David Evanshttp://www.cs.virginia.edu/evans

CS201j: Engineering SoftwareUniversity of VirginiaComputer Science

Lecture 20: Hair Dryer Attacks

Image from www.clean-funny.com, GoldenBlue LLC.

6 November 2003 CS 201J Fall 2003 2

Menu

• Array Subtyping

• Java Security Review

• Violating Type Safety with a Hair Dryer

6 November 2003 CS 201J Fall 2003 3

Subtyping and Arrays

If B <= A, should B[] <= A []?

6 November 2003 CS 201J Fall 2003 4

Array Subtypingstatic public Object getFirst (Object [] els) throws NoSuchElementException { if (els == null || els.length == 0) { throw new NoSuchElementException (); } else { return els[0]; }}

static public void main (String args[]) { try { Object o = getFirst (args); System.err.println ("The first parameter is: " + o); } catch (NoSuchElementException e) { System.err.println ("There are no parameters!"); }}

6 November 2003 CS 201J Fall 2003 5

Array Storestatic public void setFirst (Object [] els) throws NoSuchElementException { if (els == null || els.length == 0) { throw new NoSuchElementException (); } else { els[0] = new Object (); }}

static public void main (String args[]) { try { Object o = getFirst (args); System.err.println ("The first parameter is: " + o); setFirst (args); } catch (NoSuchElementException e) { System.err.println ("There are no parameters!"); }}

> javac TestArrays.java> java TestArrays testThe first parameter is: testException in thread "main" java.lang.ArrayStoreException at TestArrays.setFirst(TestArrays.java:16) at TestArrays.main(TestArrays.java:25)

6 November 2003 CS 201J Fall 2003 6

ESC/Java and Array Stores

> escjava TestArrays.javaESC/Java version 1.2.4, 27 September 2001

TestArrays ...

TestArrays: getFirst(java.lang.Object[]) ... [0.199 s] passed

TestArrays: setFirst(java.lang.Object[]) ...------------------------------------------------------------------------TestArrays.java:16: Warning: Type of right-hand side possibly not a subtype of array element type (ArrayStore) els[0] = new Object ();

6 November 2003 CS 201J Fall 2003 7

Java

• Type checking: B <= A B[] <= A[]• Need a run-time check for every array

store (to an array where the actual element type is not known)

• Better rule: no inference of array subtypes

6 November 2003 CS 201J Fall 2003 8

Java Security

6 November 2003 CS 201J Fall 2003 9

Javajavac

Compiler malcode.java

malcode.classJVML

Joe User

Java Bytecode Verifier

JavaVM

“Okay”

Invalid

STOP

Trusted Computing Base

6 November 2003 CS 201J Fall 2003 10

Bytecode Verifier• Checks JVML code satisfies safety

properties– Simulates program execution to know types

are correct, but doesn’t need to examine any instruction more than once

– After code is verified, it is trusted: is not checked for type safety at run time (except for casts, array stores)

Key assumption: when a value is written to a memory location, the value in that memory location is the same value when it is read.

6 November 2003 CS 201J Fall 2003 11

Violating the Assumption

…// The object on top of the stack is a SimObject

astore_0// There is a SimObject in location 0

aload_0// The value on top of the stack is a SimObject

If a cosmic ray hits the right bit of memory, between the store and load, the assumption might be wrong.

6 November 2003 CS 201J Fall 2003 12

Improving the Odds

• Set up memory so that a single bit error is likely to be exploitable

• Mistreat the hardware memory to increase the odds that bits will flip

Following slides adapted (with permission) from Sudhakar Govindavajhala and Andrew W. Appel, Using Memory Errors to Attack a Virtual Machine, July 2003.

6 November 2003 CS 201J Fall 2003 13

Making Bit Flips UsefulFill up memory with Filler objects, and one Pointee object:

class Filler { class Pointee { Pointee a1; Pointee a1;

Pointee a2; Pointee a2; Pointee a3; Filler f;

Pointee a4; int b; Pointee a5; Pointee a5;

Pointee a6; Pointee a6; Pointee a7; Pointee a7;} }

6 November 2003 CS 201J Fall 2003 14

Filling Up Memory

Pointee p = new Pointee ();Vector fillers = new Vector ();try { while (true) { Filler f = new Filler (); f.a1 = p; f.a2 = p; f.a3 = p; …; f.a7 =p; fillers.add (f); }} catch (OutOfMemoryException e) { ; }

a1

a2

a3

a4

a5

a6

a7

Fill

er O

bjec

t

a1

a2

f

b

a5

a6

a7

Poi

ntee

Obj

ect

a1

a2

a3

a4

Filler Object

6 November 2003 CS 201J Fall 2003 15

Wait for a bit flip…

• Remember: there are lots of Filler objects (fill up all of memory)

