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References: <20191211154556.120555-1-alainm@chromium.org>
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From:   Sonny Sasaka <sonnysasaka@chromium.org>
Date:   Wed, 11 Dec 2019 09:48:33 -0800
Message-ID: <CAOxioNngriC+_ZbGm5-OwV-wp80+ZHv_kR2dcp9XbsgVuXunJg@mail.gmail.com>
Subject: Re: [PATCH v4] Implementation of MGMT_OP_SET_BLOCKED_KEYS.
To:     Alain Michaud <alainm@chromium.org>
Cc:     BlueZ <linux-bluetooth@vger.kernel.org>,
        Marcel Holtmann <marcel@holtmann.org>
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Sender: linux-bluetooth-owner@vger.kernel.org
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Hi Alain, Marcel,

I tried to verify this patch together with its corresponding
user-space (bluetoothd) change. I found two major attack vectors of
the security issue (CVE-2019-2102) are not covered:

Attack scenario 1:
a. An LE device (say device X) is paired with BlueZ.
b. It becomes known that device X uses a hardcoded LTK, therefore X's
LTK must be blocked.
c. Add X's LTK to bluetoothd's blocked LTK list. Rebuild and restart bluetoothd.
d. An attacker tries to undermine the situation by having a malicious
device (say device M) pretending to be X, by using X's address and
also knowing the X's hardcoded LTK.
e. When X is not connected to BlueZ and device M is around and
advertising, BlueZ will automatically connect to Y (due to BlueZ's
auto-reconnection to paired devices)
f. This connection triggers BlueZ to initiate pairing to device M. The
user is presented with a UI dialog (planned as a separate fix) such as
"Alain's Logitech Mouse wants to pair, yes or no". The user, as a
non-Bluetooth engineer, is likely to accept the pairing.
g. Malicious device M has succeeded in becoming a trusted device with
BlueZ, and now is able to do the bad things it wants such as HID
injection (pretending to be a mouse/keyboard injecting inputs that
could compromise the privacy and security of the BlueZ device).

Attack scenario 2:
a. It becomes known that product X uses a hardcoded LTK, therefore
this bad LTK must be blocked.
b. Add X's LTK to bluetoothd. Rebuild and restart bluetoothd.
c. A user uses product X and have BlueZ pair with it.
d. Since product X uses its hardcoded LTK, this LTK is rejected by
BlueZ and BlueZ lets X know by sending SMP_INVALID_PARAMS. Device X
may just ignore the error and not do re-pairing.
e. Therefore BlueZ is in a state where it records device X's info but
not having LTK. Turns out that BlueZ still considers this device to
need auto-reconnection even without "paired" status nor LTK.
f. An attacker tries to undermine the situation by having a malicious
device (say device M) pretending to be X, by using 's address and also
knowing the X's hardcoded LTK.
g. When X is not connected to BlueZ and device M is around and
advertising, BlueZ will automatically connect to M (due to BlueZ's
auto-reconnection to paired devices)
h. After BlueZ is auto-connected to M, this doesn't trigger pairing at
BlueZ side because device X is recorded as "not paired". Therefore
device M can right away pretend to do the bad thing, like
mouse/keyboard HID injection, without needing pairing and therefore
without needing any user interaction.

Since the patches haven't addressed the attack vectors caused by
CVE-2019-2102, the patches still need to be revised to address those.
I don't have any particular recommendation how to, but I can help with
verifying.

I tested those cases with Ubuntu + Alain's patches.

Also, when trying to verify the patches I realized that I needed to
correct the MGMT_OP_SET_BLOCKED_KEYS_SIZE and mgmt_cp_set_blocked_keys
array. And we likely want to flag the MGMT handler with
HCI_MGMT_VAR_LEN.


On Wed, Dec 11, 2019 at 7:46 AM Alain Michaud <alainm@chromium.org> wrote:
>
> MGMT command is added to receive the list of blocked keys from
> user-space.
>
> The list is used to:
> 1) Block keys from being distributed by the device during
>    the ke distribution phase of SMP.
> 2) Filter out any keys that were previously saved so
>    they are no longer used.
