Re: [PATCH v5 3/3] Loading keys that should be blocked by the kernel.

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

Hi Alain,

On Mon, Jan 6, 2020 at 5:31 PM Alain Michaud <alainm@chromium.org> wrote:
>
> This change accomplishes 2 things:
>  1. Drop device security data from previously paired devices
>  using blocked keys.
>  2. Send the list of known bad keys that should be blocked to the kernel
>  if supported.
>
> In particular keys from the Google Titan Security key are being
> blocked.
>
> For additional background information, please see
> https://security.googleblog.com/2019/05/titan-keys-update.html
> ---
>
>  src/adapter.c | 140 +++++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 134 insertions(+), 6 deletions(-)
>
> diff --git a/src/adapter.c b/src/adapter.c
> index cef25616f..9c41ebe86 100644
> --- a/src/adapter.c
> +++ b/src/adapter.c
> @@ -99,10 +99,27 @@
>  #define DISTANCE_VAL_INVALID   0x7FFF
>  #define PATHLOSS_MAX           137
>
> +/*
> + * These are known security keys that have been compromised.
> + * If this grows or there are needs to be platform specific, it is
> + * conceivable that these could be read from a config file.
> + */
> +static const struct mgmt_blocked_key_info blocked_keys[] = {
> +       /* Google Titan Security Keys */
> +       { HCI_BLOCKED_KEY_TYPE_LTK,
> +               {0xbf, 0x01, 0xfb, 0x9d, 0x4e, 0xf3, 0xbc, 0x36,
> +                0xd8, 0x74, 0xf5, 0x39, 0x41, 0x38, 0x68, 0x4c}},
> +       { HCI_BLOCKED_KEY_TYPE_IRK,
> +               {0xa5, 0x99, 0xba, 0xe4, 0xe1, 0x7c, 0xa6, 0x18,
> +                0x22, 0x8e, 0x07, 0x56, 0xb4, 0xe8, 0x5f, 0x01}},
> +};
> +
>  static DBusConnection *dbus_conn = NULL;
>
>  static bool kernel_conn_control = false;
>
> +static bool kernel_blocked_keys_supported = false;
> +
>  static GList *adapter_list = NULL;
>  static unsigned int adapter_remaining = 0;
>  static bool powering_down = false;
> @@ -124,6 +141,7 @@ struct link_key_info {
>         unsigned char key[16];
>         uint8_t type;
>         uint8_t pin_len;
> +       bool is_blocked;
>  };
>
>  struct smp_ltk_info {
> @@ -135,12 +153,14 @@ struct smp_ltk_info {
>         uint16_t ediv;
>         uint64_t rand;
>         uint8_t val[16];
> +       bool is_blocked;
>  };
>
>  struct irk_info {
>         bdaddr_t bdaddr;
>         uint8_t bdaddr_type;
>         uint8_t val[16];
> +       bool is_blocked;
>  };
>
>  struct conn_param {
> @@ -3439,6 +3459,20 @@ static int str2buf(const char *str, uint8_t *buf, size_t blen)
>         return 0;
>  }
>
> +static bool is_blocked_key(uint8_t key_type, uint8_t *key_value)
> +{
> +       uint32_t i = 0;
> +
> +       for (i = 0; i < ARRAY_SIZE(blocked_keys); ++i) {
> +               if (key_type == blocked_keys[i].type &&
> +                               !memcmp(blocked_keys[i].val, key_value,
> +                                               sizeof(blocked_keys[i].val)))
> +                       return true;
> +       }
> +
> +       return false;
> +}
> +
>  static struct link_key_info *get_key_info(GKeyFile *key_file, const char *peer)
>  {
>         struct link_key_info *info = NULL;
> @@ -3461,6 +3495,9 @@ static struct link_key_info *get_key_info(GKeyFile *key_file, const char *peer)
>         info->pin_len = g_key_file_get_integer(key_file, "LinkKey", "PINLength",
>                                                 NULL);
>
> +       info->is_blocked = is_blocked_key(HCI_BLOCKED_KEY_TYPE_LINKKEY,
> +                                                               info->key);
> +
>  failed:
>         g_free(str);
>
> @@ -3534,6 +3571,9 @@ static struct smp_ltk_info *get_ltk(GKeyFile *key_file, const char *peer,
>         else
>                 ltk->master = master;
>
> +       ltk->is_blocked = is_blocked_key(HCI_BLOCKED_KEY_TYPE_LTK,
> +                                                               ltk->val);
> +
>  failed:
>         g_free(key);
>         g_free(rand);
> @@ -3584,6 +3624,9 @@ static struct irk_info *get_irk_info(GKeyFile *key_file, const char *peer,
>         else
>                 str2buf(&str[0], irk->val, sizeof(irk->val));
>
> +       irk->is_blocked = is_blocked_key(HCI_BLOCKED_KEY_TYPE_LINKKEY,
> +                                                               irk->val);
> +
>  failed:
>         g_free(str);
>
> @@ -4142,21 +4185,55 @@ static void load_devices(struct btd_adapter *adapter)
>                 g_key_file_load_from_file(key_file, filename, 0, NULL);
>
>                 key_info = get_key_info(key_file, entry->d_name);
> -               if (key_info)
> -                       keys = g_slist_append(keys, key_info);
>
>                 bdaddr_type = get_le_addr_type(key_file);
>
>                 ltk_info = get_ltk_info(key_file, entry->d_name, bdaddr_type);
> -               if (ltk_info)
> -                       ltks = g_slist_append(ltks, ltk_info);
>
>                 slave_ltk_info = get_slave_ltk_info(key_file, entry->d_name,
>                                                                 bdaddr_type);
> +
> +               irk_info = get_irk_info(key_file, entry->d_name, bdaddr_type);
> +
> +               // If any key for the device is blocked, we discard all.
> +               if ((key_info && key_info->is_blocked) ||
> +                               (ltk_info && ltk_info->is_blocked) ||
> +                               (slave_ltk_info &&
> +                                       slave_ltk_info->is_blocked) ||
> +                               (irk_info && irk_info->is_blocked)) {

