Any circuit that uses an IC - or anything other than resistors, capacitors and inductors - is not passive. So what you are looking for is an active circuit that uses resistors to remove excess charge from the cells.
The maximum charging voltage for LiFePO4 is 3.65V. During most of the charging cycle the voltage rises very slowly, then shoots up rapidly as each cell reaches full charge. Therefore the balancer only needs to limit the voltage on each cell to 3.65V.
With this method the cells are balanced separately without comparing one cell to another, so each cell can have its own independent balancer circuit. LiFePO4 balance modules are often supplied as a board that is simply wired across the cell (example).
The circuit is basically a voltage comparator which connects a high power resistor (or bank of resistors) across the cell when its terminals reach the threshold voltage. The resistor bypasses charging current around the cell so it charges slower than the other lower voltage cell(s). Ideally the bypass current should equal the charging current, though if the cells are not far out of balance it can be lower. An LED is often added to indicate when the balancer is active, and also tells you that the cell is fully charged.
Here is an example that uses a MAX921 comparator with voltage reference:-
For a 2S pack you need two balancers, one for each cell in series. In a 2S3P pack you can join the upper and lower rows of 3 cells in parallel, then you only need 2 balancer circuits for the whole pack.