>
> Signed-off-by: Alain Michaud <alainm@chromium.org>
> ---
>
>  include/net/bluetooth/hci_core.h | 10 ++++
>  include/net/bluetooth/mgmt.h     | 17 +++++++
>  net/bluetooth/hci_core.c         | 85 +++++++++++++++++++++++++++++---
>  net/bluetooth/hci_debugfs.c      | 17 +++++++
>  net/bluetooth/mgmt.c             | 51 +++++++++++++++++++
>  net/bluetooth/smp.c              | 18 +++++++
>  6 files changed, 190 insertions(+), 8 deletions(-)
>
> diff --git a/include/net/bluetooth/hci_core.h b/include/net/bluetooth/hci_core.h
> index b689aceb636b..9020fa3c4d30 100644
> --- a/include/net/bluetooth/hci_core.h
> +++ b/include/net/bluetooth/hci_core.h
> @@ -118,6 +118,13 @@ struct bt_uuid {
>         u8 svc_hint;
>  };
>
> +struct blocked_key {
> +       struct list_head list;
> +       struct rcu_head rcu;
> +       u8 type;
> +       u8 val[16];
> +};
> +
>  struct smp_csrk {
>         bdaddr_t bdaddr;
>         u8 bdaddr_type;
> @@ -397,6 +404,7 @@ struct hci_dev {
>         struct list_head        le_conn_params;
>         struct list_head        pend_le_conns;
>         struct list_head        pend_le_reports;
> +       struct list_head        blocked_keys;
>
>         struct hci_dev_stats    stat;
>
> @@ -1121,6 +1129,8 @@ struct smp_irk *hci_find_irk_by_addr(struct hci_dev *hdev, bdaddr_t *bdaddr,
>  struct smp_irk *hci_add_irk(struct hci_dev *hdev, bdaddr_t *bdaddr,
>                             u8 addr_type, u8 val[16], bdaddr_t *rpa);
>  void hci_remove_irk(struct hci_dev *hdev, bdaddr_t *bdaddr, u8 addr_type);
> +bool hci_is_blocked_key(struct hci_dev *hdev, u8 type, u8 val[16]);
> +void hci_blocked_keys_clear(struct hci_dev *hdev);
>  void hci_smp_irks_clear(struct hci_dev *hdev);
>
>  bool hci_bdaddr_is_paired(struct hci_dev *hdev, bdaddr_t *bdaddr, u8 type);
> diff --git a/include/net/bluetooth/mgmt.h b/include/net/bluetooth/mgmt.h
> index 9cee7ddc6741..c9b1d39d6d6c 100644
> --- a/include/net/bluetooth/mgmt.h
> +++ b/include/net/bluetooth/mgmt.h
> @@ -654,6 +654,23 @@ struct mgmt_cp_set_phy_confguration {
>  } __packed;
>  #define MGMT_SET_PHY_CONFIGURATION_SIZE        4
>
> +#define MGMT_OP_SET_BLOCKED_KEYS       0x0046
> +
> +#define HCI_BLOCKED_KEY_TYPE_LINKKEY   0x00
> +#define HCI_BLOCKED_KEY_TYPE_LTK               0x01
> +#define HCI_BLOCKED_KEY_TYPE_IRK               0x02
> +
> +struct mgmt_blocked_key_info {
> +       __u8 type;
> +       __u8 val[16];
> +} __packed;
> +
> +struct mgmt_cp_set_blocked_keys {
> +       __le16 key_count;
> +       struct mgmt_blocked_key_info keys[0];
> +} __packed;
> +#define MGMT_OP_SET_BLOCKED_KEYS_SIZE 0
> +
>  #define MGMT_EV_CMD_COMPLETE           0x0001
>  struct mgmt_ev_cmd_complete {
>         __le16  opcode;
> diff --git a/net/bluetooth/hci_core.c b/net/bluetooth/hci_core.c
> index 9e19d5a3aac8..f0298db26dc3 100644
> --- a/net/bluetooth/hci_core.c
> +++ b/net/bluetooth/hci_core.c
> @@ -2311,6 +2311,33 @@ void hci_smp_irks_clear(struct hci_dev *hdev)
>         }
>  }
>
> +void hci_blocked_keys_clear(struct hci_dev *hdev)
> +{
> +       struct blocked_key *b;
> +
> +       list_for_each_entry_rcu(b, &hdev->blocked_keys, list) {
> +               list_del_rcu(&b->list);
> +               kfree_rcu(b, rcu);
> +       }
> +}
> +
> +bool hci_is_blocked_key(struct hci_dev *hdev, u8 type, u8 val[16])
> +{
> +       bool blocked = false;
> +       struct blocked_key *b;
> +
> +       rcu_read_lock();
> +       list_for_each_entry(b, &hdev->blocked_keys, list) {
> +               if (b->type == type && !memcmp(b->val, val, sizeof(b->val))) {
> +                       blocked = true;
> +                       break;
> +               }
> +       }
> +
> +       rcu_read_unlock();
> +       return blocked;
> +}
> +
