From 2a9f6bed108c9698d4d8b358df5a2b8a06903134 Mon Sep 17 00:00:00 2001
From: Sergey Pepyakin <s.pepyakin@gmail.com>
Date: Wed, 14 Nov 2018 18:20:42 +0100
Subject: [PATCH] Fix overflow in srml-contract. (#1113)

---
 substrate/srml/contract/src/gas.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/substrate/srml/contract/src/gas.rs b/substrate/srml/contract/src/gas.rs
index 9d1978f7a5..af94ae05c1 100644
--- a/substrate/srml/contract/src/gas.rs
+++ b/substrate/srml/contract/src/gas.rs
@@ -150,9 +150,12 @@ pub fn buy_gas<T: Trait>(
 	let cost = <T::Gas as As<T::Balance>>::as_(gas_limit.clone())
 		.checked_mul(&gas_price)
 		.ok_or("overflow multiplying gas limit by price")?;
-	if b < cost + <balances::Module<T>>::existential_deposit() {
+
+	let new_balance = b.checked_sub(&cost);
+	if new_balance < Some(<balances::Module<T>>::existential_deposit()) {
 		return Err("not enough funds for transaction fee");
 	}
+
 	<balances::Module<T>>::set_free_balance(transactor, b - cost);
 	<balances::Module<T>>::decrease_total_stake_by(cost);
 	Ok(GasMeter {