Bluetooth: 6lowpan: hold L2CAP conn across debugfs control

Bluetooth: 6lowpan: hold L2CAP conn across debugfs control

get_l2cap_conn() looks up an LE hci_conn under hdev protection, but
then drops that protection before reading hcon->l2cap_data and before
lowpan_control_write() later dereferences conn->hcon.  A disconnect or
device close can tear down the same L2CAP connection in that window.

The buggy scenario involves two paths, with each column showing the order
within that path:

6LoWPAN control write:              HCI disconnect/device close:
  1. get_l2cap_conn() finds hcon      1. hci_disconn_cfm() dispatches
     and hcon->l2cap_data.               the L2CAP disconnect callback.
  2. get_l2cap_conn() drops hdev      2. l2cap_conn_del() clears
     protection and returns conn.        hcon->l2cap_data and drops the
                                         L2CAP connection reference.
  3. lowpan_control_write() reads     3. hci_conn_del() removes and drops
     conn->hcon.                         the HCI connection.

Take a reference to the L2CAP connection with
l2cap_conn_hold_unless_zero() while hdev is still locked, and drop that
reference after the debugfs command's last use of conn.  This mirrors the
existing L2CAP ACL receive-side handoff and keeps the connection
dereferenceable after leaving hdev protection.  Export the existing helper
so the bluetooth_6lowpan module can use the same lifetime primitive.

Validation reproduced this kernel report:
BUG: KASAN: slab-use-after-free in lowpan_control_write+0x374/0x520
The buggy address belongs to the object at ffff888111b9d000 which belongs
to the cache kmalloc-1k of size 1024
The buggy address is located 0 bytes inside of freed 1024-byte region
[ffff888111b9d000, ffff888111b9d400)
Read of size 8
Call trace:
  dump_stack_lvl+0x66/0xa0
  print_report+0xce/0x5f0
  lowpan_control_write+0x374/0x520 (net/bluetooth/6lowpan.c:1131)
  srso_alias_return_thunk+0x5/0xfbef5
  __virt_addr_valid+0x19f/0x330
  kasan_report+0xe0/0x110
  __debugfs_file_get+0xf7/0x400
  full_proxy_write+0x9e/0xd0
  vfs_write+0x1b0/0x810
  ksys_write+0xd2/0x170
  dnotify_flush+0x32/0x220
  do_syscall_64+0x115/0x6a0 (arch/x86/entry/syscall_64.c:87)
  entry_SYSCALL_64_after_hwframe+0x77/0x7f
Allocated by task stack:
  kasan_save_stack+0x33/0x60
  kasan_save_track+0x17/0x60
  __kasan_kmalloc+0xaa/0xb0
  l2cap_conn_add+0x45/0x520
  l2cap_chan_connect+0xac6/0xd90
  l2cap_sock_connect+0x216/0x350
  __sys_connect+0x101/0x130
  __x64_sys_connect+0x40/0x50
  do_syscall_64+0x115/0x6a0 (arch/x86/entry/syscall_64.c:87)
  entry_SYSCALL_64_after_hwframe+0x77/0x7f
Freed by task stack:
  kasan_save_stack+0x33/0x60
  kasan_save_track+0x17/0x60
  kasan_save_free_info+0x3b/0x60
  __kasan_slab_free+0x5f/0x80
  kfree+0x313/0x590
  hci_conn_hash_flush+0xc0/0x140
  hci_dev_close_sync+0x41a/0xb00
  hci_dev_close+0x12f/0x160
  hci_sock_ioctl+0x157/0x570
  sock_do_ioctl+0xf7/0x210
  sock_ioctl+0x32f/0x490
  __x64_sys_ioctl+0xc7/0x110
  do_syscall_64+0x115/0x6a0 (arch/x86/entry/syscall_64.c:87)
  entry_SYSCALL_64_after_hwframe+0x77/0x7f
  kasan_record_aux_stack+0xa7/0xc0
  insert_work+0x32/0x100
  __queue_work+0x262/0xa60
  queue_work_on+0xad/0xb0
  l2cap_connect_cfm+0x4ef/0x670
  hci_le_remote_feat_complete_evt+0x247/0x430
  hci_event_packet+0x360/0x6f0
  hci_rx_work+0x2ae/0x7a0
  process_one_work+0x4fd/0xbc0
  worker_thread+0x2d8/0x570
  kthread+0x1ad/0x1f0
  ret_from_fork+0x3c9/0x540
  ret_from_fork_asm+0x1a/0x30

Fixes: 6b8d4a6a0314 ("Bluetooth: 6LoWPAN: Use connected oriented channel instead of fixed one")
Assisted-by: Codex:gpt-5.5
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>