Re: [PATCH v2] Bluetooth: Check for encryption key size on connect

[Luiz Augusto von Dentz <luiz.dentz@gmail.com>]

Hi Archie,


On Mon, Sep 21, 2020 at 12:56 AM Archie Pusaka <apusaka@google.com> wrote:
>
> From: Archie Pusaka <apusaka@chromium.org>
>
> 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>
> Reviewed-by: Alain Michaud <alainm@chromium.org>
>
> ---
> Btw, it looks like the patch sent by Alex Lu with the title
> [PATCH] Bluetooth: Fix the vulnerable issue on enc key size
> also solves the exact same issue.
>
> Changes in v2:
> * Add btmon trace to the commit message
>
>  net/bluetooth/l2cap_core.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
> index ade83e224567..b4fc0ad38aaa 100644
> --- a/net/bluetooth/l2cap_core.c
> +++ b/net/bluetooth/l2cap_core.c
> @@ -4101,7 +4101,8 @@ static struct l2cap_chan *l2cap_connect(struct l2cap_conn *conn,
>
>         /* Check if the ACL is secure enough (if not SDP) */
>         if (psm != cpu_to_le16(L2CAP_PSM_SDP) &&
> -           !hci_conn_check_link_mode(conn->hcon)) {
> +           (!hci_conn_check_link_mode(conn->hcon) ||
> +           !l2cap_check_enc_key_size(conn->hcon))) {

I wonder if we couldn't incorporate the check of key size into
hci_conn_check_link_mode, like I said in the first patch checking the
enc key size should not be specific to L2CAP.

>                 conn->disc_reason = HCI_ERROR_AUTH_FAILURE;
>                 result = L2CAP_CR_SEC_BLOCK;
>                 goto response;
> --
> 2.28.0.681.g6f77f65b4e-goog
>


--
Luiz Augusto von Dentz