Perhaps it would be more efficient if we don't add is_blocked as a
member of these keys and instead pass bool variable to get_*_info
which just set it in case the key is blocked, that way if is_blocked
is set for get_key_info you don't have to free anything. Also it might
be a good idea to move the handling of loading the keys to another
function i.e. load_keys to make it simpler to bail out, etc. Have you
though about the possibility of just using a the device block API when
you detect it is using compromised keys? That way the application can
detect the device is in fact blocked and can either remove the device,
which would clean up the storage as well (see my comments bellow), or
unblock it in order to continue using the device.

> +                       if (key_info) {
> +                               g_free(key_info);
> +                               key_info = NULL;
> +                       }
> +
> +                       if (ltk_info) {
> +                               g_free(ltk_info);
> +                               ltk_info = NULL;
> +                       }
> +
> +                       if (slave_ltk_info) {
> +                               g_free(slave_ltk_info);
> +                               slave_ltk_info = NULL;
> +                       }
> +
> +                       if (irk_info) {
> +                               g_free(irk_info);
> +                               irk_info = NULL;
> +                       }
> +
> +                       goto free;
> +               }
> +
> +               if (key_info)
> +                       keys = g_slist_append(keys, key_info);
> +
> +               if (ltk_info)
> +                       ltks = g_slist_append(ltks, ltk_info);
> +

Perhaps I missing something but don't we need to clear these keys form
the storage once we figure they are blocked or there is any reason to
leave them in there so we can unblock?

>                 if (slave_ltk_info)
>                         ltks = g_slist_append(ltks, slave_ltk_info);
>
> -               irk_info = get_irk_info(key_file, entry->d_name, bdaddr_type);
>                 if (irk_info)
>                         irks = g_slist_append(irks, irk_info);
>
> @@ -8568,6 +8645,42 @@ static bool set_static_addr(struct btd_adapter *adapter)
>         return false;
>  }
>
> +static void set_blocked_keys_complete(uint8_t status, uint16_t length,
> +                                       const void *param, void *user_data)
> +{
> +       struct btd_adapter *adapter = user_data;
> +
> +       if (status != MGMT_STATUS_SUCCESS) {
> +               btd_error(adapter->dev_id,
> +                               "Failed to set blocked keys: %s (0x%02x)",
> +                               mgmt_errstr(status), status);
> +               return;
> +       }
> +
> +       DBG("Successfully set blocked keys for index %u", adapter->dev_id);
> +}
> +
> +static bool set_blocked_keys(struct btd_adapter *adapter)
> +{
> +       uint8_t buffer[sizeof(struct mgmt_cp_set_blocked_keys) +
> +                                       sizeof(blocked_keys)] = { 0 };
> +       struct mgmt_cp_set_blocked_keys *cp =
> +                               (struct mgmt_cp_set_blocked_keys *)buffer;
> +       int i;
> +
> +       cp->key_count = ARRAY_SIZE(blocked_keys);
> +       for (i = 0; i < cp->key_count; ++i) {
> +               cp->keys[i].type = blocked_keys[i].type;
> +               memcpy(cp->keys[i].val, blocked_keys[i].val,
> +                                               sizeof(cp->keys[i].val));
> +       }
> +
> +       return mgmt_send(mgmt_master, MGMT_OP_SET_BLOCKED_KEYS, adapter->dev_id,
> +                                               sizeof(buffer), buffer,
> +                                               set_blocked_keys_complete,
> +                                               adapter, NULL);
> +}
> +
>  static void read_info_complete(uint8_t status, uint16_t length,
>                                         const void *param, void *user_data)
>  {
> @@ -8795,6 +8908,13 @@ static void read_info_complete(uint8_t status, uint16_t length,
>
>         set_name(adapter, btd_adapter_get_name(adapter));
>
> +       if (kernel_blocked_keys_supported && !set_blocked_keys(adapter)) {
> +               btd_error(adapter->dev_id,
> +                               "Failed to set blocked keys for index %u",
> +                               adapter->dev_id);
> +               goto failed;
> +       }
> +
>         if (main_opts.pairable &&
>                         !(adapter->current_settings & MGMT_SETTING_BONDABLE))
>                 set_mode(adapter, MGMT_OP_SET_BONDABLE, 0x01);
> @@ -8972,9 +9092,17 @@ static void read_commands_complete(uint8_t status, uint16_t length,
>         for (i = 0; i < num_commands; i++) {
>                 uint16_t op = get_le16(rp->opcodes + i);
>
> -               if (op == MGMT_OP_ADD_DEVICE) {
> +               switch (op) {
> +               case MGMT_OP_ADD_DEVICE:
>                         DBG("enabling kernel-side connection control");
>                         kernel_conn_control = true;
> +                       break;
> +               case MGMT_OP_SET_BLOCKED_KEYS:
> +                       DBG("kernel supports the set_blocked_keys op");
> +                       kernel_blocked_keys_supported = true;
> +                       break;
> +               default:
> +                       break;
>                 }
>         }
>  }
> --
> 2.24.1.735.g03f4e72817-goog
>


-- 
Luiz Augusto von Dentz