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 though a contract containing them ought to very probably present malfunctions in exams.
Daenam Kim with assist from Nguyen Pham, each from Curvegrid found a difficulty the place invalid knowledge is saved in reference to arrays of signed integers.
This bug has been current since Solidity 0.4.7 and we contemplate it the extra critical of the 2. If these arrays use unfavorable integers in a sure scenario, it’ll trigger knowledge corruption and thus the bug ought to be simple to detect.
By way of the Ethereum bug bounty program, we obtained a report a couple of flaw throughout the new experimental ABI encoder (known as ABIEncoderV2). The brand new ABI encoder continues to be marked as experimental, however we nonetheless suppose that this deserves a outstanding 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 launch accommodates the fixes to the bugs.
In the meanwhile, we don’t plan to publish a repair to the legacy 0.4.x sequence of Solidity, however we would if there’s fashionable demand.
Each bugs ought to be simply seen in exams that contact the related code paths.
Particulars concerning the two bugs might be discovered beneath.
Signed Integer Array Bug
Who ought to be involved
When you have deployed contracts which use signed integer arrays in storage and both instantly assign
- a literal array with at the least one unfavorable worth in it (x = [-1, -2, -3];) or
- an current array of a totally different signed integer sort
to it, it will result in knowledge corruption within the storage array.
Contracts that solely assign particular person array components (i.e. with x[2] = -1;) will not be affected.
How one can test if contract is susceptible
Should you use signed integer arrays in storage, attempt to run exams the place you employ unfavorable values. The impact ought to be that the precise worth saved is optimistic as a substitute of unfavorable.
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 safety@ethereum.org.
Technical particulars
Storage arrays might be assigned from arrays of various sort. Throughout this copy and project operation, a kind 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. Particularly, the signal bit might be zero which makes the worth optimistic.
ABIEncoderV2 Array Bug
Who ought to be involved
When you have deployed contracts which use the experimental ABI encoder V2, then these could be affected. Which means that solely contracts which use the next directive throughout the supply code might be affected:
pragma experimental ABIEncoderV2;
Moreover, there are a variety of necessities for the bug to set off. See technical particulars additional beneath for extra data.
How one can test if contract is susceptible
The bug solely manifests itself when the entire following situations are met:
- Storage knowledge involving arrays or structs is distributed on to an exterior operate name, to abi.encode or to occasion knowledge with out prior project to an area (reminiscence) variable AND
- this knowledge both accommodates an array of structs or an array of statically-sized arrays (i.e. at the least two-dimensional).
Along with that, within the following scenario, your code is NOT affected:
- if you happen to solely return such knowledge and don’t use it in abi.encode, exterior calls or occasion knowledge.
Attainable penalties
Naturally, any bug can have wildly various penalties relying on this system management circulation, however we anticipate that that is extra prone to result in malfunction than exploitability.
The bug, when triggered, will below sure circumstances ship corrupt parameters on technique 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 aspect 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 will not be affected.
The particular impact you will note is that knowledge 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 will probably be encoded as [[1, 2], [2, 3], [3, 4]] as a result of the encoder solely advances by a single slot between components as a substitute of two.
This put up was collectively composed by @axic, @chriseth, @holiman