From 405738c79170e17ae280db0b67041d7aa0c00605 Mon Sep 17 00:00:00 2001
From: Rim Rakhimov <rimrakhimov@gmail.com>
Date: Tue, 30 Nov 2021 10:33:40 +0300
Subject: [PATCH] Fix for modexp overflow in debug mode (#578)

* fix modexp overflow panic in debug mode

* Added one more unit test for modexp

* Remove redundant bytes from modexp unit test

Co-authored-by: POA <33550681+poa@users.noreply.github.com>
---
 crates/vm/builtin/src/lib.rs | 63 ++++++++++++++++++++++++++++++++++--
 1 file changed, 61 insertions(+), 2 deletions(-)

diff --git a/crates/vm/builtin/src/lib.rs b/crates/vm/builtin/src/lib.rs
index 3583c2d69..c54d910f8 100644
--- a/crates/vm/builtin/src/lib.rs
+++ b/crates/vm/builtin/src/lib.rs
@@ -203,11 +203,11 @@ impl ModexpPricer {
         let (base_len, exp_len) = (base_len_u256.low_u64(), exp_len_u256.low_u64());
 
         // read fist 32-byte word of the exponent.
-        let exp_low = if base_len + 96 >= input.len() as u64 {
+        let exp_low = if base_len.wrapping_add(96) >= input.len() as u64 {
             U256::zero()
         } else {
             buf.iter_mut().for_each(|b| *b = 0);
-            let mut reader = input[(96 + base_len as usize)..].chain(io::repeat(0));
+            let mut reader = input[(base_len as usize).wrapping_add(96)..].chain(io::repeat(0));
             let len = min(exp_len, 32) as usize;
             reader
                 .read_exact(&mut buf[(32 - len)..])
@@ -1728,6 +1728,33 @@ mod tests {
             native: EthereumBuiltin::from_str("modexp").unwrap(),
         };
 
+        // test for potential base len overflow
+        {
+            let input = hex!(
+                "
+                00000000000000000000000000000000ffffffffffffffffffffffffffffffff
+                0000000000000000000000000000000000000000000000000000000000000001
+                0000000000000000000000000000000000000000000000000000000000000001
+                ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+                03
+                ff"
+            );
+            let expected_cost = U256::max_value();
+            assert_eq!(f.cost(&input[..], 0), expected_cost);
+        }
+
+        // another test for potential base len overflow
+        {
+            let input = hex!(
+                "
+                00000000000000000000000000000000ffffffffffffffffffffffffffffffff
+                0000000000000000000000000000000000000000000000000000000000000020
+                0000000000000000000000000000000000000000000000000000000000000020"
+            );
+            let expected_cost = U256::max_value();
+            assert_eq!(f.cost(&input[..], 0), expected_cost);
+        }
+
         // test for potential gas cost multiplication overflow
         {
             let input = hex!("0000000000000000000000000000000000000000000000000000000000000001000000000000000000000000000000000000000000000000000000003b27bafd00000000000000000000000000000000000000000000000000000000503c8ac3");
@@ -1840,6 +1867,38 @@ mod tests {
         }
     }
 
+    #[test]
+    fn modexp2565() {
+        let pricer = Modexp2565Pricer {};
+
+        // test for potential base len overflow
+        {
+            let input = hex!(
+                "
+                00000000000000000000000000000000ffffffffffffffffffffffffffffffff
+                0000000000000000000000000000000000000000000000000000000000000001
+                0000000000000000000000000000000000000000000000000000000000000001
+                ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+                03
+                ff"
+            );
+            let expected_cost = U256::max_value();
+            assert_eq!(pricer.cost(&input[..]), expected_cost);
+        }
+
+        // another test for potential base len overflow
+        {
+            let input = hex!(
+                "
+                00000000000000000000000000000000ffffffffffffffffffffffffffffffff
+                0000000000000000000000000000000000000000000000000000000000000020
+                0000000000000000000000000000000000000000000000000000000000000020"
+            );
+            let expected_cost = U256::max_value();
+            assert_eq!(pricer.cost(&input[..]), expected_cost);
+        }
+    }
+
     #[test]
     fn bn128_add() {
         let f = Builtin {