Add function selector check for slow vm

rvgo/slow/vm.go

@@ -1,6 +1,7 @@
 package slow
 
 import (
+	"bytes"
 	"encoding/binary"
 	"fmt"
 
@@ -121,6 +122,12 @@ func Step(calldata []byte, po PreimageOracle) (stateHash common.Hash, outErr err
 		return
 	}
 
+	// First 4 bytes of keccak256("step(bytes,bytes,bytes32)")
+	expectedSelector := []byte{0xe1, 0x4c, 0xed, 0x32}
+	if len(calldata) < 4 || !bytes.Equal(calldata[:4], expectedSelector) {
+		panic("invalid function selector")
+	}
+
 	stateContentOffset := uint8(4 + 32 + 32 + 32 + 32)
 	if iszero(eq(b32asBEWord(calldataload(toU64(4+32*3))), shortToU256(stateSize))) {
 		// user-provided state size must match expected state size