vyper performs multiple eval of `sqrt()` argument built in

Summary

Using the sqrt builtin can result in multiple eval evaluation of side effects when the argument has side-effects. The bug is more difficult (but not impossible!) to trigger as of 0.3.4, when the unique symbol fence was introduced (https://github.com/vyperlang/vyper/pull/2914).

A contract search was performed and no vulnerable contracts were found in production.

Details

It can be seen that the build_IR function of the sqrt builtin doesn't cache the argument to the stack: https://github.com/vyperlang/vyper/blob/4595938734d9988f8e46e8df38049ae0559abedb/vyper/builtins/functions.py#L2151

As such, it can be evaluated multiple times (instead of retrieving the value from the stack).

PoC

With at least Vyper version 0.2.15+commit.6e7dba7 the following contract:

c: uint256

@internal
def some_decimal() -> decimal:
    self.c += 1
    return 1.0

@external
def foo() -> uint256:
    k: decimal = sqrt(self.some_decimal())
    return self.c

passes the following test:

// SPDX-License-Identifier: MIT
pragma solidity >=0.8.13;

import "../../lib/ds-test/test.sol";
import "../../lib/utils/Console.sol";
import "../../lib/utils/VyperDeployer.sol";

import "../ITest.sol";

contract ConTest is DSTest {
    VyperDeployer vyperDeployer = new VyperDeployer();

    ITest t;

    function setUp() public {
        t = ITest(vyperDeployer.deployContract("Test"));
    }

    function testFoo() public {
        uint256 val = t.foo();
        console.log(val);
        assert (val == 4);
    }
}

Patches

Patched in https://github.com/vyperlang/vyper/pull/3976.

Impact

No vulnerable production contracts were found.