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@xxxxxxxxxxxx>
>
> ---
>
> 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