>  struct link_key *hci_find_link_key(struct hci_dev *hdev, bdaddr_t *bdaddr)
>  {
>         struct link_key *k;
> @@ -2319,6 +2346,16 @@ struct link_key *hci_find_link_key(struct hci_dev *hdev, bdaddr_t *bdaddr)
>         list_for_each_entry_rcu(k, &hdev->link_keys, list) {
>                 if (bacmp(bdaddr, &k->bdaddr) == 0) {
>                         rcu_read_unlock();
> +
> +                       if (hci_is_blocked_key(hdev,
> +                                              HCI_BLOCKED_KEY_TYPE_LINKKEY,
> +                                              k->val)) {
> +                               bt_dev_warn_ratelimited(hdev,
> +                                                       "Link key blocked for %pMR",
> +                                                       &k->bdaddr);
> +                               return NULL;
> +                       }
> +
>                         return k;
>                 }
>         }
> @@ -2387,6 +2424,15 @@ struct smp_ltk *hci_find_ltk(struct hci_dev *hdev, bdaddr_t *bdaddr,
>
>                 if (smp_ltk_is_sc(k) || ltk_role(k->type) == role) {
>                         rcu_read_unlock();
> +
> +                       if (hci_is_blocked_key(hdev, HCI_BLOCKED_KEY_TYPE_LTK,
> +                                              k->val)) {
> +                               bt_dev_warn_ratelimited(hdev,
> +                                                       "LTK blocked for %pMR",
> +                                                       &k->bdaddr);
> +                               return NULL;
> +                       }
> +
>                         return k;
>                 }
>         }
> @@ -2397,31 +2443,42 @@ struct smp_ltk *hci_find_ltk(struct hci_dev *hdev, bdaddr_t *bdaddr,
>
>  struct smp_irk *hci_find_irk_by_rpa(struct hci_dev *hdev, bdaddr_t *rpa)
>  {
> +       struct smp_irk *irk_to_return = NULL;
>         struct smp_irk *irk;
>
>         rcu_read_lock();
>         list_for_each_entry_rcu(irk, &hdev->identity_resolving_keys, list) {
>                 if (!bacmp(&irk->rpa, rpa)) {
> -                       rcu_read_unlock();
> -                       return irk;
> +                       irk_to_return = irk;
> +                       goto done;
>                 }
>         }
>
>         list_for_each_entry_rcu(irk, &hdev->identity_resolving_keys, list) {
>                 if (smp_irk_matches(hdev, irk->val, rpa)) {
>                         bacpy(&irk->rpa, rpa);
> -                       rcu_read_unlock();
> -                       return irk;
> +                       irk_to_return = irk;
> +                       goto done;
>                 }
>         }
> +
> +done:
> +       if (irk_to_return && hci_is_blocked_key(hdev, HCI_BLOCKED_KEY_TYPE_IRK,
> +                                               irk_to_return->val)) {
> +               bt_dev_warn_ratelimited(hdev, "Identity key blocked for %pMR",
> +                                       &irk_to_return->bdaddr);
> +               irk_to_return = NULL;
> +       }
> +
>         rcu_read_unlock();
>
> -       return NULL;
> +       return irk_to_return;
>  }
>
>  struct smp_irk *hci_find_irk_by_addr(struct hci_dev *hdev, bdaddr_t *bdaddr,
>                                      u8 addr_type)
>  {
> +       struct smp_irk *irk_to_return = NULL;
>         struct smp_irk *irk;
>
>         /* Identity Address must be public or static random */
> @@ -2432,13 +2489,23 @@ struct smp_irk *hci_find_irk_by_addr(struct hci_dev *hdev, bdaddr_t *bdaddr,
>         list_for_each_entry_rcu(irk, &hdev->identity_resolving_keys, list) {
>                 if (addr_type == irk->addr_type &&
>                     bacmp(bdaddr, &irk->bdaddr) == 0) {
> -                       rcu_read_unlock();
