1. Language-Based Security Reference Monitors Greg Morrisett Cornell University

2. June 2001Lang. Based Security2 Papers for this lecture F.B.Schneider, “Enforceable Security Policies.” In ACM Trans. on Information and System Security, 2(4), March 2000. R.Wahbe et al. “Efficient Software-Based Fault Isolation.” In Proceedings of the 14th ACM Symp. on Operating Systems Principles, pages 203--216, December 1993. U.Erlingsson and F.B.Schneider. “SASI Enforcement of Security Policies: A Retrospective.” Cornell Technical Report TR99- 1758. U.Erlingsson and F.B.Schneider. IRM Enforcement of Java Stack Inspection. Cornell Technical Report TR2000-1786, January 2000. D.Evans and A.Twynman. “Flexible Policy-Directed Code Safety.” In 1999 IEEE Symposium on Security and Privacy, Oakland, CA, May 1999. [sp99.ps]

3. June 2001Lang. Based Security3 A Reference Monitor Observes the execution of a program and halts the program if it’s going to violate the security policy. Common Examples: –operating system (hardware-based) –interpreters (software-based) –firewalls Claim: majority of today’s enforcement mechanisms are instances of reference monitors.

4. June 2001Lang. Based Security4 Reference Monitors Outline Analysis of power and limitations [Schneider] –What is a security policy? –What policies can reference monitors enforce? Traditional Operating Systems. –Policies and practical issues –Hardware-enforcement of OS policies. Software-enforcement of OS policies. –Why? –Software-Based Fault Isolation [Wahbe et al.] –Inlined Reference Monitors [Urlingsson & Schneider]

5. June 2001Lang. Based Security5 Requirements for a Monitor Must have (reliable) access to information about what the program is about to do. –e.g., what instruction is it about to execute? Must have the ability to “stop” the program –can’t stop a program running on another machine that you don’t own. –really, stopping isn’t necessary, but transition to a “good” state. Must protect the monitor’s state and code from tampering. –key reason why a kernel’s data structures and code aren’t accessible by user code. In practice, must have low overhead.

6. June 2001Lang. Based Security6 What Policies? We’ll see that under quite liberal assumptions: –there’s a nice class of policies that reference monitors can enforce. –there are desirable policies that no reference monitor can enforce precisely. rejects a program if and only if it violates the policy Assumptions: –monitor can have access to entire state of computation. –monitor can have infinite state. –but monitor can’t guess the future – the predicate it uses to determine whether to halt a program must be computable.

7. June 2001Lang. Based Security7 Some Definitions Model programs as labeled transition systems: –  : set of possible program states –  0   : set of possible initial states –  : set of labels –  1   2   x  x  : transition relation –  : infinite sequence of states and labels (  0, 0 ),(  1, 1 ),(  2, 2 ),... such that  0  0  1  1  2  2... and  0  0 –  [..i]: the first i pairs in the sequence  –  : set of all sequences –   : set of all sequences starting with 

8. June 2001Lang. Based Security8 Example: A Unix Process on an x86 machine: –  : memory of the machine, contents of the disk, program counter, registers, etc. – : read(addr), write(addr), jump(addr), syscall(args), other. –transition relation would need to faithfully model the x86 instructions by transforming the state and emitting the appropriate label. if pc = 50, Mem[50] = “movl ebx,[eax+4]”, eax=12, and ebx=3, then we’d transition to a state where Mem[16] = 3 and pc = 54 and the label was “write(16)”.

9. June 2001Lang. Based Security9 Security Policies Schneider defines a policy P to be a predicate on sets of sequences. P : 2   bool A program  satisfies a policy P iff P(   ) is true.

10. June 2001Lang. Based Security10 Policies P = false P = true P = false

11. June 2001Lang. Based Security11 Why Sets of Sequences? There are some conceivable policies (e.g., confidentiality in an information theoretic setting) that involve statistical properties of sets of execution sequences. let Secret = 42 in Out := rand()... out = 0 out = 1 out = 2 out = 41 out = 42out = 43 We don’t want to reveal anything about the secret. P({out:=42}) should be false P(  ) should be true (i.e., admit all executions)

12. June 2001Lang. Based Security12 Back to Reference Monitors A reference monitor only sees one execution sequence of a program. So we can only enforce policies P s.t.: (1) P(S) =  S. P (  ) where P is a predicate on individual sequences. A set of execution sequences S is a property if membership is determined solely by the sequence and not the other members in the set.

13. June 2001Lang. Based Security13 Properties So, reference monitors can’t enforce security policies that aren’t properties. And we’ve already seen one policy regarding information leakage that can’t be described in terms of single execution sequences. So reference monitors can’t enforce every conceivable security policy. Oh well...

