Pools with a large total supply causes SWAP functions to always revert

Severity: Low Difficulty: High

Description

A revert caused during the computations performed of SWAP functions can stop users to call these functions, if the initial supply of the token is large.

When a pool is finalized, the initial supply of shares is created. There is a lower bound to the initial supply, but there is no upper bound:

https://github.com/balancer-labs/balancer-core/blob/942a51e202cc5bf9158bad77162bc72aa0a8afaf/contracts/BPool.sol#L224-L238

The total supply is used in several places, for instance in the joinswapExternAmountIn function which calls calcPoolOutGivenSingleIn:

https://github.com/balancer-labs/balancer-core/blob/942a51e202cc5bf9158bad77162bc72aa0a8afaf/contracts/BMath.sol#L142

The multiplications are done through the fixed-point arithmetic bmul:

https://github.com/balancer-labs/balancer-core/blob/942a51e202cc5bf9158bad77162bc72aa0a8afaf/contracts/BNum.sol#L63-L73

An overflow in this computation will revert, regardless of the input values used in the SWAP functions.

Exploit Scenario

Bob creates a pool with a very large number of token shares. Alice will try to call a SWAP function, but will only revert regardless of the input values she is using. As a result of that, she will be unable to use the pool as expected.

Recommendation

Short term:

Add an upper bound to the initial total supply and do not allow the total supply to grow beyond that limit.
Alternatively, document this behavior and make sure the users are aware of it.

Long term:

Consider using Echidna and Manticore to detect this kind of issues in the codebase.

balancer / balancer-core