GHSA-6m97-7527-mh74

Impact

contracts containing large arrays might underallocate the number of slots they need. prior to v0.3.8, the calculation to determine how many slots a storage variable needed used math.ceil(type_.size_in_bytes / 32):

https://github.com/vyperlang/vyper/blob/6020b8bbf66b062d299d87bc7e4eddc4c9d1c157/vyper/semantics/validation/data_positions.py#L197

the intermediate floating point step can produce a rounding error if there are enough bits set in the IEEE-754 mantissa. roughly speaking, if type_.size_in_bytes is large (> 2**46), and slightly less than a power of 2, the calculation can overestimate how many slots are needed. if type_.size_in_bytes is slightly more than a power of 2, the calculation can underestimate how many slots are needed.

the following two example contracts can result in overwriting of the variable vulnerable:

large_array: address[2**64 + 1]  # type_.size_in_bytes == 32 * (2**64 + 1); math.ceil(type_.size_in_bytes / 32) < 2**64 + 1
vulnerable: uint256

# writing to self.large_array[2**64] will overwrite self.vulnerable

large_dynarray: DynArray[address, 2**64]  # Dynarray has a length word in front, its size in bytes is 32 * (2**64 + 1)
vulnerable: uint256

# writing to self.large_dynarray[2**64 - 1] will overwrite self.vulnerable

note that in the latter case, the risk of vulnerable being overwritten is relatively small, since it would cost roughly $1.45 million trillion USD at today's gas prices (gas price 20gwei, ETH ~= $1800) in order to extend the DynArray to its full container size.

Patches

patched by v0.3.8, specifically in commit https://github.com/vyperlang/vyper/commit/0bb7203b584e771b23536ba065a6efda457161bb.