CVE-2023-46247: Vyper has incorrect storage layout for contracts containing large arrays

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.sizeinbytes / 32):

https://github.com/vyperlang/vyper/blob/6020b8bbf66b062d299d87bc7e4eddc4c9d1c157/vyper/semantics/validation/datapositions.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.sizeinbytes is large (> 246), and slightly less than a power of 2, the calculation can overestimate how many slots are needed. if type.sizeinbytes 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: vyper largearray: address[264 + 1] # type.sizeinbytes == 32 (264 + 1); math.ceil(type.sizeinbytes / 32) < 264 + 1 vulnerable: uint256

writing to self.largearray[264] will overwrite self.vulnerable vyper largedynarray: DynArray[address, 264] # Dynarray has a length word in front, its size in bytes is 32 (264 + 1) vulnerable: uint256

writing to self.largedynarray[264 - 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.