Vyper's external calls can overflow return data to return input buffer

Summary

When calls to external contracts are made, we write the input buffer starting at byte 28, and allocate the return buffer to start at byte 0 (overlapping with the input buffer). When checking RETURNDATASIZE for dynamic types, the size is compared only to the minimum allowed size for that type, and not to the returned value's length. As a result, malformed return data can cause the contract to mistake data from the input buffer for returndata.

This advisory is given a severity of "Low" because when the called contract returns invalid ABIv2 encoded data, the calling contract can read different invalid data (from the dirty buffer) than the called contract returned.

Details

When arguments are packed for an external call, we create a buffer of size max(args, return_data) + 32. The input buffer is placed in this buffer (starting at byte 28), and the return buffer is allocated to start at byte 0. The assumption is that we can reuse the memory becase we will not be able to read past RETURNDATASIZE.

if fn_type.return_type is not None:
    return_abi_t = calculate_type_for_external_return(fn_type.return_type).abi_type

    # we use the same buffer for args and returndata,
    # so allocate enough space here for the returndata too.
    buflen = max(args_abi_t.size_bound(), return_abi_t.size_bound())
else:
    buflen = args_abi_t.size_bound()

buflen += 32  # padding for the method id

When data is returned, we unpack the return data by starting at byte 0. We check that RETURNDATASIZE is greater than the minimum allowed for the returned type:

if not call_kwargs.skip_contract_check:
    assertion = IRnode.from_list(
        ["assert", ["ge", "returndatasize", min_return_size]],
        error_msg="returndatasize too small",
    )
    unpacker.append(assertion)

This check ensures that any dynamic types returned will have a size of at least 64. However, it does not verify that RETURNDATASIZE is as large as the length word of the dynamic type.

As a result, if a contract expects a dynamic type to be returned, and the part of the return data that is read as length includes a size that is larger than the actual RETURNDATASIZE, the return data read from the buffer will overrun the actual return data size and read from the input buffer.

Proof of Concept

This contract calls an external contract with two arguments. As the call is made, the buffer includes:

• byte 28: method_id
• byte 32: first argument (0)
• byte 64: second argument (hash)