XPath: Restrict AST depth to prevent stack overflow

XPath parser and execution engine isn't stackless; the depth of the
query controls the amount of C stack space required.

This change instruments places in the parser where the control flow can
recurse, requiring too much C stack space to produce an AST, or where a
stackless parse is used to produce arbitrarily deep AST which will
create issues for downstream processing.

As a result XPath parser should now be fuzz safe for malicious inputs.

src/pugixml.cpp

@@ -11143,6 +11143,14 @@ PUGI__NS_BEGIN
 		}
 	};
 
+	static const size_t xpath_ast_depth_limit =
+	#ifdef PUGIXML_XPATH_DEPTH_LIMIT
+		PUGIXML_XPATH_DEPTH_LIMIT
+	#else
+		4096
+	#endif
+		;
+
 	struct xpath_parser
 	{
 		xpath_allocator* _alloc;
@@ -11155,6 +11163,8 @@ PUGI__NS_BEGIN
 
 		char_t _scratch[32];
 
+		size_t _depth;
+
 		xpath_ast_node* error(const char* message)
 		{
 			_result->error = message;
@@ -11171,6 +11181,11 @@ PUGI__NS_BEGIN
 			return 0;
 		}
 
+		xpath_ast_node* error_rec()
+		{
+			return error("Exceeded maximum allowed query depth");
+		}
+
 		void* alloc_node()
 		{
 			return _alloc->allocate(sizeof(xpath_ast_node));
@@ -11563,10 +11578,15 @@ PUGI__NS_BEGIN
 			xpath_ast_node* n = parse_primary_expression();
 			if (!n) return 0;
 
+			size_t old_depth = _depth;
+
 			while (_lexer.current() == lex_open_square_brace)
 			{
 				_lexer.next();
 
+				if (++_depth > xpath_ast_depth_limit)
+					return error_rec();
+
 				if (n->rettype() != xpath_type_node_set)
 					return error("Predicate has to be applied to node set");
 
@@ -11582,6 +11602,8 @@ PUGI__NS_BEGIN
 				_lexer.next();
 			}
 
+			_depth = old_depth;
+
 			return n;
 		}
 
@@ -11733,12 +11755,17 @@ PUGI__NS_BEGIN
 			xpath_ast_node* n = alloc_node(ast_step, set, axis, nt_type, nt_name_copy);
 			if (!n) return 0;
 
+			size_t old_depth = _depth;
+
 			xpath_ast_node* last = 0;
 
 			while (_lexer.current() == lex_open_square_brace)
 			{
 				_lexer.next();
 
+				if (++_depth > xpath_ast_depth_limit)
+					return error_rec();
+
 				xpath_ast_node* expr = parse_expression();
 				if (!expr) return 0;
 
@@ -11755,6 +11782,8 @@ PUGI__NS_BEGIN
 				last = pred;
 			}
 
+			_depth = old_depth;
+
 			return n;
 		}
 
@@ -11764,11 +11793,16 @@ PUGI__NS_BEGIN
 			xpath_ast_node* n = parse_step(set);
 			if (!n) return 0;
 
+			size_t old_depth = _depth;
+
 			while (_lexer.current() == lex_slash || _lexer.current() == lex_double_slash)
 			{
 				lexeme_t l = _lexer.current();
 				_lexer.next();
 
+				if (++_depth > xpath_ast_depth_limit)
+					return error_rec();
+
 				if (l == lex_double_slash)
 				{
 					n = alloc_node(ast_step, n, axis_descendant_or_self, nodetest_type_node, 0);
@@ -11779,6 +11813,8 @@ PUGI__NS_BEGIN
 				if (!n) return 0;
 			}
 
+			_depth = old_depth;
+
 			return n;
 		}
 
@@ -11964,6 +12000,9 @@ PUGI__NS_BEGIN
 			{
 				_lexer.next();
 
+				if (++_depth > xpath_ast_depth_limit)
+					return error_rec();
+
 				xpath_ast_node* rhs = parse_path_or_unary_expression();
 				if (!rhs) return 0;
 
@@ -12009,13 +12048,22 @@ PUGI__NS_BEGIN
 		//						  | MultiplicativeExpr 'mod' UnaryExpr
 		xpath_ast_node* parse_expression(int limit = 0)
 		{
+			size_t old_depth = _depth;
+
+			if (++_depth > xpath_ast_depth_limit)
+				return error_rec();
+
 			xpath_ast_node* n = parse_path_or_unary_expression();
 			if (!n) return 0;
 
-			return parse_expression_rec(n, limit);
+			n = parse_expression_rec(n, limit);
+
+			_depth = old_depth;
+
+			return n;
 		}
 
-		xpath_parser(const char_t* query, xpath_variable_set* variables, xpath_allocator* alloc, xpath_parse_result* result): _alloc(alloc), _lexer(query), _query(query), _variables(variables), _result(result)
+		xpath_parser(const char_t* query, xpath_variable_set* variables, xpath_allocator* alloc, xpath_parse_result* result): _alloc(alloc), _lexer(query), _query(query), _variables(variables), _result(result), _depth(0)
 		{
 		}
 
@@ -12024,6 +12072,8 @@ PUGI__NS_BEGIN
 			xpath_ast_node* n = parse_expression();
 			if (!n) return 0;
 
+			assert(_depth == 0);
+
 			// check if there are unparsed tokens left
 			if (_lexer.current() != lex_eof)
 				return error("Incorrect query");

tests/test_xpath_parse.cpp

@@ -381,6 +381,28 @@ TEST(xpath_parse_oom_propagation)
 	}
 }
 
+static std::basic_string<char_t> rep(const std::basic_string<char_t>& base, size_t count)
+{
+	std::basic_string<char_t> result;
+	result.reserve(base.size() * count);
+
+	for (size_t i = 0; i < count; ++i)
+		result += base;
+
+	return result;
+}
+
+TEST(xpath_parse_depth_limit)
+{
+	const size_t limit = 5000;
+
+	CHECK_XPATH_FAIL((rep(STR("("), limit) + "1" + rep(STR(")"), limit)).c_str());
+	CHECK_XPATH_FAIL(("(id('a'))" + rep(STR("[1]"), limit)).c_str());
+	CHECK_XPATH_FAIL(("/foo" + rep(STR("[1]"), limit)).c_str());
+	CHECK_XPATH_FAIL(("/foo" + rep(STR("/x"), limit)).c_str());
+	CHECK_XPATH_FAIL(("1" + rep(STR("+1"), limit)).c_str());
+}
+
 TEST_XML(xpath_parse_location_path, "<node><child/></node>")
 {
 	CHECK_XPATH_NODESET(doc, STR("/node")) % 2;