14. June 2001Lang. Based Security14 More Constraints on Monitors Shouldn’t be able to “see” the future. –Assumption: must make decisions in finite time. –Suppose P (  ) is true but P (  [..i] ) is false for some prefix  [..i] of . When the monitor sees  [..i] it can’t tell whether or not the execution will yield  or some other sequence, so the best it can do is rule out all sequences involving  [..i] including . So in some sense, P must be continuous: (2)  . P (  )  (  i. P (  [..i] ))

15. June 2001Lang. Based Security15 Safety Properties A predicate P on sets of sequences s.t. (1) P(S) =  S. P (  ) (2)  . P (  )  (  i. P (  [..i] )) is a safety property: “no bad thing will happen.” Conclusion: a reference monitor can’t enforce a policy P unless it’s a safety property. In fact, Schneider shows that reference monitors can (in theory) implement any safety property.

16. June 2001Lang. Based Security16 Safety vs. Security Safety is what we can implement, but is it what we want? –already saw “lack of info. flow” isn’t a property. Safety ensures something bad won’t happen, but it doesn’t ensure something good will eventually happen: –program will terminate –program will eventually release the lock –user will eventually make payment These are examples of liveness properties. –policies involving availability aren’t safety prop. –so a ref. monitor can’t handle denial-of-service?

17. June 2001Lang. Based Security17 Safety Is Nice Safety does have its benefits: –They compose: if P and Q are safety properties, then P & Q is a safety property (just the intersection of allowed traces.) –Safety properties can approximate liveness by setting limits. e.g., we can determine that a program terminates within k steps. –We can also approximate many other security policies (e.g., info. flow) by simply choosing a stronger safety property.

18. June 2001Lang. Based Security18 Practical Issues In theory, a monitor could: –examine the entire history and the entire machine state to decide whether or not to allow a transition. –perform an arbitrary computation to decide whether or not to allow a transition. In practice, most systems: –keep a small piece of state to track history –only look at labels on the transitions –have small labels –perform simple tests Otherwise, the overheads would be overwhelming. –so policies are practically limited by the vocabulary of labels, the complexity of the tests, and the state maintained by the monitor.

19. June 2001Lang. Based Security19 Reference Monitors Outline Analysis of the power and limitations. What is a security policy? What policies can reference monitors enforce? Traditional Operating Systems. –Policies and practical issues –Hardware-enforcement of OS policies. Software-enforcement of OS policies. –Why? –Software-Based Fault Isolation –Inlined Reference Monitors

20. June 2001Lang. Based Security20 Operating Systems circa ‘75 Simple Model: system is a collection of running processes and files. –processes perform actions on behalf of a user. open, read, write files read, write, execute memory, etc. –files have access control lists dictating which users can read/write/execute/etc. the file. (Some) High-Level Policy Goals: –Integrity: one user’s processes shouldn’t be able to corrupt the code, data, or files of another user. –Availability: processes should eventually gain access to resources such as the CPU or disk. –Secrecy? Confidentiality? Access control?

21. June 2001Lang. Based Security21 What Can go Wrong? –read/write/execute or change ACL of a file for which process doesn’t have proper access. check file access against ACL –process writes into memory of another process isolate memory of each process (& the OS!) –process pretends it is the OS and execute its code maintain process ID and keep certain operations privileged --- need some way to transition. –process never gives up the CPU force process to yield in some finite time –process uses up all the memory or disk enforce quotas –OS or hardware is buggy...

22. June 2001Lang. Based Security22 Key Mechanisms in Hardware –Translation Lookaside Buffer (TLB) provides an inexpensive check for each memory access. maps virtual address to physical address –small, fully associative cache (8-10 entries) –cache miss triggers a trap (see below) –granularity of map is a page (4-8KB) –Distinct user and supervisor modes certain operations (e.g., reload TLB, device access) require supervisor bit is set. –Invalid operations cause a trap set supervisor bit and transfer control to OS routine. –Timer triggers a trap for preemption.

23. June 2001Lang. Based Security23 Steps in a System Call Time calls f=fopen(“foo”) User Process library executes “break” Kernel trap saves context, flushes TLB, etc. checks UID against ACL, sets up IO buffers & file context, pushes ptr to context on user’s stack, etc. restores context, clears supervisor bit calls fread(f,n,&buf) library executes “break” saves context, flushes TLB, etc. checks f is a valid file context, does disk access into local buffer, copies results into user’s buffer, etc. restores context, clears supervisor bit

24. June 2001Lang. Based Security24 Hardware Trends The functionality provided by the hardware hasn’t changed much over the years. Clearly, the raw performance in terms of throughput has. Certain trends are clear: –small => large # of registers: 8 16-bit =>128 64-bit –small => large pages: 4 KB => 16 KB –flushing TLB, caches is increasingly expensive –computed jumps are increasingly expensive –copying data to/from memory is increasingly expensive So a trap into a kernel is costing more over time.

