Skip to content
This repository has been archived by the owner on Nov 6, 2020. It is now read-only.

Fixed potential modexp exp len overflow #6686

Merged
merged 1 commit into from
Oct 11, 2017
Merged

Fixed potential modexp exp len overflow #6686

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
34 changes: 25 additions & 9 deletions ethcore/src/builtin.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -101,18 +101,17 @@ impl Pricer for ModexpPricer {
let exp_len = read_len();
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

It won't let me add comments on the actual line, but on line 92 above here: Why do we zero-extend the input and read_exact instead of just using read? The buffer's already filled with zeroes.

let mod_len = read_len();

if mod_len.is_zero() && base_len.is_zero() {
return U256::zero()
}

let max_len = U256::from(u32::max_value() / 2);
if base_len > max_len || mod_len > max_len {
if base_len > max_len || mod_len > max_len || exp_len > max_len {
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

does this work properly with base = 1, mod = 1, exp = 2^256-1?

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

@arkpar ^^^

Copy link
Collaborator Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Yes, according to the spec it should fail because gas calculation formula would return a huge value.

return U256::max_value();
}
let (base_len, exp_len, mod_len) = (base_len.low_u64(), exp_len.low_u64(), mod_len.low_u64());

let base_len = base_len.low_u64();
let exp_len = exp_len.low_u64();
let mod_len = mod_len.low_u64();
let m = max(mod_len, base_len);
if m == 0 {
return U256::zero();
}
// read fist 32-byte word of the exponent.
Copy link
Contributor

@eira-fransham eira-fransham Oct 11, 2017 •
edited
Loading

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

👊 fist 👊

let exp_low = if base_len + 96 >= input.len() as u64 { U256::zero() } else {
let mut buf = [0; 32];
Expand All @@ -133,8 +132,7 @@ impl ModexpPricer {
let bit_index = if exp_low.is_zero() { 0 } else { (255 - exp_low.leading_zeros()) as u64 };
if len <= 32 {
bit_index
}
else {
} else {
Copy link
Collaborator

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

👍 Was hurting my eyes a bit :)

8 * (len - 32) + bit_index
}
}
Expand Down Expand Up @@ -707,6 +705,24 @@ mod tests {
native: ethereum_builtin("modexp"),
activate_at: 0,
};

// test for potential exp len overflow
{
let input = FromHex::from_hex("\
00000000000000000000000000000000000000000000000000000000000000ff\
2a1e530000000000000000000000000000000000000000000000000000000000\
0000000000000000000000000000000000000000000000000000000000000000"
).unwrap();

let mut output = vec![0u8; 32];
let expected = FromHex::from_hex("0000000000000000000000000000000000000000000000000000000000000000").unwrap();
let expected_cost = U256::max_value();

f.execute(&input[..], &mut BytesRef::Fixed(&mut output[..])).expect("Builtin should fail");
assert_eq!(output, expected);
assert_eq!(f.cost(&input[..]), expected_cost.into());
}

// fermat's little theorem example.
{
let input = FromHex::from_hex("\
Expand Down