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From:   Yun-hao Chung <howardchung@google.com>
Date:   Tue, 7 Jan 2020 11:30:24 +0800
Message-ID: <CAPHZWUfMZAqkj++icECVPs48=S4SEHsLz-A=kUcoOVp8A-Mu7Q@mail.gmail.com>
Subject: Re: [Bluez PATCH v1] bluetooth: secure bluetooth stack from bluedump attack
To:     Matias Karhumaa <matias.karhumaa@gmail.com>
Cc:     chromeos-bluetooth-upstreaming@chromium.org,
        "David S. Miller" <davem@davemloft.net>,
        Johan Hedberg <johan.hedberg@gmail.com>,
        linux-bluetooth@vger.kernel.org, linux-kernel@vger.kernel.org,
        Marcel Holtmann <marcel@holtmann.org>, netdev@vger.kernel.org
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Sender: linux-bluetooth-owner@vger.kernel.org
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Hi Matias,

Thanks for the comment.
I think in the just-work case, if we ask user for confirmation, they
will likely just accept it.
Rejecting the pairing immediately is more secure.

Thanks,
Howard

On Mon, Jan 6, 2020 at 7:44 PM Matias Karhumaa
<matias.karhumaa@gmail.com> wrote:
>
> Hi Howard,
>
> Re-sending as plain text.
>
> This same attack scenario works also against Ubuntu 18.04 at least.
>
> ma 6. tammik. 2020 klo 12.17 howardchung@google.com
> (howardchung@google.com) kirjoitti:
> >
> > From: howardchung <howardchung@google.com>
> >
> > Attack scenario:
> > 1. A Chromebook (let's call this device A) is paired to a legitimate
> >    Bluetooth classic device (e.g. a speaker) (let's call this device
> >    B).
> > 2. A malicious device (let's call this device C) pretends to be the
> >    Bluetooth speaker by using the same BT address.
> > 3. If device A is not currently connected to device B, device A will
> >    be ready to accept connection from device B in the background
> >    (technically, doing Page Scan).
> > 4. Therefore, device C can initiate connection to device A
> >    (because device A is doing Page Scan) and device A will accept the
> >    connection because device A trusts device C's address which is the
> >    same as device B's address.
> > 5. Device C won't be able to communicate at any high level Bluetooth
> >    profile with device A because device A enforces that device C is
> >    encrypted with their common Link Key, which device C doesn't have.
> >    But device C can initiate pairing with device A with just-works
> >    model without requiring user interaction (there is only pairing
> >    notification). After pairing, device A now trusts device C with a
> >    new different link key, common between device A and C.
> > 6. From now on, device A trusts device C, so device C can at anytime
> >    connect to device A to do any kind of high-level hijacking, e.g.
> >    speaker hijack or mouse/keyboard hijack.
> >
> > To fix this, reject the pairing if all the conditions below are met.
> > - the pairing is initialized by peer
> > - the authorization method is just-work
> > - host already had the link key to the peer
> >
> > Also create a debugfs option to permit the pairing even the
> > conditions above are met.
> >
> > Signed-off-by: howardchung <howardchung@google.com>
> > ---
> >
> >  include/net/bluetooth/hci.h |  1 +
> >  net/bluetooth/hci_core.c    | 47 +++++++++++++++++++++++++++++++++++++
> >  net/bluetooth/hci_event.c   | 12 ++++++++++
> >  3 files changed, 60 insertions(+)
> >
> > diff --git a/include/net/bluetooth/hci.h b/include/net/bluetooth/hci.h
> > index 07b6ecedc6ce..4918b79baa41 100644
> > --- a/include/net/bluetooth/hci.h
> > +++ b/include/net/bluetooth/hci.h
> > @@ -283,6 +283,7 @@ enum {
> >         HCI_FORCE_STATIC_ADDR,
> >         HCI_LL_RPA_RESOLUTION,
> >         HCI_CMD_PENDING,
> > +       HCI_PERMIT_JUST_WORK_REPAIR,
> >
> >         __HCI_NUM_FLAGS,
> >  };
> > diff --git a/net/bluetooth/hci_core.c b/net/bluetooth/hci_core.c
> > index 9e19d5a3aac8..9014aa567e7b 100644