25. June 2001Lang. Based Security25 OS Trends In the 1980’s, a big push for microkernels: –Mach, Spring, etc. –Only put the bare minimum into the kernel. context switching code, TLB management trap and interrupt handling device access –Run everything else as a process. file system(s) networking protocols page replacement algorithm –Sub-systems communicate via remote procedure call (RPC) –Reasons: Increase Flexibility, Minimize the TCB

26. June 2001Lang. Based Security26 A System Call in Mach Time f=fopen(“foo”) User Process “break” Kernel saves context checks capabilities, copies arguments switches to Unix server context Unix Server checks ACL, sets up buffers, etc. “returns” to user. saves context checks capabilities, copies results restores user’s context

27. June 2001Lang. Based Security27 Microkernels Claim was that flexibility and increased assurance would win out. –But performance overheads were non-trivial –Many PhD’s on minimizing overheads of communication –Even highly optimized implementations of RPC cost 2-3 orders of magnitude more than a procedure call. Result: a backlash against the approach. –Windows, Linux, Solaris continue the monolithic tradition. and continue to grow for performance reasons (e.g., GUI) and for functionality gains (e.g., specialized file systems.) –Mac OS X, some embedded or specialized kernels (e.g., Exokernel) are exceptions. VMware achieves multiple personalities but has monolithic personalities sitting on top.

28. June 2001Lang. Based Security28 Performance Matters The hit of crossing the kernel boundary: –Original Apache forked a process to run each CGI: could attenuate file access for sub-process protected memory/data of server from rogue script i.e., closer to least privilege –Too expensive for a small script: fork, exec, copy data to/from the server, etc. –So current push is to run the scripts in the server. i.e., throw out least privilege Similar situation with databases, web browsers, file systems, etc.

29. June 2001Lang. Based Security29 The Big Question? From a least privilege perspective, many systems should be decomposed into separate processes. But if the overheads of communication (i.e., traps, copying, flushing TLB) are too great, programmers won’t do it. Can we achieve isolation and cheap communication?

30. June 2001Lang. Based Security30 Reference Monitors Outline Analysis of the power and limitations. What is a security policy? What policies can reference monitors enforce? Traditional Operating Systems. Policies and practical issues Hardware-enforcement of OS policies. Software-enforcement of OS policies. Why? –Software-Based Fault Isolation –Inlined Reference Monitors

31. June 2001Lang. Based Security31 Software Fault Isolation (SFI) Wahbe et al. (SOSP’93) Keep software components in same hardware-based address space. Use a software-based reference monitor to isolate components into logical address spaces. –conceptually: check each read, write, & jump to make sure it’s within the component’s logical address space. –hope: communication as cheap as procedure call. –worry: overheads of checking will swamp the benefits of communication. Note: doesn’t deal with other policy issues –e.g., availability of CPU

32. June 2001Lang. Based Security32 One Way to SFI void interp(int pc, reg[], mem[], code[], memsz, codesz) { while (true) { if (pc >= codesz) exit(1); int inst = code[pc], rd = RD(inst), rs1 = RS1(inst), rs2 = RS2(inst), immed = IMMED(inst); switch (opcode(inst)) { case ADD: reg[rd] = reg[rs1] + reg[rs2]; break; case LD: int addr = reg[rs1] + immed; if (addr >= memsz) exit(1); reg[rd] = mem[addr]; break; case JMP: pc = reg[rd]; continue;... } pc++; }} 0: add r1,r2,r3 1: ld r4,r3(12) 2: jmp r4

33. June 2001Lang. Based Security33 Pros & Cons of Interpreter Pros: –easy to implement (small TCB.) –works with binaries (high-level language- independent.) –easy to enforce other aspects of OS policy Cons: –terribly execution overhead (x25? x70?) but it’s a start.

34. June 2001Lang. Based Security34 Partial Evaluation (PE) A technique for speeding up interpreters. –we know what the code is. –specialize the interpreter to the code. unroll the loop – one copy for each instruction specialize the switch to the instruction compile the resulting code PE is a technology that was primarily developed here in Copenhagen.