> -                       return irk;
> +                       irk_to_return = irk;
> +                       goto done;
>                 }
>         }
> +
> +done:
> +
> +       if (irk_to_return && hci_is_blocked_key(hdev, HCI_BLOCKED_KEY_TYPE_IRK,
> +                                               irk_to_return->val)) {
> +               bt_dev_warn_ratelimited(hdev, "Identity key blocked for %pMR",
> +                                       &irk_to_return->bdaddr);
> +               irk_to_return = NULL;
> +       }
> +
>         rcu_read_unlock();
>
> -       return NULL;
> +       return irk_to_return;
>  }
>
>  struct link_key *hci_add_link_key(struct hci_dev *hdev, struct hci_conn *conn,
> @@ -3244,6 +3311,7 @@ struct hci_dev *hci_alloc_dev(void)
>         INIT_LIST_HEAD(&hdev->pend_le_reports);
>         INIT_LIST_HEAD(&hdev->conn_hash.list);
>         INIT_LIST_HEAD(&hdev->adv_instances);
> +       INIT_LIST_HEAD(&hdev->blocked_keys);
>
>         INIT_WORK(&hdev->rx_work, hci_rx_work);
>         INIT_WORK(&hdev->cmd_work, hci_cmd_work);
> @@ -3443,6 +3511,7 @@ void hci_unregister_dev(struct hci_dev *hdev)
>         hci_bdaddr_list_clear(&hdev->le_resolv_list);
>         hci_conn_params_clear_all(hdev);
>         hci_discovery_filter_clear(hdev);
> +       hci_blocked_keys_clear(hdev);
>         hci_dev_unlock(hdev);
>
>         hci_dev_put(hdev);
> diff --git a/net/bluetooth/hci_debugfs.c b/net/bluetooth/hci_debugfs.c
> index 402e2cc54044..1c8100bc4e04 100644
> --- a/net/bluetooth/hci_debugfs.c
> +++ b/net/bluetooth/hci_debugfs.c
> @@ -152,6 +152,21 @@ static int blacklist_show(struct seq_file *f, void *p)
>
>  DEFINE_SHOW_ATTRIBUTE(blacklist);
>
> +static int blocked_keys_show(struct seq_file *f, void *p)
> +{
> +       struct hci_dev *hdev = f->private;
> +       struct blocked_key *key;
> +
> +       rcu_read_lock();
> +       list_for_each_entry_rcu(key, &hdev->blocked_keys, list)
> +               seq_printf(f, "%u %*phN\n", key->type, 16, key->val);
> +       rcu_read_unlock();
> +
> +       return 0;
> +}
> +
> +DEFINE_SHOW_ATTRIBUTE(blocked_keys);
> +
>  static int uuids_show(struct seq_file *f, void *p)
>  {
>         struct hci_dev *hdev = f->private;
> @@ -308,6 +323,8 @@ void hci_debugfs_create_common(struct hci_dev *hdev)
>                             &device_list_fops);
>         debugfs_create_file("blacklist", 0444, hdev->debugfs, hdev,
>                             &blacklist_fops);
> +       debugfs_create_file("blocked_keys", 0444, hdev->debugfs, hdev,
> +                           &blocked_keys_fops);
>         debugfs_create_file("uuids", 0444, hdev->debugfs, hdev, &uuids_fops);
>         debugfs_create_file("remote_oob", 0400, hdev->debugfs, hdev,
>                             &remote_oob_fops);
> diff --git a/net/bluetooth/mgmt.c b/net/bluetooth/mgmt.c
> index acb7c6d5643f..77cba2603e2a 100644
> --- a/net/bluetooth/mgmt.c
> +++ b/net/bluetooth/mgmt.c
> @@ -106,6 +106,7 @@ static const u16 mgmt_commands[] = {
>         MGMT_OP_START_LIMITED_DISCOVERY,
>         MGMT_OP_READ_EXT_INFO,
>         MGMT_OP_SET_APPEARANCE,
> +       MGMT_OP_SET_BLOCKED_KEYS,
>  };
>
>  static const u16 mgmt_events[] = {
> @@ -3531,6 +3532,55 @@ static int set_phy_configuration(struct sock *sk, struct hci_dev *hdev,
>         return err;
>  }
>
> +static int set_blocked_keys(struct sock *sk, struct hci_dev *hdev, void *data,
> +                           u16 len)
> +{
> +       int err = MGMT_STATUS_SUCCESS;
> +       struct mgmt_cp_set_blocked_keys *keys = data;
> +       const u16 max_key_count = ((U16_MAX - sizeof(*keys)) /
> +                                  sizeof(struct mgmt_blocked_key_info));
> +       u16 key_count, expected_len;