> > --- a/net/bluetooth/hci_core.c
> > +++ b/net/bluetooth/hci_core.c
> > @@ -172,10 +172,57 @@ static const struct file_operations vendor_diag_fops = {
> >         .llseek         = default_llseek,
> >  };
> >
> > +static ssize_t permit_just_work_repair_read(struct file *file,
> > +                                           char __user *user_buf,
> > +                                           size_t count, loff_t *ppos)
> > +{
> > +       struct hci_dev *hdev = file->private_data;
> > +       char buf[3];
> > +
> > +       buf[0] = hci_dev_test_flag(hdev, HCI_PERMIT_JUST_WORK_REPAIR) ? 'Y'
> > +                                                                     : 'N';
> > +       buf[1] = '\n';
> > +       buf[2] = '\0';
> > +       return simple_read_from_buffer(user_buf, count, ppos, buf, 2);
> > +}
> > +
> > +static ssize_t permit_just_work_repair_write(struct file *file,
> > +                                            const char __user *user_buf,
> > +                                            size_t count, loff_t *ppos)
> > +{
> > +       struct hci_dev *hdev = file->private_data;
> > +       char buf[32];
> > +       size_t buf_size = min(count, (sizeof(buf) - 1));
> > +       bool enable;
> > +
> > +       if (copy_from_user(buf, user_buf, buf_size))
> > +               return -EFAULT;
> > +
> > +       buf[buf_size] = '\0';
> > +       if (strtobool(buf, &enable))
> > +               return -EINVAL;
> > +
> > +       if (enable)
> > +               hci_dev_set_flag(hdev, HCI_PERMIT_JUST_WORK_REPAIR);
> > +       else
> > +               hci_dev_clear_flag(hdev, HCI_PERMIT_JUST_WORK_REPAIR);
> > +
> > +       return count;
> > +}
> > +
> > +static const struct file_operations permit_just_work_repair_fops = {
> > +       .open           = simple_open,
> > +       .read           = permit_just_work_repair_read,
> > +       .write          = permit_just_work_repair_write,
> > +       .llseek         = default_llseek,
> > +};
> > +
> >  static void hci_debugfs_create_basic(struct hci_dev *hdev)
> >  {
> >         debugfs_create_file("dut_mode", 0644, hdev->debugfs, hdev,
> >                             &dut_mode_fops);
> > +       debugfs_create_file("permit_just_work_repair", 0644, hdev->debugfs,
> > +                           hdev, &permit_just_work_repair_fops);
> >
> >         if (hdev->set_diag)
> >                 debugfs_create_file("vendor_diag", 0644, hdev->debugfs, hdev,
> > diff --git a/net/bluetooth/hci_event.c b/net/bluetooth/hci_event.c
> > index 6ddc4a74a5e4..898e347e19e0 100644
> > --- a/net/bluetooth/hci_event.c
> > +++ b/net/bluetooth/hci_event.c
> > @@ -4539,6 +4539,18 @@ static void hci_user_confirm_request_evt(struct hci_dev *hdev,
> >                 goto unlock;
> >         }
> >
> > +       /* If there already exists link key in local host, terminate the
> > +        * connection by default since the remote device could be malicious.
> > +        * Permit the connection if permit_just_work_repair is enabled.
> > +        */
> > +       if (!hci_dev_test_flag(hdev, HCI_PERMIT_JUST_WORK_REPAIR) &&
> > +           hci_find_link_key(hdev, &ev->bdaddr)) {
> > +               BT_DBG("Rejecting request: local host already have link key");
> > +               hci_send_cmd(hdev, HCI_OP_USER_CONFIRM_NEG_REPLY,
>
> Why wouldn't we just request authorization from userspace in case we
> already have link key? I think that is how it works on other
> platforms.
> >
> > +                            sizeof(ev->bdaddr), &ev->bdaddr);
> > +               goto unlock;
> > +       }
> > +
> >         /* If no side requires MITM protection; auto-accept */
> >         if ((!loc_mitm || conn->remote_cap == HCI_IO_NO_INPUT_OUTPUT) &&
> >             (!rem_mitm || conn->io_capability == HCI_IO_NO_INPUT_OUTPUT)) {
> > --
> > 2.24.1.735.g03f4e72817-goog
>
>
> Best regard,
> Matias Karhumaa