35. June 2001Lang. Based Security35 Example PE Specialized interpreter: reg[1] = reg[2] + reg[3]; addr = reg[3] + 12; if (addr >= memsz) exit(1); reg[4] = mem[addr]; pc = reg[4] 0: add r1,r2,r3 1: ld r4,r3(12) 2: jmp r4... Original Binary: while (true) { if (pc >= codesz) exit(1); int inst = code[pc];... } Interpreter 0: add r1,r2,r3 1: addi r5,r3,12 2: subi r6,r5,memsz 3: jab _exit 4: ld r4,r5(0)... Resulting Compiled Code

36. June 2001Lang. Based Security36 SFI in Practice Used a hand-written specializer or rewriter. –Code and data for a domain in one contiguous segment. upper bits are all the same and form a segment id. separate code space to ensure code is not modified. –Inserts code to ensure stores [optionally loads] are in the logical address space. force the upper bits in the address to be the segment id no branch penalty – just mask the address may have to re-allocate registers and adjust PC-relative offsets in code. simple analysis used to eliminate unnecessary masks –Inserts code to ensure jump is to a valid target must be in the code segment for the domain must be the beginning of the translation of a source instruction in practice, limited to instructions with labels.

37. June 2001Lang. Based Security37 More on Jumps PC-relative jumps are easy: –just adjust to the new instruction’s offset. Computed jumps are not: –must ensure code doesn’t jump into or around a check or else that it’s safe for code to do the jump. –for this paper, they ensured the latter: a dedicated register is used to hold the address that’s going to be written – so all writes are done using this register. only inserted code changes this value, and it’s always changed (atomically) with a value that’s in the data segment. so at all times, the address is “valid” for writing. works with little overhead for almost all computed jumps.

38. June 2001Lang. Based Security38 More SFI Details Protection vs. Sandboxing: –Protection covers loads as well as stores: stronger security guarantees (e.g., reads) required 5 dedicated registers, 4 instruction sequence 20% overhead on 1993 RISC machines –Sandboxing covers only stores requires only 2 registers, 2 instruction sequence 5% overhead Remote Procedure Call: –10x cost of a procedure call –10x faster than a really good OS RPC Sequoia DB benchmarks: 2-7% overhead for SFI compared to 18-40% overhead for OS.

39. June 2001Lang. Based Security39 Questions What happens on the x86? –small # of registers –variable-length instruction encoding What happens with discontiguous hunks of memory? What would happen if we really didn’t trust the extension? –i.e., check the arguments to an RPC? –timeouts on upcalls? Does this really scale to secure systems?

40. June 2001Lang. Based Security40 Inlined Reference Monitors Schneider & Erlingsson Generalize the idea of SFI: –write specification of desired policy as a security automaton. (Possibly infinite number of) states used to track history/context of computation. Transitions correspond to labels on computation’s sequence. No transition in automaton means the sequence should be rejected. In principle, can enforce any safety property. –general re-writing tool enforces the policy by inserting appropriate state and checks.

41. June 2001Lang. Based Security41 Example Policy start has read “Applet can’t send a message after reading a file.” read not(read) not(send)

42. June 2001Lang. Based Security42 SFI Policy (retOp() && validRetPt(topOfStack)) || (callOp() && validCallPt()) || (jumpOp() && validJumpPt()) || (readOp() && canRead(sourceAddr)) || (writeOp() && canWrite(targetAddr))

43. June 2001Lang. Based Security43 1 st Prototype SASI: –Security Automata-based Software fault Isolation. –One version for x86 native code, another for JVM bytecodes. x86: labels correspond to opcode & operands of an instruction, sets of addresses that can be read/written/jumped to. JVM: class name, method name, method type signature, JVML opcode, instruction operands, JVM state in which operation will be executed. –SAL: Security Automata specification Language finite list of states, list of (deterministic) transitions simple predicate language for transitions

44. June 2001Lang. Based Security44 Example SAL /* Macros */ MathodCall(name) ::= op == “invokevirtual” && param[1] == name; FileRead() ::= MethodCall(“java/io/FileInputStream/read()I”); Send() := MethodCall(“java/net/SocketOutputStream/write(I)V”); /* Security Automaton */ start ::= !FileRead() -> start FileRead() -> hasRead; hasRead ::= !Send() -> hasRead;

45. June 2001Lang. Based Security45 x86 Prototype Leveraged GCC –code had to come from gcc assumes programs don’t care about nop’s. assumes branch targets restricted to the set of labels identified by GCC during compilation. Relative to original code (std. dev) BenchmarkMiSFITSASI page evicition hostlist 2.378 (0.3%)3.643 (2.6%) logical log- structured disk 1.576 (0.3%)1.654 (0.5%) MD5 message digest 1.331 (1.4%)1.363 (0.1%)

