Solidity Storage Array Bug Announcement
This weblog put up is about two bugs related to storage arrays that are in any other case unrelated. Each have been current within the compiler for a very long time and have solely been found now although a contract containing them ought to very possible present malfunctions in assessments.
Daenam Kim with assist from Nguyen Pham, each from Curvegrid found a difficulty the place invalid information is saved in reference to arrays of signed integers.
This bug has been current since Solidity 0.4.7 and we take into account it the extra severe of the 2. If these arrays use destructive integers in a sure state of affairs, it should trigger information corruption and thus the bug must be simple to detect.
By way of the Ethereum bug bounty program, we acquired a report a few flaw throughout the new experimental ABI encoder (known as ABIEncoderV2). The brand new ABI encoder remains to be marked as experimental, however we nonetheless assume that this deserves a distinguished announcement since it’s already used on mainnet.
Credit to Ming Chuan Lin (of https://www.secondstate.io) for each discovering and fixing the bug!
The 0.5.10 release accommodates the fixes to the bugs.
In the mean time, we don’t plan to publish a repair to the legacy 0.4.x sequence of Solidity, however we’d if there’s well-liked demand.
Each bugs must be simply seen in assessments that contact the related code paths.
Particulars concerning the two bugs could be discovered under.
Signed Integer Array Bug
Who must be involved
When you have deployed contracts which use signed integer arrays in storage and both straight assign
- a literal array with not less than one destructive worth in it (x = [-1, -2, -3];) or
- an present array of a totally different signed integer sort
to it, this may result in information corruption within the storage array.
Contracts that solely assign particular person array components (i.e. with x[2] = -1;) aren’t affected.
Methods to verify if contract is susceptible
In the event you use signed integer arrays in storage, attempt to run assessments the place you utilize destructive values. The impact must be that the precise worth saved is constructive as an alternative of destructive.
When you have a contract that meets these situations, and wish to confirm whether or not the contract is certainly susceptible, you may attain out to us through [email protected].
Technical particulars
Storage arrays could be assigned from arrays of various sort. Throughout this copy and project operation, a sort conversion is carried out on every of the weather. Along with the conversion, particularly if the signed integer sort is shorter than 256 bits, sure bits of the worth need to be zeroed out in preparation for storing a number of values in the identical storage slot.
Which bits to zero out was incorrectly decided from the supply and never the goal sort. This results in too many bits being zeroed out. Specifically, the signal bit will probably be zero which makes the worth constructive.
ABIEncoderV2 Array Bug
Who must be involved
When you have deployed contracts which use the experimental ABI encoder V2, then these could be affected. Which means solely contracts which use the next directive throughout the supply code could be affected:
pragma experimental ABIEncoderV2;
Moreover, there are a selection of necessities for the bug to set off. See technical particulars additional under for extra data.
Methods to verify if contract is susceptible
The bug solely manifests itself when all the following situations are met:
- Storage information involving arrays or structs is shipped on to an exterior perform name, to abi.encode or to occasion information with out prior project to an area (reminiscence) variable AND
- this information both accommodates an array of structs or an array of statically-sized arrays (i.e. not less than two-dimensional).
Along with that, within the following state of affairs, your code is NOT affected:
- when you solely return such information and don’t use it in abi.encode, exterior calls or occasion information.
Attainable penalties
Naturally, any bug can have wildly various penalties relying on this system management stream, however we anticipate that that is extra more likely to result in malfunction than exploitability.
The bug, when triggered, will beneath sure circumstances ship corrupt parameters on methodology invocations to different contracts.
Technical particulars
In the course of the encoding course of, the experimental ABI encoder doesn’t correctly advance to the following factor in an array in case the weather occupy greater than a single slot in storage.
That is solely the case for components which can be structs or statically-sized arrays. Arrays of dynamically-sized arrays or of elementary datatypes aren’t affected.
The precise impact you will notice is that information is “shifted” within the encoded array: When you have an array of sort uint[2][] and it accommodates the information
[[1, 2], [3, 4], [5, 6]], then it will likely be encoded as [[1, 2], [2, 3], [3, 4]] as a result of the encoder solely advances by a single slot between components as an alternative of two.
This put up was collectively composed by @axic, @chriseth, @holiman