Memory Protection in the Java Virtual Machine

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1 1 / 31 Memory Protection in the Java Virtual Machine Philip W. L. Fong Department of Computer Science University of Calgary Calgary, Alberta, Canada CPSC 525/625 (Fall 2016)

2 Requirement Evolution Useful software: Users want new features. Requirement evolution 2 / 31

3 Requirement Evolution Useful software: Users want new features. Requirement evolution Supporting requirement evolution software extensions plug-in architecture scripting languages 2 / 31

4 Dynamic Loading of Software Extension (1) 3 / 31

5 Dynamic Loading of Software Extension (2) Code fragment: with free variables 4 / 31

6 Dynamic Loading of Software Extension (3) Linking free variables to local resources 5 / 31

7 Essence of Code Mobility Essence of Code Mobility Running a code fragment (i.e., with free variables) in an environment owned by code consumer. 6 / 31

8 Code Mobility 2 types of code mobility: Weak mobility: migration of code only 7 / 31

9 Code Mobility 2 types of code mobility: Weak mobility: migration of code only 1 Code-on-Demand (consumer initiated) 7 / 31

10 Code Mobility 2 types of code mobility: Weak mobility: migration of code only 1 Code-on-Demand (consumer initiated) 2 Remote Evaluation (producer initiated) 7 / 31

11 Code Mobility 2 types of code mobility: Weak mobility: migration of code only 1 Code-on-Demand (consumer initiated) 2 Remote Evaluation (producer initiated) Strong mobility: migration of code and data 1 Process Migration (code + state) 7 / 31

12 Motivations of Code Mobility 1 Real-time interaction with remote resources 8 / 31

13 Motivations of Code Mobility 1 Real-time interaction with remote resources 2 Reduction of communication traffic 8 / 31

14 Motivations of Code Mobility 1 Real-time interaction with remote resources 2 Reduction of communication traffic 3 Customization and extension of code-consumer capability 8 / 31

15 Motivations of Code Mobility 1 Real-time interaction with remote resources 2 Reduction of communication traffic 3 Customization and extension of code-consumer capability 4 Avoiding distribution of state 8 / 31

16 Security Challenges for Code Mobility Host security protecting host from applications 9 / 31

17 Security Challenges for Code Mobility Host security protecting host from applications Application security protecting applications from host 9 / 31

18 Aspects of Host Security Memory protection Access control 10 / 31

19 Focus of This Lecture Question How is the Java platform designed to achieve memory protection? 11 / 31

20 Outline 1 Memory Protection by Abstract Interpreter 2 Structure of the JVM, Class File Format, Bytecode Instructions 3 Class Loading, Bytecode Verification, and Symbol Resolution 12 / 31

21 13 / 31 Memory Protection by Abstract Interpreter The Java Virtual Machine C.java Compiler (javac) C.class JVM (java) Java source language JVM bytecode

22 2 Protection Features 1 Restricted expressiveness 2 Dynamic checking 14 / 31

23 Restricted Expressiveness JVM bytecode is a strongly typed intermediate representation 15 / 31

24 Restricted Expressiveness JVM bytecode is a strongly typed intermediate representation unsafe instructions cannot be expressed 15 / 31

25 Restricted Expressiveness JVM bytecode is a strongly typed intermediate representation unsafe instructions cannot be expressed no pointer arithmetic: cannot forge illegal pointer 15 / 31

26 Restricted Expressiveness JVM bytecode is a strongly typed intermediate representation unsafe instructions cannot be expressed no pointer arithmetic: cannot forge illegal pointer no type confusion: encapsulation is real 15 / 31

27 Dynamic Checking untrusted code interacts with the CPU through the mediation of the JVM 16 / 31

28 Dynamic Checking untrusted code interacts with the CPU through the mediation of the JVM screen out or detect dangerous actions 16 / 31

29 Dynamic Checking untrusted code interacts with the CPU through the mediation of the JVM screen out or detect dangerous actions null pointer dereferencing out-of-bound array access illegal type-cast 16 / 31

30 More on Encapsulation Encapsulation is actually enforced at run-time Private fields & methods are truly inaccessible 17 / 31

