Vyper has a double eval in For List Iter

Multiple evaluation of a single expression is possible in the iterator target of a for loop. While the iterator expression cannot produce multiple writes, it can consume side effects produced in the loop body (e.g. read a storage variable updated in the loop body) and thus lead to unexpected program behavior. Specifically, reads in iterators which contain an ifexp (e.g. for s: uint256 in ([read(), read()] if True else [])) may interleave reads with writes in the loop body.

The fix is tracked in https://github.com/vyperlang/vyper/pull/4488.

Vulnerability Details

Vyper for loops allow two kinds of iterator targets, namely the range() builtin and an iterable type, like SArray and DArray.

During codegen, iterable lists are required to not produce any side-effects (in the following code, range_scope forces iter_list to be parsed in a constant context, which is checked against is_constant).

def _parse_For_list(self):
    with self.context.range_scope():
        iter_list = Expr(self.stmt.iter, self.context).ir_node
    ...

def range_scope(self):
    prev_value = self.in_range_expr
    self.in_range_expr = True
    yield
    self.in_range_expr = prev_value

def is_constant(self):
    return self.constancy is Constancy.Constant or self.in_range_expr

However, this does not prevent the iterator from consuming side effects provided by the body of the loop. For dynamic arrays, the compiler simply panics:

x: DynArray[uint256, 3]

@external
def test():
    for i: uint256 in (self.usesideeffect() if True else self.usesideeffect()):
        pass

@view
def usesideeffect() -> DynArray[uint256, 3]:
    return self.x

For SArrays on the other hand, iter_list is instantiated in the body of a repeat ir, so it can be evaluated several times.

Here are three illustrating examples. In the first example, the following test case pre-evaluates the iter list and stores the result to a temporary list in memory. So the list is only evaluated once, before entry into the loop body, and the log output will be 0, 0, 0.

event I:
    i: uint256

x: uint256

@deploy
def __init__():
    self.x = 0

@external
def test():
    for i: uint256 in [self.usesideeffect(), self.usesideeffect(), self.usesideeffect()]:
        self.x += 1
        log I(i)

@view
def usesideeffect() -> uint256:
    return self.x

However, in the next two examples, because the iterator target is not a list literal, it will be evaluated in the loop body. In the second example, iter_list is an ifexp, thus it will be evaluated lazily in the loop body. The log output will be 0, 1, 2 due to consumption of side effects.

event I:
    i: uint256

x: uint256

@deploy
def __init__():
    self.x = 0

@external
def test():
    for i: uint256 in ([self.usesideeffect(), self.usesideeffect(), self.usesideeffect()] if True else self.otherclause()):
        self.x += 1
        log I(i)

@view
def usesideeffect() -> uint256:
    return self.x

@view
def otherclause() -> uint256[3]:
    return [0, 0, 0]

In the third example, iter_list is also an ifexp, thus it will only be evaluated in the loop body. The log output will be 0, 1, 2 due to consumption of side effects.

event I:
    i: uint256

x: uint256[3]

@deploy
def __init__():
    self.x = [0, 0, 0]

@external
def test():
    for i: uint256 in (self.usesideeffect() if True else self.otherclause()):
        self.x[0] += 1
        self.x[1] += 1
        self.x[2] += 1
        log I(i)

@view
def usesideeffect() -> uint256[3]:
    return self.x

@view
def otherclause() -> uint256[3]:
    return [0, 0, 0]