Bluetooth: secure bluetooth stack from bluedump attack

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.

Since we don't know whether the repairing is legitimate or not,
leave the decision to user space 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
Signed-off-by: Howard Chung <howardchung@google.com>
Acked-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/hci_event.c

@@ -4557,6 +4557,16 @@ static void hci_user_confirm_request_evt(struct hci_dev *hdev,
 			goto confirm;
 		}
 
+		/* If there already exists link key in local host, leave the
+		 * decision to user space since the remote device could be
+		 * legitimate or malicious.
+		 */
+		if (hci_find_link_key(hdev, &ev->bdaddr)) {
+			bt_dev_dbg(hdev, "Local host already has link key");
+			confirm_hint = 1;
+			goto confirm;
+		}
+
 		BT_DBG("Auto-accept of user confirmation with %ums delay",
 		       hdev->auto_accept_delay);
 

net/bluetooth/smp.c

@@ -2168,6 +2168,25 @@ static u8 smp_cmd_pairing_random(struct l2cap_conn *conn, struct sk_buff *skb)
 		smp_send_cmd(conn, SMP_CMD_PAIRING_RANDOM, sizeof(smp->prnd),
 			     smp->prnd);
 		SMP_ALLOW_CMD(smp, SMP_CMD_DHKEY_CHECK);
+
+		/* Only Just-Works pairing requires extra checks */
+		if (smp->method != JUST_WORKS)
+			goto mackey_and_ltk;
+
+		/* If there already exists long term key in local host, leave
+		 * the decision to user space since the remote device could
+		 * be legitimate or malicious.
+		 */
+		if (hci_find_ltk(hcon->hdev, &hcon->dst, hcon->dst_type,
+				 hcon->role)) {
+			err = mgmt_user_confirm_request(hcon->hdev, &hcon->dst,
+							hcon->type,
+							hcon->dst_type,
+							passkey, 1);
+			if (err)
+				return SMP_UNSPECIFIED;
+			set_bit(SMP_FLAG_WAIT_USER, &smp->flags);
+		}
 	}
 
 mackey_and_ltk: