Flush secureRandom buffer on fork

Summary: On fork, flush the secureRandom buffer, so that we don't share entropy between the parent and child.

Reviewed By: ricklavoie

Differential Revision: D9196474

fbshipit-source-id: 12ff8488d814466186df61328a5f1d4000beb27f

folly/Random.cpp

@@ -72,6 +72,7 @@ void readRandomDevice(void* data, size_t size) {
 
 class BufferedRandomDevice {
  public:
+  static once_flag flag;
   static constexpr size_t kDefaultBufferSize = 128;
 
   explicit BufferedRandomDevice(size_t bufferSize = kDefaultBufferSize);
@@ -97,10 +98,28 @@ class BufferedRandomDevice {
   unsigned char* ptr_;
 };
 
+once_flag BufferedRandomDevice::flag;
+struct RandomTag {};
+
 BufferedRandomDevice::BufferedRandomDevice(size_t bufferSize)
   : bufferSize_(bufferSize),
     buffer_(new unsigned char[bufferSize]),
     ptr_(buffer_.get() + bufferSize) {  // refill on first use
+  call_once(flag, [this]() {
+    detail::AtFork::registerHandler(
+        this,
+        /*prepare*/ []() { return true; },
+        /*parent*/ []() {},
+        /*child*/
+        []() {
+          using Single = SingletonThreadLocal<BufferedRandomDevice, RandomTag>;
+          auto& t = Single::get();
+          // Clear out buffered data on fork.
+          //
+          // Ensure child and parent do not share same entropy pool.
+          t.ptr_ = t.buffer_.get() + t.bufferSize_;
+        });
+  });
 }
 
 void BufferedRandomDevice::getSlow(unsigned char* data, size_t size) {
@@ -124,8 +143,6 @@ void BufferedRandomDevice::getSlow(unsigned char* data, size_t size) {
   ptr_ += size;
 }
 
-struct RandomTag {};
-
 } // namespace
 
 void Random::secureRandom(void* data, size_t size) {

folly/test/RandomTest.cpp

@@ -138,3 +138,24 @@ TEST(Random, sanity) {
         std::unordered_set<uint64_t>(vals.begin(), vals.end()).size());
   }
 }
+
+TEST(Random, SecureFork) {
+  unsigned char buffer = 0;
+  // Init random buffer
+  folly::Random::secureRandom(&buffer, 1);
+
+  auto pid = fork();
+  EXPECT_NE(pid, -1);
+  if (pid) {
+    // parent
+    int status = 0;
+    folly::Random::secureRandom(&buffer, 1);
+    auto pid2 = wait(&status);
+    EXPECT_NE(WEXITSTATUS(status), buffer);
+    EXPECT_EQ(pid, pid2);
+  } else {
+    // child
+    folly::Random::secureRandom(&buffer, 1);
+    exit(buffer); // Do not print gtest results
+  }
+}