Secure Execution of Untrusted Code

Motivation Mobile code scenarios Java applet / Javascript in web page Postscript / PDF file Email attachment BPF Active networks Agent technology Untrusted Code My Secure PC

Challenges Untrusted code may Read sensitive memory regions (password, cryptographic keys, db records) Write memory areas, violate integrity (overwrite public verification key) Jump to arbitrary location (jump past password verification) Perform arbitrary system calls (execute arbitrary local programs, access file system) Waste resources, DoS (memory, computation, network bandwidth), thrash OS (while 1 do fork()) Crash program, OS

Solution 1: Separate Process Run untrusted code in separate process Advantages Memory separation through VM system Disadvantages Inter-Process Communication (IPC) is expensive Context switch has high overhead (TLB, cache) Untrusted code can do any system calls To protect file system, use chroot

Solution 2: Signed Code Only accept code digitally signed by trusted entity Advantages Simple security policy Can be secure if trusted entity creates “secure code” Disadvantages Security relies on secrecy of private key Security relies on authenticity of public key Coarse-grained access control

Solution 3: Sandboxing Run code in limited environment Verify each system call Verify each memory reference Advantage Fine-grained control Disadvantages Efficiency Expressing security policy Sandbox may have security vulnerabilities Sandbox Untrusted Code Application

Solution 4: SFI Provide memory safety efficiently Can only r/w legitimate memory locations Software fault isolation (SFI): transform untrusted code into safe code Compiler: generates safe code Ship source code, or Generate code that verifier can easily check Loader (binary patching): rewrite untrusted code

SFI Load each module into its own fault domain Two parts Code is in read-only part Module can r/w anything within fault domain Code can only jump within fault domain Two parts Efficient Software Encapsulation Fast Communication Across Fault Domains Application Untrusted Code Untrusted Code Untrusted Code

Efficient Software Encapsulation Goal Only allow writes within data part of fault domain Only allow jumps within code segment of fault domain Mechanisms Split memory space into segments, each segment has a segment identifier (high-order bits of address), and comprises all addresses for any low-order bits Separate code and data segments Dedicated registers are only used by inserted code, cannot be modified by untrusted code Tame unsafe instructions by segment matching or address sandboxing PC-relative jumps often safe, static stores are safe Other stores and jumps are unsafe

RISC Instruction Set Simplifies SFI Memory accessed with load / store instruction (no add to memory) All instructions have fixed size, and are aligned Simplifies instruction verification Simplifies jump target verification Usually large number of registers Can tolerate giving up 5 dedicated registers

Segment Matching Verifies correctness of memory accesses Advantage Can pinpoint address of offending instruction, useful in debugging Disadvantage Slow dedicated-reg  target address scratch-reg  (dedicated-reg >> shift reg) compare scratch-reg and segment-reg trap if not equal store instruction uses dedicated-reg

Address Sandboxing Set segment identifier for each memory access / jump instruction Overwrite segment identifier with fault domain’s segment identifier Advantage: efficient Disadvantage: cannot pinpoint flaw, only enforces that accesses are within domain dedicated-reg  target-reg & and-mask-reg dedicated-reg  dedicated-reg | segment-reg store instruction uses dedicated-reg

Puzzles What about the following transformation, not using dedicated register? target-reg  target-reg & and-mask-reg target-reg  target-reg | segment-reg store instruction uses target-reg What about jumps into SFI verification instructions? Could we jump outside of fault domain? Could we write to code segment? Could we jump to data segment? What if we only used 4 dedicated registers (sandboxing case, i.e., 1 dedicated-reg for code/data)?