31 More on Encapsulation Encapsulation is actually enforced at run-time Private fields & methods are truly inaccessible Contrast with C++ class C { private: int secret; } class D { private: int exposed; } D* attack = (D*) C_ptr; 17 / 31

32 More on Encapsulation Encapsulation is actually enforced at run-time Private fields & methods are truly inaccessible Contrast with C++ class C { private: int secret; } class D { private: int exposed; } D* attack = (D*) C_ptr; To protect the SecurityManager class i.e., the reference monitor for Java 17 / 31

33 Outline 1 Memory Protection by Abstract Interpreter 2 Structure of the JVM, Class File Format, Bytecode Instructions 3 Class Loading, Bytecode Verification, and Symbol Resolution 18 / 31

34 Structure of the JVM Run-time stack Structure of stack frames local variable array (fixed size): analogous to registers operand stack (fixed size) current class & method program counter 19 / 31

35 Structure of the JVM Run-time stack Structure of stack frames local variable array (fixed size): analogous to registers operand stack (fixed size) current class & method program counter Method area code segment in Unix terminology inaccessible by Java references 19 / 31

36 Structure of the JVM Run-time stack Structure of stack frames local variable array (fixed size): analogous to registers operand stack (fixed size) current class & method program counter Method area code segment in Unix terminology inaccessible by Java references Heap dynamic memory allocation object creation garbage collected no need to worry about forgetting to deallocate add to memory safety 19 / 31

37 Structure of the JVM Run-time stack Structure of stack frames local variable array (fixed size): analogous to registers operand stack (fixed size) current class & method program counter Method area code segment in Unix terminology inaccessible by Java references Heap dynamic memory allocation object creation garbage collected no need to worry about forgetting to deallocate add to memory safety Run-time constant pools i.e., symbol tables one for each loaded class 19 / 31

38 Classfile Format one class file for each declared type (class/interface) symbolic names for class, fields, methods access flags for class, fields, methods (e.g., public, protected, private) symbolic references to super class and super interfaces method argument and return type sizes of local variable array and operand stack bytecode exceptions thrown by this method constant pool in class file and in bytecode, symbolic references to declared types constant pool is a symbol table for these external references 20 / 31

39 Lesson Lesson All source-level typing information is preserved in the class file format. 21 / 31

40 Bytecode Instructions iadd aload invokevirtual iastore checkcast 22 / 31

41 Highlight Highlight The assumed types of operands in local variable array and operand stack, and the assumed sizes of the latter two must be honored. Ensuring the above is the responsiblity of bytecode verification. 23 / 31

42 Outline 1 Memory Protection by Abstract Interpreter 2 Structure of the JVM, Class File Format, Bytecode Instructions 3 Class Loading, Bytecode Verification, and Symbol Resolution 24 / 31

43 Class Loading Classes are loaded into the JVM in an incremental manner laziness speeds up application start-up time 25 / 31

44 Class Loading Classes are loaded into the JVM in an incremental manner laziness speeds up application start-up time Class loading involves more than downloading the class file from a remote host creating an internal representation of the class in the JVM 25 / 31

45 Class Loading Classes are loaded into the JVM in an incremental manner laziness speeds up application start-up time Class loading involves more than downloading the class file from a remote host creating an internal representation of the class in the JVM bytecode verification 25 / 31

46 Class Loading Classes are loaded into the JVM in an incremental manner laziness speeds up application start-up time Class loading involves more than downloading the class file from a remote host creating an internal representation of the class in the JVM bytecode verification initialization calling the static initializer method of the class 25 / 31

47 Bytecode Verification Basic well-formedness checks 26 / 31

48 Bytecode Verification Basic well-formedness checks Static type checking based on source-level typing information 26 / 31

49 Bytecode Verification Basic well-formedness checks Static type checking based on source-level typing information Because JVM bytecode is an unstructured language (i.e., with goto ), type checking involves a non-trivial data flow analysis. 26 / 31

50 Bytecode Verification Basic well-formedness checks Static type checking based on source-level typing information Because JVM bytecode is an unstructured language (i.e., with goto ), type checking involves a non-trivial data flow analysis. Symbolically simulates the execution of bytecode to ensure / 31

