Re: [PATCH v3] Bluetooth: Check for encryption key size on connect

[Marcel Holtmann <marcel@holtmann.org>]

Hi Archie,

> When receiving connection, we only check whether the link has been
> encrypted, but not the encryption key size of the link.
> 
> This patch adds check for encryption key size, and reject L2CAP
> connection which size is below the specified threshold (default 7)
> with security block.
> 
> Here is some btmon trace.
> @ MGMT Event: New Link Key (0x0009) plen 26    {0x0001} [hci0] 5.847722
>        Store hint: No (0x00)
>        BR/EDR Address: 38:00:25:F7:F1:B0 (OUI 38-00-25)
>        Key type: Unauthenticated Combination key from P-192 (0x04)
>        Link key: 7bf2f68c81305d63a6b0ee2c5a7a34bc
>        PIN length: 0
>> HCI Event: Encryption Change (0x08) plen 4        #29 [hci0] 5.871537
>        Status: Success (0x00)
>        Handle: 256
>        Encryption: Enabled with E0 (0x01)
> < HCI Command: Read Encryp... (0x05|0x0008) plen 2  #30 [hci0] 5.871609
>        Handle: 256
>> HCI Event: Command Complete (0x0e) plen 7         #31 [hci0] 5.872524
>      Read Encryption Key Size (0x05|0x0008) ncmd 1
>        Status: Success (0x00)
>        Handle: 256
>        Key size: 3
> 
> ////// WITHOUT PATCH //////
>> ACL Data RX: Handle 256 flags 0x02 dlen 12        #42 [hci0] 5.895023
>      L2CAP: Connection Request (0x02) ident 3 len 4
>        PSM: 4097 (0x1001)
>        Source CID: 64
> < ACL Data TX: Handle 256 flags 0x00 dlen 16        #43 [hci0] 5.895213
>      L2CAP: Connection Response (0x03) ident 3 len 8
>        Destination CID: 64
>        Source CID: 64
>        Result: Connection successful (0x0000)
>        Status: No further information available (0x0000)
> 
> ////// WITH PATCH //////
>> ACL Data RX: Handle 256 flags 0x02 dlen 12        #42 [hci0] 4.887024
>      L2CAP: Connection Request (0x02) ident 3 len 4
>        PSM: 4097 (0x1001)
>        Source CID: 64
> < ACL Data TX: Handle 256 flags 0x00 dlen 16        #43 [hci0] 4.887127
>      L2CAP: Connection Response (0x03) ident 3 len 8
>        Destination CID: 0
>        Source CID: 64
>        Result: Connection refused - security block (0x0003)
>        Status: No further information available (0x0000)
> 
> Signed-off-by: Archie Pusaka <apusaka@chromium.org>
> 
> ---
> 
> Changes in v3:
> * Move the check to hci_conn_check_link_mode()
> 
> Changes in v2:
> * Add btmon trace to the commit message
> 
> net/bluetooth/hci_conn.c | 4 ++++
> 1 file changed, 4 insertions(+)
> 
> diff --git a/net/bluetooth/hci_conn.c b/net/bluetooth/hci_conn.c
> index 9832f8445d43..89085fac797c 100644
> --- a/net/bluetooth/hci_conn.c
> +++ b/net/bluetooth/hci_conn.c
> @@ -1348,6 +1348,10 @@ int hci_conn_check_link_mode(struct hci_conn *conn)
> 	    !test_bit(HCI_CONN_ENCRYPT, &conn->flags))
> 		return 0;
> 
> +	if (test_bit(HCI_CONN_ENCRYPT, &conn->flags) &&
> +	    conn->enc_key_size < conn->hdev->min_enc_key_size)
> +		return 0;
> +
> 	return 1;
> }

I am a bit concerned since we had that check and I on purpose moved it. See commit 693cd8ce3f88 for the change where I removed and commit d5bb334a8e17 where I initially added it.

Naively adding the check in that location caused a major regression with Bluetooth 2.0 devices. This makes me a bit reluctant to re-add it here since I restructured the whole change to check the key size a different location.

Now I have to ask, are you running an upstream kernel with both commits above that address KNOB vulnerability?

Regards

Marcel