Fix stack overflow (#807)

Fix stack overflow in HandleNode by explicitly limiting the depth of recursion.

include/yaml-cpp/depthguard.h

+#ifndef DEPTH_GUARD_H_00000000000000000000000000000000000000000000000000000000
+#define DEPTH_GUARD_H_00000000000000000000000000000000000000000000000000000000
+
+#if defined(_MSC_VER) ||                                            \
+    (defined(__GNUC__) && (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || \
+     (__GNUC__ >= 4))  // GCC supports "pragma once" correctly since 3.4
+#pragma once
+#endif
+
+#include "exceptions.h"
+
+namespace YAML {
+
+/**
+ * @brief The DeepRecursion class
+ *  An exception class which is thrown by DepthGuard. Ideally it should be
+ * a member of DepthGuard. However, DepthGuard is a templated class which means
+ * that any catch points would then need to know the template parameters. It is
+ * simpler for clients to not have to know at the catch point what was the
+ * maximum depth.
+ */
+class DeepRecursion : public ParserException {
+public:
+  virtual ~DeepRecursion() = default;
+
+  DeepRecursion(int depth, const Mark& mark_, const std::string& msg_);
+
+  // Returns the recursion depth when the exception was thrown
+  int depth() const {
+    return m_depth;
+  }
+
+private:
+  int m_depth = 0;
+};
+
+/**
+ * @brief The DepthGuard class
+ *  DepthGuard takes a reference to an integer. It increments the integer upon
+ * construction of DepthGuard and decrements the integer upon destruction.
+ *
+ * If the integer would be incremented past max_depth, then an exception is
+ * thrown. This is ideally geared toward guarding against deep recursion.
+ *
+ * @param max_depth
+ *  compile-time configurable maximum depth.
+ */
+template <int max_depth = 2000>
+class DepthGuard final {
+public:
+  DepthGuard(int & depth_, const Mark& mark_, const std::string& msg_) : m_depth(depth_) {
+    ++m_depth;
+    if ( max_depth <= m_depth ) {
+        throw DeepRecursion{m_depth, mark_, msg_};
+    }
+  }
+
+  DepthGuard(const DepthGuard & copy_ctor) = delete;
+  DepthGuard(DepthGuard && move_ctor) = delete;
+  DepthGuard & operator=(const DepthGuard & copy_assign) = delete;
+  DepthGuard & operator=(DepthGuard && move_assign) = delete;
+
+  ~DepthGuard() {
+    --m_depth;
+  }
+
+  int current_depth() const {
+    return m_depth;
+  }
+
+private:
+    int & m_depth;
+};
+
+} // namespace YAML
+
+#endif // DEPTH_GUARD_H_00000000000000000000000000000000000000000000000000000000

src/depthguard.cpp

+#include "yaml-cpp/depthguard.h"
+
+namespace YAML {
+
+DeepRecursion::DeepRecursion(int depth, const Mark& mark_, const std::string& msg_)
+    : ParserException(mark_, msg_),
+      m_depth(depth) {
+}
+
+} // namespace YAML

src/singledocparser.cpp

@@ -7,6 +7,7 @@
 #include "singledocparser.h"
 #include "tag.h"
 #include "token.h"
+#include "yaml-cpp/depthguard.h"
 #include "yaml-cpp/emitterstyle.h"
 #include "yaml-cpp/eventhandler.h"
 #include "yaml-cpp/exceptions.h"  // IWYU pragma: keep
@@ -47,6 +48,8 @@ void SingleDocParser::HandleDocument(EventHandler& eventHandler) {
 }
 
 void SingleDocParser::HandleNode(EventHandler& eventHandler) {
+  DepthGuard<2000> depthguard(depth, m_scanner.mark(), ErrorMsg::BAD_FILE);
+
   // an empty node *is* a possibility
   if (m_scanner.empty()) {
     eventHandler.OnNull(m_scanner.mark(), NullAnchor);

src/singledocparser.h

@@ -15,6 +15,7 @@
 
 namespace YAML {
 class CollectionStack;
+template <int> class DepthGuard; // depthguard.h
 class EventHandler;
 class Node;
 class Scanner;
@@ -55,6 +56,7 @@ class SingleDocParser {
   anchor_t LookupAnchor(const Mark& mark, const std::string& name) const;
 
  private:
+  int depth = 0;
   Scanner& m_scanner;
   const Directives& m_directives;
   std::unique_ptr<CollectionStack> m_pCollectionStack;

test/parser_test.cpp

+#include <yaml-cpp/depthguard.h>
 #include "yaml-cpp/parser.h"
+#include "yaml-cpp/exceptions.h"
 #include "mock_event_handler.h"
 #include "gtest/gtest.h"
 
 using YAML::Parser;
 using YAML::MockEventHandler;
+using ::testing::NiceMock;
 using ::testing::StrictMock;
 
 TEST(ParserTest, Empty) {
@@ -14,3 +17,48 @@ TEST(ParserTest, Empty) {
     StrictMock<MockEventHandler> handler;
     EXPECT_FALSE(parser.HandleNextDocument(handler));
 }
+
+TEST(ParserTest, CVE_2017_5950) {
+    std::string excessive_recursion;
+    for (auto i = 0; i != 16384; ++i)
+        excessive_recursion.push_back('[');
+    std::istringstream input{excessive_recursion};
+    Parser parser{input};
+
+    NiceMock<MockEventHandler> handler;
+    EXPECT_THROW(parser.HandleNextDocument(handler), YAML::DeepRecursion);
+}
+
+TEST(ParserTest, CVE_2018_20573) {
+    std::string excessive_recursion;
+    for (auto i = 0; i != 20535; ++i)
+        excessive_recursion.push_back('{');
+    std::istringstream input{excessive_recursion};
+    Parser parser{input};
+
+    NiceMock<MockEventHandler> handler;
+    EXPECT_THROW(parser.HandleNextDocument(handler), YAML::DeepRecursion);
+}
+
+TEST(ParserTest, CVE_2018_20574) {
+    std::string excessive_recursion;
+    for (auto i = 0; i != 21989; ++i)
+        excessive_recursion.push_back('{');
+    std::istringstream input{excessive_recursion};
+    Parser parser{input};
+
+    NiceMock<MockEventHandler> handler;
+    EXPECT_THROW(parser.HandleNextDocument(handler), YAML::DeepRecursion);
+}
+
+TEST(ParserTest, CVE_2019_6285) {
+    std::string excessive_recursion;
+    for (auto i = 0; i != 23100; ++i)
+        excessive_recursion.push_back('[');
+    excessive_recursion.push_back('f');
+    std::istringstream input{excessive_recursion};
+    Parser parser{input};
+
+    NiceMock<MockEventHandler> handler;
+    EXPECT_THROW(parser.HandleNextDocument(handler), YAML::DeepRecursion);
+}