51 Bytecode Verification Basic well-formedness checks Static type checking based on source-level typing information Because JVM bytecode is an unstructured language (i.e., with goto ), type checking involves a non-trivial data flow analysis. Symbolically simulates the execution of bytecode to ensure... when a value is loaded from local variable array to the operand stack (or store from operand stack to local variable array), the right type of data will be there 26 / 31

52 Bytecode Verification Basic well-formedness checks Static type checking based on source-level typing information Because JVM bytecode is an unstructured language (i.e., with goto ), type checking involves a non-trivial data flow analysis. Symbolically simulates the execution of bytecode to ensure... when a value is loaded from local variable array to the operand stack (or store from operand stack to local variable array), the right type of data will be there local variable array and operand stack are large enough to hold all data 26 / 31

53 Symbol Resolution Symbolic references in constant pools are incrementally resolved into pointers 27 / 31

54 Symbol Resolution Symbolic references in constant pools are incrementally resolved into pointers Induces more class loading 27 / 31

55 Symbol Resolution Symbolic references in constant pools are incrementally resolved into pointers Induces more class loading Some additional type checking occurs at this stage 27 / 31

56 Class Loader To implement namespace partitioning at run-time same symbolic class name defined (i.e., loaded) in 2 different class loaders represent 2 distinct types 28 / 31

57 Class Loader To implement namespace partitioning at run-time same symbolic class name defined (i.e., loaded) in 2 different class loaders represent 2 distinct types to support application separation each application is loaded by a different class loader 28 / 31

58 Class Loader Hierarchy Class loaders are hierarchically organized parent-child relationship 29 / 31

59 Class Loader Hierarchy Class loaders are hierarchically organized parent-child relationship If a class name cannont be resolved in a child class loader (i.e., namespace) then JVM will recursively attempt to resolve it in the parent class loader 29 / 31

60 Class Loader Hierarchy Class loaders are hierarchically organized parent-child relationship If a class name cannont be resolved in a child class loader (i.e., namespace) then JVM will recursively attempt to resolve it in the parent class loader Root class loader (most trusted, for bootstrapping) 29 / 31

61 Class Loader Hierarchy Class loaders are hierarchically organized parent-child relationship If a class name cannont be resolved in a child class loader (i.e., namespace) then JVM will recursively attempt to resolve it in the parent class loader Root class loader (most trusted, for bootstrapping) When a constant pool entry is resolved, it is resolved in the class loader that loaded the class 29 / 31

62 Class Loader Hierarchy Class loaders are hierarchically organized parent-child relationship If a class name cannont be resolved in a child class loader (i.e., namespace) then JVM will recursively attempt to resolve it in the parent class loader Root class loader (most trusted, for bootstrapping) When a constant pool entry is resolved, it is resolved in the class loader that loaded the class To support class sharing (e.g., java.lang.string) 29 / 31

63 Class Loader Hierarchy Class loaders are hierarchically organized parent-child relationship If a class name cannont be resolved in a child class loader (i.e., namespace) then JVM will recursively attempt to resolve it in the parent class loader Root class loader (most trusted, for bootstrapping) When a constant pool entry is resolved, it is resolved in the class loader that loaded the class To support class sharing (e.g., java.lang.string) A form of dynamic binding 29 / 31

64 History The 1st JVM specification got the class loader part wrong Identify a class only by its class name 30 / 31

65 History The 1st JVM specification got the class loader part wrong Identify a class only by its class name Could lead to type confusion 30 / 31

66 History The 1st JVM specification got the class loader part wrong Identify a class only by its class name Could lead to type confusion E.g., impersonating the Security Manager 30 / 31

67 History The 1st JVM specification got the class loader part wrong Identify a class only by its class name Could lead to type confusion E.g., impersonating the Security Manager Illustrate why English is a terrible specification language 30 / 31

68 History The 1st JVM specification got the class loader part wrong Identify a class only by its class name Could lead to type confusion E.g., impersonating the Security Manager Illustrate why English is a terrible specification language The 2nd edition of the JVM specification had it fixed. 30 / 31

69 Mini Quiz What techniques are employed to achieve memory protection in the JVM? 31 / 31

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