46. June 2001Lang. Based Security46 Java Prototype Implemented JDK 1.1 Security Manager –more on this later in the course –essentially: check that certain methods are called in the right “stack” context Same cost as Sun’s implementation –primarily because of partial evaluation Easier to see the policy –for Sun, it’s baked into the code

47. June 2001Lang. Based Security47 Some Pros and Cons: Policy is separate from implementation –easier to reason about and compose application-specific policies and get it right because things are automated –e.g., SFI is a half-page policy and there were bugs in MiSFIT Analysis and optimization can get rid of overheads –but this increases the size of the TCB Stopping a “bad” program might not be “good” –suppose it holds locks... Vocabulary is limited by what can be observed. –what’s a “procedure call”? what’s a “file descriptor”? –more structure on the code (e.g., types) leads to a bigger vocabulary (Java vs. x86) –but then it’s not working on native code

48. June 2001Lang. Based Security48 2 nd Prototype: PoET/PSLang PoET: Policy Enforcement Toolkit –similar to SASI but specific to Java and the JVM –organized around JVM “events” e.g., method calls, instructions, returns, etc. similar to ATOM and other toolkits for monitoring performance also similar to “aspect-oriented” programming. PSLang: Policy Specification Language –has full power for Java for defining events and transitions –spec is at the Java level, but rewriting is done at the JVM level.

49. June 2001Lang. Based Security49 Ex: “at most 10 windows” IMPORT LIBRARY Lock; ADD SECURITY STATE { int openWindows = 0; Object lock = Lock.create(); } ON EVENT begin method WHEN Event.fullMethodNameIs(“void java.awt.Window.show()”) PERFORM SECURITY UPDATE { Lock.acquire(lock); if (openWindows = 10) { HALT[ “Too many open GUI windows” ]; } openWindows = openWindows + 1; Lock.release(lock); } ON EVENT begin method WHEN Event.fullMethodNameIs(“void java.awt.Window.dispose()”) PERFORM SECURITY UPDATE { Lock.acquire(lock); openWindows := openWindows – 1; Lock.release(lock); }

50. June 2001Lang. Based Security50 Java 2 Security Model Each hunk of code is associated with a protection domain. e.g., applet vs. trusted local-host code Each protection domain can only perform certain operations. e.g., applets can only access /tmp while trusted local-host code can access any file. Both direct and indirect access control: applet can’t directly call load(“/etc/passwd”) nor can it call a trusted piece of code that calls load(“etc/passwd”). avoids accidentally leaking access through a method. But sometimes, we need indirect access this is achieved through amplification (similar idea to setuid)

51. June 2001Lang. Based Security51 Example Security Domains display:... load(“thesis.txt”) use plain font show on screen... Untrusted Applet: /home/* use plain font:... doPrivileged { load(“Courier”); }... GUI Library: /fonts/* load(file F):... checkPermission(F,read)... File Library: *

52. June 2001Lang. Based Security52 Java 2 Stack Inspection A richer notion of “context” than just the domain of the executing code. –on whose behalf is the code executing? doPrivileged{S} –switches from the caller’s protection domain to the method’s protection domain while executing the statement S. checkPermission(P) –examines the “stack” from most recent to least recent. –if it encounters a frame from a protection domain that does not have permission P, throws a security exception. –stops when it reaches the end of the stack or a doPrivileged command.

53. June 2001Lang. Based Security53 3 Implementations Sun’s implementation –no overhead on method call, doPrivileged –crawls the stack on checkPermission –hard to see what the policy is Naive PSLang –use an external stack object –push the domain upon each method call, pop upon return. –push a different domain upon doPrivileged. –crawl the stack object on checkPermission. Optimized PSLang –takes advantage of reflection facilities to crawl the stack in a fashion similar to Sun’s

54. June 2001Lang. Based Security54 Performance Overhead JVMIRM (naive) IRM Jigsaw6.2%20.1%6.4% javac2.9%46.2%2.0% tar10.1%3.0%5.4% MPEG0.9%72.5%0.4% Naive approach is very expensive because there are a lot of method calls relative to domain crossings. IRM approach can beat JVM because of static (local) optimization.

55. June 2001Lang. Based Security55 Summary Reference monitors –implement safety policies OS policies –designed in a different era –leverage hardware for efficient checking –but overheads discourage “least privilege” Software Fault Isolation –small overheads, but limited protection –not clear it scales in many directions Inlined Reference Monitors (SASI/PoET) –competitive with hand-rolled systems –more flexible, easier to reason about policies –limited by “vocabulary” and structure of the code