We have a BT headset (Lenovo Thinkplus XT99), the pairing and
connecting has no problem, once this headset is paired, bluez will
remember this device and will auto re-connect it whenever the device
is powered on. The auto re-connecting works well with Windows and
Android, but with Linux, it always fails. Through debugging, we found
at the rfcomm connection stage, the bluetooth stack reports
"Connection refused - security block (0x0003)".

For this device, the re-connecting negotiation process is different
from other BT headsets, it sends the Link_KEY_REQUEST command before
the CONNECT_REQUEST completes, and it doesn't send ENCRYPT_CHANGE
command during the negotiation. When the device sends the "connect
complete" to hci, the ev->encr_mode is 1.

So here in the conn_complete_evt(), if ev->encr_mode is 1, link type
is ACL and HCI_CONN_ENCRYPT is not set, we set HCI_CONN_ENCRYPT to
this conn, and update conn->enc_key_size accordingly.

After this change, this BT headset could re-connect with Linux
successfully. This is the btmon log after applying the patch, after
receiving the "Connect Complete" with "Encryption: Enabled", will send
the command to read encryption key size:
> HCI Event: Connect Request (0x04) plen 10
        Address: 8C:3C:AA:D8:11:67 (OUI 8C-3C-AA)
        Class: 0x240404
          Major class: Audio/Video (headset, speaker, stereo, video, vcr)
          Minor class: Wearable Headset Device
          Rendering (Printing, Speaker)
          Audio (Speaker, Microphone, Headset)
        Link type: ACL (0x01)
...
> HCI Event: Link Key Request (0x17) plen 6
        Address: 8C:3C:AA:D8:11:67 (OUI 8C-3C-AA)
< HCI Command: Link Key Request Reply (0x01|0x000b) plen 22
        Address: 8C:3C:AA:D8:11:67 (OUI 8C-3C-AA)
        Link key: ${32-hex-digits-key}
...
> HCI Event: Connect Complete (0x03) plen 11
        Status: Success (0x00)
        Handle: 256
        Address: 8C:3C:AA:D8:11:67 (OUI 8C-3C-AA)
        Link type: ACL (0x01)
        Encryption: Enabled (0x01)
< HCI Command: Read Encryption Key... (0x05|0x0008) plen 2
        Handle: 256
< ACL Data TX: Handle 256 flags 0x00 dlen 10
      L2CAP: Information Request (0x0a) ident 1 len 2
        Type: Extended features supported (0x0002)
> HCI Event: Command Complete (0x0e) plen 7
      Read Encryption Key Size (0x05|0x0008) ncmd 1
        Status: Success (0x00)
        Handle: 256
        Key size: 16

Cc: stable@vger.kernel.org
Link: https://github.com/bluez/bluez/issues/704Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-by: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Signed-off-by: Hui Wang <hui.wang@canonical.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>