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Subject[Bluez PATCH v1] bluetooth: secure bluetooth stack from bluedump attack
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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,
+ 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
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