[bug] Make RefundGas safer (potential issue)

[Unique-Divine]

The Ask

Commit and Scope: The RefundGas function from evm/keeper/gas_fees.go
Deadline: 2024-10-15

• Inspect and verify the behavior of RefundGas with its own tests.
• The gethcore.Message uses raw gas price instead of effective gas price. We need to switch it effective.
• It should not be possible to refund negative gas
• RefundGas with 0 should be a no-op

Edit 2024-10-12: Added details from C4 audit

Warning Generated by Coderabbit

[Click to Expand]

Prevent negative gas refund calculation

The calculation msg.Gas() - evmResp.GasUsed could result in a negative value if evmResp.GasUsed exceeds msg.Gas(). This could lead to unintended behavior or panic due to negative gas values.

Consider ensuring the refund amount is non-negative:

+refundGas := msg.Gas() - evmResp.GasUsed
+if refundGas < 0 {
+    refundGas = 0
+}
 if err = k.RefundGas(ctx, msg, refundGas, evmConfig.Params.EvmDenom); err != nil {
     return nil, errors.Wrapf(err, "failed to refund leftover gas to sender %s", msg.From())
 }

📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

	refundGas := msg.Gas() - evmResp.GasUsed
	if refundGas < 0 {
		refundGas = 0
	}
	if err = k.RefundGas(ctx, msg, refundGas, evmConfig.Params.EvmDenom); err != nil {
		return nil, errors.Wrapf(err, "failed to refund leftover gas to sender %s", msg.From())
	}

Originally posted by @coderabbitai[bot] in #2053 (comment)

[Unique-Divine]

Related comment on the part that references a gethcore.Message

RefundGas() refunds the remaining gas via a bank coin transfer in L60

However, refundedCoins uses denom = "micronibi" with remaining being denominated in wei instead of micronibi. That's because msg.GasPrice() is in wei.

Recap: 1 NIBI = 1e6 micronibi

As a result, refunds are massively overinflated.

E.g., let's assume the transaction's gasPrice = 1e12 = 1 micronibi (gas price is in wei and has 18 decimals. 1e18 = 1 NIBI) and leftover gas is 50_000 -> remaining = 50_000 * 1e12 = 5e16

This would result in transferring 5e16 micronibi, which is equal to 5e10 NIBI and massively more than anticipated.

@Unique-Divine

Originally posted by @berndartmueller in https://github.com/code-423n4/2024-10-nibiru-zenith/pull/19#discussion_r1796721161

[Unique-Divine]

The recent PR addresses each point highlighted in the original issue