• If a bit flips, good chance (~70%) it will be in a field of a Filler object and it will now point to a Filler object instead of a Pointee object

a1

a2

a3

a4

a5

a6

a7

Fill

er O

bjec

t

a1

a2

f

b

a5

a6

a7

Poi

ntee

Obj

ect

a1

a2

a3

a4

Filler Object

6 November 2003 CS 201J Fall 2003 16

Type Violation

After the bit flip, the

value of f.a2 is a

Filler object, but

f.a2 was declared

as a Pointee object!

a1

a2

a3

a4

a5

a6

a7

Fill

er O

bjec

t

a1

a2

f

b

a5

a6

a7

Poi

ntee

Obj

ect

a1

a2

a3

a4

Filler Object

Can we exploit this?

6 November 2003 CS 201J Fall 2003 17

Finding the Bit Flip

while (true) { for (Enumeration e = fillers.elements (); e.hasMoreElements () ; ) { Filler f = (Filler) e.nextElement (); if (f.a1 != p) { // bit flipped! … } else if (f.a2 != p) { … }}

Pointee p = new Pointee ();Vector fillers = new Vector ();try { while (true) { Filler f = new Filler (); f.a1 = p; f.a2 = p; f.a3 = p; …; f.a7 =p; fillers.add (f); }} catch (OutOfMemoryException e) { ; }

6 November 2003 CS 201J Fall 2003 18

Violating Type Safety

Filler f = (Filler) e.nextElement (); if (f.a1 != p) { // bit flipped! Object r = f.a1; // Filler fr = (Filler) r; // Cast is checked at run-time

class Filler { class Pointee { Pointee a1; Pointee a1; Pointee a2; Pointee a2; Pointee a3; Filler f; Pointee a4; int b; Pointee a5; Pointee a5;

Pointee a6; Pointee a6; Pointee a7; Pointee a7;} }

Declared Typef.a1 Pointee f.a1.b intfr == f.a1 Fillerfr.a4 == f.a1.b Pointee

6 November 2003 CS 201J Fall 2003 19

Violating Type Safety

Filler f = (Filler) e.nextElement (); if (f.a1 != p) { // bit flipped! Object r = f.a1; // Filler fr = (Filler) r; // Cast is checked at run-time f.a1.b = 69473248; // Address of bank balance object fr.a4.a1 = p.a5; // Set it to a new value

class Filler { class Pointee { Pointee a1; Pointee a1; Pointee a2; Pointee a2; Pointee a3; Filler f; Pointee a4; int b; Pointee a5; Pointee a5;

Pointee a6; Pointee a6; Pointee a7; Pointee a7;} }

6 November 2003 CS 201J Fall 2003 20

Violating Type Safety

Filler f = (Filler) e.nextElement (); if (f.a1 != p) { // bit flipped! Object r = f.a1; // Filler fr = (Filler) r; // Cast is checked at run-time f.a1.b = 1524383; // Address of the SecurityManager fr.a4.a1 = null; // Set it to a null // Do whatever you want! There’s no security policy now… new File (“C:\thesis.doc”).delete ();

class Filler { class Pointee { Pointee a1; Pointee a1; Pointee a2; Pointee a2; Pointee a3; Filler f; Pointee a4; int b; Pointee a5; Pointee a5;

Pointee a6; Pointee a6; Pointee a7; Pointee a7;} }

6 November 2003 CS 201J Fall 2003 21

Getting a Bit Flip• Wait for a Cosmic Ray

– You have to be really, really patient… (or move machine out of Earth’s atmosphere)

• X-Rays– Expensive, not enough power to generate bit-flip

• High energy protons and neutrons– Work great - but, you need a particle accelerator

• Hmm….

6 November 2003 CS 201J Fall 2003 22

Using Heat 50-watt spotlight

bulb Between 80° -

100°C, memory starts to have a few failures

Attack applet is successful (at least half the time)!

Hairdryer works too, but it fries too many bits at once

Picture from Sudhakar Govindavajhala

6 November 2003 CS 201J Fall 2003 23

Should Anyone be Worried?

Java virtual machine

6 November 2003 CS 201J Fall 2003 24

Recap• Verifier assumes the value you write is the

same value when you read it

• By flipping bits, we can violate this assumption

• By violating this assumption, we can violate type safety: get two references to the same storage that have inconsistent types

• By violating type safety, we can get around all other security measures

• For details, see paper linked from notes

6 November 2003 CS 201J Fall 2003 25

Charge: Problem Set 6• Violate type safety to steal an election

• No need to open up ITC machines to try to heat up memory– Please don’t try this in lab (but try it at home if

you want)

• Instead, use java –noverify to get around type checking: allows byte codes to run without verifying them