[BUG] Bluetooth: KASAN null-ptr-deref in klist_put during HCI connection sysfs teardown

Hi,

I am reporting a KASAN null-ptr-deref triggered by a syzkaller
reproducer on Linux 7.2-rc3.

The crash happens during Bluetooth HCI/vhci teardown. The reproducer
creates Bluetooth L2CAP/BNEP sockets and issues BNEPCONNADD. When the
syz-executor exits, vhci_release() unregisters the HCI device, which
flushes HCI connections and reaches hci_conn_del_sysfs(). During this
sysfs cleanup path, device_move() tries to move a child device away
from the HCI connection device, and the kernel crashes in klist_put()
while removing the device from the old parent's klist.

This looks like a Bluetooth HCI connection sysfs lifetime / teardown
ordering issue, rather than a generic klist problem.

Reproducer:
syz reproducer: https://pastebin.com/raw/n2k0FdAq
kernel config: https://pastebin.com/raw/j608iPvW
console output: https://pastebin.com/raw/Sy0Am14Y

Kernel:
HEAD commit: 1137d8b5df06137fb49513cc923b3b24d94cb809
git tree: torvalds/linux
kernel version: 7.2.0-rc3 #1 PREEMPT(full) (QEMU Ubuntu 24.10)

Crash log excerpt:

Oops: general protection fault, probably for non-canonical address
0xdffffc000000000b: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000058-0x000000000000005f]
CPU: 1 UID: 0 PID: 22881 Comm: syz-executor Not tainted 7.2.0-rc3 #1
PREEMPT(full)
Hardware name: QEMU Ubuntu 24.10 PC (i440FX + PIIX, 1996), BIOS
1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010+0x4b/0x170

Call Trace:
<TASK>
klist_remove+0x12a/0x2d0
device_move+0x136/0xf10
hci_conn_del_sysfs+0x87/0x1b0
hci_conn_del+0x5ca/0x11e0
hci_conn_hash_flush+0x184/0x290
hci_dev_close_sync+0x656/0x1350
hci_dev_do_close+0x31/0xc0
hci_unregister_dev+0x23f/0x6a0
vhci_release+0x181/0x230
__fput+0x401/0xb50
task_work_run+0x172/0x280
do_exit+0xa54/0x3030
do_group_exit+0xd3/0x2a0
get_signal+0x26db/0x2790
arch_do_signal_or_restart+0x80/0x910
exit_to_user_mode_loop+0x115/0x660
do_syscall_64+0x5ba/0x7a0
entry_SYSCALL_64_after_hwframe+0x76/0x7e
</TASK>

Registers around the fault:

RAX: dffffc0000000000 RBX: 0000000000000001 RCX: 0000000000000000
RDX: 000000000000000b RSI: ffff8880006d5940 RDI: 0000000000000058
RBP: ffff888075ce9c60 R12: 0000000000000000

The NULL-deref range and RDI=0x58 suggest that klist_put() is
dereferencing a field through a NULL klist pointer while device_move()
is removing the device from its old parent's klist. Since this is
reached from hci_conn_del_sysfs(), it may indicate that a child device
under the HCI connection sysfs device is being moved or removed after
its parent klist state has already been cleared, or that the same
child/connection sysfs relation can be torn down more than once during
HCI device unregister.


Thanks