CS711: Reference Monitors Part 1: OS & SFI

1. CS711: Reference MonitorsPart 1: OS & SFI Greg Morrisett Cornell University

2. 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. Lang. Based Security

3. 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 • Java and CLR Stack Inspection • Inlined Reference Monitors Lang. Based Security

4. 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. Lang. Based Security

5. What Policies? We’ll see that under quite liberal assumptions: • there’s a nice class of policies that reference monitors can enforce (safety properties). • 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. Lang. Based Security

6. Schneider's Formalism 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 propertyifmembership is determined solely by the sequence and not the other members in the set. Lang. Based Security

7. 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])) Lang. Based Security

8. 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. Lang. Based Security

9. Safety vs. Security Safety is what we can implement, but is it what we want? • “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? Lang. Based Security

10. 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. Lang. Based Security

11. 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. Lang. Based Security

12. 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 Lang. Based Security

13. 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? Lang. Based Security

14. 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... Lang. Based Security

15. 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. Lang. Based Security

16. Steps in a System Call Time User Process Kernel calls f=fopen(“foo”) library executes “break” saves context, flushes TLB, etc. trap 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 Lang. Based Security

17. 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. Lang. Based Security

18. 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 Lang. Based Security

19. A System Call in Mach Time User Process Kernel Unix Server f=fopen(“foo”) “break” saves context checks capabilities,copies arguments switches to Unixserver context checks ACL, sets upbuffers, etc. “returns” to user. saves context checks capabilities, copies results restores user’s context Lang. Based Security

20. 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. Lang. Based Security

21. 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. Lang. Based Security

22. 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? Lang. Based Security

23. 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 • Java Stack Inspection • Inlined Reference Monitors Lang. Based Security

24. 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 Lang. Based Security

25. 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 Lang. Based Security

26. 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. Lang. Based Security

27. 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 For a cool example of this, see Fred Smith's thesis (hanging off my web page.) Lang. Based Security

28. Example PE Original Binary: Interpreter 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 ... while (true) { if (pc >= codesz) exit(1); int inst = code[pc]; ... } Resulting Compiled Code 0: add r1,r2,r3 1: addi r5,r3,12 2: subi r6,r5,memsz 3: jab _exit 4: ld r4,r5(0) ... Lang. Based Security

29. 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. Lang. Based Security

30. 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. Lang. Based Security

31. More SFI Details Protection vs. Sandboxing: • Protection is fail-stop: • 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. Lang. Based Security

32. 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? Lang. Based Security