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

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