Bluetooth: Enforce key size of 16 bytes on FIPS level

According to the spec Ver 5.2, Vol 3, Part C, Sec 5.2.2.8:
Device in security mode 4 level 4 shall enforce:
128-bit equivalent strength for link and encryption keys required
using FIPS approved algorithms (E0 not allowed, SAFER+ not allowed,
and P-192 not allowed; encryption key not shortened)

This patch rejects connection with key size below 16 for FIPS
level services.
Signed-off-by: Archie Pusaka <apusaka@chromium.org>
Reviewed-by: Alain Michaud <alainm@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/l2cap_core.c

@@ -1515,8 +1515,14 @@ static bool l2cap_check_enc_key_size(struct hci_conn *hcon)
 	 * that have no key size requirements. Ensure that the link is
 	 * actually encrypted before enforcing a key size.
 	 */
+	int min_key_size = hcon->hdev->min_enc_key_size;
+
+	/* On FIPS security level, key size must be 16 bytes */
+	if (hcon->sec_level == BT_SECURITY_FIPS)
+		min_key_size = 16;
+
 	return (!test_bit(HCI_CONN_ENCRYPT, &hcon->flags) ||
-		hcon->enc_key_size >= hcon->hdev->min_enc_key_size);
+		hcon->enc_key_size >= min_key_size);
 }
 
 static void l2cap_do_start(struct l2cap_chan *chan)