> +       int i;
> +
> +       BT_DBG("request for %s", hdev->name);
> +
> +       key_count = __le16_to_cpu(keys->key_count);
> +       if (key_count > max_key_count) {
> +               bt_dev_err(hdev, "too big key_count value %u", key_count);
> +               return mgmt_cmd_status(sk, hdev->id, MGMT_OP_SET_BLOCKED_KEYS,
> +                                      MGMT_STATUS_INVALID_PARAMS);
> +       }
> +
> +       expected_len = struct_size(keys, keys, key_count);
> +       if (expected_len != len) {
> +               bt_dev_err(hdev, "expected %u bytes, got %u bytes",
> +                          expected_len, len);
> +               return mgmt_cmd_status(sk, hdev->id, MGMT_OP_SET_BLOCKED_KEYS,
> +                                      MGMT_STATUS_INVALID_PARAMS);
> +       }
> +
> +       hci_dev_lock(hdev);
> +
> +       hci_blocked_keys_clear(hdev);
> +
> +       for (i = 0; i < keys->key_count; ++i) {
> +               struct blocked_key *b = kzalloc(sizeof(*b), GFP_KERNEL);
> +
> +               if (!b) {
> +                       err = MGMT_STATUS_NO_RESOURCES;
> +                       break;
> +               }
> +
> +               b->type = keys->keys[i].type;
> +               memcpy(b->val, keys->keys[i].val, sizeof(b->val));
> +               list_add_rcu(&b->list, &hdev->blocked_keys);
> +       }
> +       hci_dev_unlock(hdev);
> +
> +       return mgmt_cmd_complete(sk, hdev->id, MGMT_OP_SET_BLOCKED_KEYS,
> +                               err, NULL, 0);
> +}
> +
>  static void read_local_oob_data_complete(struct hci_dev *hdev, u8 status,
>                                          u16 opcode, struct sk_buff *skb)
>  {
> @@ -6914,6 +6964,7 @@ static const struct hci_mgmt_handler mgmt_handlers[] = {
>         { set_appearance,          MGMT_SET_APPEARANCE_SIZE },
>         { get_phy_configuration,   MGMT_GET_PHY_CONFIGURATION_SIZE },
>         { set_phy_configuration,   MGMT_SET_PHY_CONFIGURATION_SIZE },
> +       { set_blocked_keys,        MGMT_OP_SET_BLOCKED_KEYS_SIZE },
>  };
>
>  void mgmt_index_added(struct hci_dev *hdev)
> diff --git a/net/bluetooth/smp.c b/net/bluetooth/smp.c
> index 6b42be4b5861..4ece170c518e 100644
> --- a/net/bluetooth/smp.c
> +++ b/net/bluetooth/smp.c
> @@ -2453,6 +2453,15 @@ static int smp_cmd_encrypt_info(struct l2cap_conn *conn, struct sk_buff *skb)
>         if (skb->len < sizeof(*rp))
>                 return SMP_INVALID_PARAMS;
>
> +       /* Pairing is aborted if any blocked keys are distributed */
> +       if (hci_is_blocked_key(conn->hcon->hdev, HCI_BLOCKED_KEY_TYPE_LTK,
> +                              rp->ltk)) {
> +               bt_dev_warn_ratelimited(conn->hcon->hdev,
> +                                       "LTK blocked for %pMR",
> +                                       &conn->hcon->dst);
> +               return SMP_INVALID_PARAMS;
> +       }
> +
>         SMP_ALLOW_CMD(smp, SMP_CMD_MASTER_IDENT);
>
>         skb_pull(skb, sizeof(*rp));
> @@ -2509,6 +2518,15 @@ static int smp_cmd_ident_info(struct l2cap_conn *conn, struct sk_buff *skb)
>         if (skb->len < sizeof(*info))
>                 return SMP_INVALID_PARAMS;
>
> +       /* Pairing is aborted if any blocked keys are distributed */
> +       if (hci_is_blocked_key(conn->hcon->hdev, HCI_BLOCKED_KEY_TYPE_IRK,
> +                              info->irk)) {
> +               bt_dev_warn_ratelimited(conn->hcon->hdev,
> +                                       "Identity key blocked for %pMR",
> +                                       &conn->hcon->dst);
> +               return SMP_INVALID_PARAMS;
> +       }
> +
>         SMP_ALLOW_CMD(smp, SMP_CMD_IDENT_ADDR_INFO);
>
>         skb_pull(skb, sizeof(*info));
> --
> 2.24.0.525.g8f36a354ae-goog
>