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Change execution order to avoid reentry through the _beforeTokenTransfer hook #3611

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Aug 19, 2022
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Change execution order to avoid reentry through the _beforeTokenTransfer hook #3611

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6c3ea96
Change execution order to avoid reentry through the beforeTokenTransf…
Amxx Aug 11, 2022
52058b2
Add changelog entry
Amxx Aug 11, 2022
2c5204a
Update CHANGELOG.md
Amxx Aug 11, 2022
24996ed
Update CHANGELOG.md
Amxx Aug 17, 2022
abae8ca
Update ERC721.sol
Amxx Aug 17, 2022
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add comment
frangio Aug 17, 2022
90f3c16
Update CHANGELOG.md
frangio Aug 17, 2022
ec24469
Apply strict check-effect
Amxx Aug 19, 2022
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minimize diff
Amxx Aug 19, 2022
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3 changes: 2 additions & 1 deletion CHANGELOG.md
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Original file line number Diff line number Diff line change
Expand Up @@ -5,10 +5,11 @@
* `ERC165Checker`: add `supportsERC165InterfaceUnchecked` for consulting individual interfaces without the full ERC165 protocol. ([#3339](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3339))
* `Address`: optimize `functionCall` by calling `functionCallWithValue` directly. ([#3468](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3468))
* `Address`: optimize `functionCall` functions by checking contract size only if there is no returned data. ([#3469](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3469))
* `GovernorCompatibilityBravo`: remove unused `using` statements ([#3506](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3506))
* `GovernorCompatibilityBravo`: remove unused `using` statements. ([#3506](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3506))
* `ERC20`: optimize `_transfer`, `_mint` and `_burn` by using `unchecked` arithmetic when possible. ([#3513](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3513))
* `ERC721`: optimize transfers by making approval clearing implicit instead of emitting an event. ([#3481](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3481))
* `ERC721`: optimize burn by making approval clearing implicit instead of emitting an event. ([#3538](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3538))
* `ERC721`: Change execution order to preserve correct balances with custom `_beforeTokenTransfer` hooks. ([#3611](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3611))
* `ReentrancyGuard`: Reduce code size impact of the modifier by using internal functions. ([#3515](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3515))
* `SafeCast`: optimize downcasting of signed integers. ([#3565](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3565))
* `ERC20FlashMint`: add an internal `_flashFee` function for overriding. ([#3551](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3551))
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23 changes: 13 additions & 10 deletions contracts/token/ERC721/ERC721.sol
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Expand Up @@ -277,11 +277,11 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
* Emits a {Transfer} event.
*/
function _mint(address to, uint256 tokenId) internal virtual {
_beforeTokenTransfer(address(0), to, tokenId);

require(to != address(0), "ERC721: mint to the zero address");
require(!_exists(tokenId), "ERC721: token already minted");

_beforeTokenTransfer(address(0), to, tokenId);

_balances[to] += 1;
_owners[tokenId] = to;

Expand All @@ -302,19 +302,22 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
* Emits a {Transfer} event.
*/
function _burn(uint256 tokenId) internal virtual {
address owner = ERC721.ownerOf(tokenId);
address from = ERC721.ownerOf(tokenId);

_beforeTokenTransfer(from, address(0), tokenId);

_beforeTokenTransfer(owner, address(0), tokenId);
// The token may have been transferred in the before hook so we have to re-read the current owner.
from = ERC721.ownerOf(tokenId);

// Clear approvals
delete _tokenApprovals[tokenId];

_balances[owner] -= 1;
_balances[from] -= 1;
delete _owners[tokenId];

emit Transfer(owner, address(0), tokenId);
emit Transfer(from, address(0), tokenId);

_afterTokenTransfer(owner, address(0), tokenId);
_afterTokenTransfer(from, address(0), tokenId);
}

/**
Expand All @@ -333,11 +336,11 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
address to,
uint256 tokenId
) internal virtual {
require(ERC721.ownerOf(tokenId) == from, "ERC721: transfer from incorrect owner");
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@frangio frangio Aug 17, 2022 •
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It is wrong to remove this from here... We need to check that from is the owner at the time of the transfer, don't we? We should do the same we did in _burn, and re-read the owner after the hook...

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My impression was that it would be checked after the hook, and that would revert in the owner is not correct.

The risk is that the hook would be executed with an invalid from, and the revert would come after.

There is a world in which the from is not valid, the before hook move the token from its current owner to the from, and then the test passes and we do what we have to do... The code would have to be really f****d up, but its possible.

My concern is that doing the test twice increase transfer cost by >100 gas (on probably the most common and scrutinized operation)

require(to != address(0), "ERC721: transfer to the zero address");

_beforeTokenTransfer(from, to, tokenId);

require(from == ERC721.ownerOf(tokenId), "ERC721: transfer from incorrect owner");
require(to != address(0), "ERC721: transfer to the zero address");

// Clear approvals from the previous owner
delete _tokenApprovals[tokenId];

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