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limit modexp call to 8192 bit inputs #1391

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limit modexp call to 8192 bit inputs #1391

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e60a53f
limit modexp call to 8192 bit inputs
hexoscott Oct 30, 2024
39bd2dc
logic change to mod exp revert rules
hexoscott Oct 31, 2024
3865086
mod len 0 logic in modExp
hexoscott Oct 31, 2024
83cc2a0
remove comment from modexp
hexoscott Oct 31, 2024
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57 changes: 47 additions & 10 deletions core/vm/contracts_zkevm.go
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Original file line number Diff line number Diff line change
Expand Up @@ -299,6 +299,27 @@ func (c *bigModExp_zkevm) RequiredGas(input []byte) uint64 {
} else {
input = input[:0]
}

// Retrieve the operands and execute the exponentiation
var (
base = new(big.Int).SetBytes(getData(input, 0, baseLen.Uint64()))
exp = new(big.Int).SetBytes(getData(input, baseLen.Uint64(), expLen.Uint64()))
mod = new(big.Int).SetBytes(getData(input, baseLen.Uint64()+expLen.Uint64(), modLen.Uint64()))
baseBitLen = base.BitLen()
expBitLen = exp.BitLen()
modBitLen = mod.BitLen()
)

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if modulo size is 0, the precompile returns 0 and it consumes GAS. This case is not handled.
if base size is 0 & modsize < 8192, it should also return 0 and consume GAS. This case is not handled.

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image These are the revert cases to return a 0 gas so that we don't consume any. Anything else consumes gas as it would normally, unless I'm missing something?

// special revert case for ZK
if baseBitLen == 0 && modBitLen > 8192 {
return 0
}

// limit to 8192 bits for base, exp, and mod in ZK - revert if we go over
if baseBitLen > 8192 || expBitLen > 8192 || modBitLen > 8192 {
return 0
}

// Retrieve the head 32 bytes of exp for the adjusted exponent length
var expHead *big.Int
if big.NewInt(int64(len(input))).Cmp(baseLen) <= 0 {
Expand Down Expand Up @@ -373,21 +394,37 @@ func (c *bigModExp_zkevm) Run(input []byte) ([]byte, error) {
} else {
input = input[:0]
}
// Handle a special case when both the base and mod length is zero
if baseLen == 0 && modLen == 0 {
return []byte{}, nil
}

// Retrieve the operands and execute the exponentiation
var (
base = new(big.Int).SetBytes(getData(input, 0, baseLen))
exp = new(big.Int).SetBytes(getData(input, baseLen, expLen))
mod = new(big.Int).SetBytes(getData(input, baseLen+expLen, modLen))
v []byte
base = new(big.Int).SetBytes(getData(input, 0, baseLen))
exp = new(big.Int).SetBytes(getData(input, baseLen, expLen))
mod = new(big.Int).SetBytes(getData(input, baseLen+expLen, modLen))
v []byte
baseBitLen = base.BitLen()
expBitLen = exp.BitLen()
modBitLen = mod.BitLen()
)
switch {
case mod.BitLen() == 0:

if modBitLen == 0 {
// Modulo 0 is undefined, return zero
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comment is not right

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I have removed this comment. It was there from the original erigon implementation.

return common.LeftPadBytes([]byte{}, int(modLen)), nil
}

if baseBitLen == 0 {
if modBitLen > 8192 {
return nil, ErrExecutionReverted
} else {
return []byte{}, nil
}
}

// limit to 8192 bits for base, exp, and mod in ZK
if baseBitLen > 8192 || expBitLen > 8192 || modBitLen > 8192 {
return nil, ErrExecutionReverted
}

switch {
case base.Cmp(libcommon.Big1) == 0:
//If base == 1, then we can just return base % mod (if mod >= 1, which it is)
v = base.Mod(base, mod).Bytes()
Expand Down
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