Re: [Bluez PATCH v1] bluetooth: secure bluetooth stack from bluedump attack

[Alain Michaud <alainmichaud@google.com>]

Hi Johan, Luiz,

Did you have additional feedback on this before we can send a new
version to address Marcel's comments?

Marcel, you are right, LE likely will need a similar fix.  Given we
currently have SC disabled on chromium, we can probably submit this as
a separate patch unless someone else would like to contribute it
sooner.

Thanks,
Alain

On Wed, Jan 8, 2020 at 4:02 PM Marcel Holtmann <marcel@holtmann.org> wrote:
>
> Hi Howard,
>
> > 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>
>
> we prefer full name signed-off-by signatures.
>
> > ---
> >
> > 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");
>
> Can we use bt_dev_warn() here.
>
> > +             hci_send_cmd(hdev, HCI_OP_USER_CONFIRM_NEG_REPLY,
> > +                          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)) {
>
> What about the LE cases?
>
> In addition, I like to get a pair of second eyes from Johan and Luiz on this one.
>
> Regards
>
> Marcel
>
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