 48 c7 c7 c1 4d 66 88 31 c0 e8 25 19 e8 fd <0f> 0b eb df 90  
55 48 89 e5 41 57 41 56 41 55 41 54 53 48 83 e4 e0
RSP: 0018:ffff88809fa9f7f0 EFLAGS: 00010246
RAX: 5c3359f64ee59e00 RBX: ffff88821ae62bf8 RCX: ffff88808682c6c0
RDX: 0000000000000000 RSI: 0000000080000000 RDI: 0000000000000000
RBP: ffff88809fa9f7f8 R08: ffffffff815e87d4 R09: ffffed1015d66063
R10: ffffed1015d66063 R11: 1ffff11015d66062 R12: ffff88809ec74480
R13: 1ffff11013d8e890 R14: dffffc0000000000 R15: ffff88821ae62bf8
 kref_get include/linux/kref.h:45 [inline]
 kobject_get lib/kobject.c:642 [inline]
 kset_get include/linux/kobject.h:214 [inline]
 kobj_kset_join lib/kobject.c:194 [inline]
 kobject_add_internal+0x8c4/0xd50 lib/kobject.c:246
 kobject_add_varg lib/kobject.c:390 [inline]
 kobject_add+0x138/0x200 lib/kobject.c:442
 class_dir_create_and_add drivers/base/core.c:1732 [inline]
 get_device_parent+0x48b/0x4e0 drivers/base/core.c:1787
 device_add+0x3d6/0x1570 drivers/base/core.c:2048
 hci_register_dev+0x331/0x6c0 net/bluetooth/hci_core.c:3305
 __vhci_create_device drivers/bluetooth/hci_vhci.c:124 [inline]
 vhci_create_device+0x2d1/0x510 drivers/bluetooth/hci_vhci.c:148
 vhci_get_user drivers/bluetooth/hci_vhci.c:204 [inline]
 vhci_write+0x3ac/0x440 drivers/bluetooth/hci_vhci.c:284
 call_write_iter include/linux/fs.h:1872 [inline]
 new_sync_write fs/read_write.c:483 [inline]
 __vfs_write+0x617/0x7d0 fs/read_write.c:496
 vfs_write+0x227/0x510 fs/read_write.c:558
 ksys_write+0x16b/0x2a0 fs/read_write.c:611
 __do_sys_write fs/read_write.c:623 [inline]
 __se_sys_write fs/read_write.c:620 [inline]
 __x64_sys_write+0x7b/0x90 fs/read_write.c:620
 do_syscall_64+0xfe/0x140 arch/x86/entry/common.c:301
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x441289
Code: e8 ac e8 ff ff 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7  
48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff  
ff 0f 83 eb 08 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffe62540878 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000441289
RDX: 0000000000000002 RSI: 0000000020000000 RDI: 0000000000000003
RBP: 000000000000e96e R08: 00000000004002c8 R09: 00000000004002c8
R10: 0000000000000004 R11: 0000000000000246 R12: 0000000000402000
R13: 0000000000402090 R14: 0000000000000000 R15: 0000000000000000
Kernel Offset: disabled
Rebooting in 86400 seconds..


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