Solidity Storage Array Bug Announcement
This weblog submit 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 despite the fact that a contract containing them ought to very doubtless 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 think about it the extra severe of the 2. If these arrays use unfavorable integers in a sure state of affairs, it’ll trigger knowledge corruption and thus the bug needs to be simple to detect.
By way of the Ethereum bug bounty program, we acquired 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 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 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 may be well-liked demand.
Each bugs needs to be simply seen in exams that contact the related code paths.
Particulars in regards to the two bugs might be discovered under.
Signed Integer Array Bug
Who needs to be involved
When you’ve got deployed contracts which use signed integer arrays in storage and both instantly assign
- a literal array with not less than one unfavorable worth in it (x = [-1, -2, -3];) or
- an current array of a completely different signed integer sort
to it, this can result in knowledge corruption within the storage array.
Contracts that solely assign particular person array components (i.e. with x[2] = -1;) should not affected.
The way to test if contract is weak
Should you use signed integer arrays in storage, attempt to run exams the place you employ unfavorable values. The impact needs to be that the precise worth saved is optimistic as an alternative of unfavorable.
When you’ve got a contract that meets these circumstances, and need to confirm whether or not the contract is certainly weak, you possibly can attain out to us through [email protected].
Technical particulars
Storage arrays might be assigned from arrays of various sort. Throughout this copy and task 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 can be zero which makes the worth optimistic.
ABIEncoderV2 Array Bug
Who needs to be involved
When you’ve got deployed contracts which use the experimental ABI encoder V2, then these is perhaps 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 selection of necessities for the bug to set off. See technical particulars additional under for extra info.
The way to test if contract is weak
The bug solely manifests itself when the entire following circumstances are met:
- Storage knowledge involving arrays or structs is shipped on to an exterior perform name, to abi.encode or to occasion knowledge with out prior task to an area (reminiscence) variable AND
- this knowledge 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:
- in case you solely return such knowledge and don’t use it in abi.encode, exterior calls or occasion knowledge.
Potential penalties
Naturally, any bug can have wildly various penalties relying on this system management move, however we anticipate that that is extra more likely to result in malfunction than exploitability.
The bug, when triggered, will underneath 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 subsequent ingredient in an array in case the weather occupy greater than a single slot in storage.
That is solely the case for components which might be structs or statically-sized arrays. Arrays of dynamically-sized arrays or of elementary datatypes should not affected.
The precise impact you will notice is that knowledge is “shifted” within the encoded array: When you’ve got an array of sort uint[2][] and it accommodates the info
[[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 submit was collectively composed by @axic, @chriseth, @holiman