4-Register Attack jump target-reg dedicated-reg  target-reg & and-mask-reg dedicated-reg  dedicated-reg | segment-reg-c jump instruction uses dedicated-reg target-reg = r2 dedicated-reg  dedicated-reg | segment-reg-d store instruction uses dedicated-reg

Verification of Instrumented Code Problem: given instrumented binary, how can we verify that all rules are met? Verify that all unsafe stores and jumps use dedicated registers to form address Verify that all inserted code has correct pattern (identify inserted code by use of dedicated registers)

Fast Communication Across Fault Domains Use Jump Table with legal entry points outside fault domain For each call between fault domains, a customized stub procedure is created Stubs run unprotected outside fault domain Trusted stub directly copies arguments to destination domain Switch execution stack Set up register context and validate dedicated registers

SFI Discussion System calls from fault domain? Issues and solutions Allowing arbitrary memory reads? Could we improve the granulartiy of memory protection? Missing read protection a problem? Is granularity of jump protection sufficient? Flexibility for segment size? Internal fragmentation? Can SFI prevent buffer overruns? Is self-modifying code possible? SFI on CISC processors? Can we change compiler once verifier is deployed?

Solution 5: PCC Proof-carrying code (PCC) developed @ CMU by George Necula and Peter Lee Goal: safely run code in unsafe languages Approach: create efficiently verifiable proof that untrusted code satisfies safety policy May not need instrumentation code (no execution overhead!) if we can prove code safety Slides based on George Necula’s OSDI96 talk

Proof-Carrying Code: An Analogy Legend: code proof oracle bits

Proof-Carrying Code (PCC) untrusted application code certification code producer formal safety policy code consumer native code safety proof PCC binary proof validation CPU

Benefits of PCC Wide range of safety policies Wide range of languages memory safety resource usage guarantees (CPU, locks, etc.) concurrency properties data abstraction boundaries Wide range of languages assembly languages high-level languages Simple, fast, easy-to-trust validation Tamper-proof

Experimentation Goal: Choose simple but practical applications Test feasibility of PCC concept Measure costs (proof size and validation time) Choose simple but practical applications Network Packet Filters IP Checksum Extensions to the TIL run-time system for Standard ML

Experimentation (2) Use DEC Alpha assembly language (hand-optimized for speed) Network Packet Filters BPF safety policy: “The packet is read-only and the scratch memory is read-write. No backward branches. Only aligned memory accesses.”

PCC Implementation (1) Formalize the safety policy: Use first-order predicate logic extended with can_rd(addr) and can_wr(addr) Kernel specifies safety preconditions Calling convention Guaranteed by the kernel to hold on entry Use tagged memory to define access

PCC Implementation (2) Compute a safety predicate for the code Use Floyd-style verification conditions (VCgen) One pass through the code, for example: For each LD r,n[rb] add can_rd(rb+n) For each ST r,n[rb] add can_wr(rb+n) Prove the safety predicate Use a general purpose theorem prover

PCC Implementation (3) Formal proofs are trees: the leaves are axiom instances the internal nodes are inference rule instances at the root is the proved predicate Example:

PCC Implementation (4) Proof Representation: Edinburgh Logical Framework (LF) Proofs encoded as LF expressions Proof Checking is LF type checking simple, fast and easy-to-trust (14 rules) 5 pages of C code independent of the safety policy or application based on well-established results from type-theory and logic Large design space, not yet explored

Packet Filter Experiments 4 assembly language packet filters (hand-optimized for speed): Accepts IP packets (8 instr.) Accepts IP packets for 128.2.206 (15 instr.) IP or ARP between 128.2.206 and 128.2.209 TCP/IP packets for FTP (28 instr.) Compared with: Run-Time Checking: Software Fault Isolation Safe Language: Modula-3 Interpretation: Berkeley Packet Filter

Performance Comparison Off-line packet trace on a DEC Alpha 175MHz PCC packet filters: fastest possible on the architecture The point: Safety without sacrificing performance!

Cost of PCC for Packet Filters Proofs are approx. 3 times larger than the code Validation time: 0.3-1.8ms

Validation Cost (Filter 3) Conclusion: One-time validation cost amortized quickly

PCC for Memory Safety Continuum of choices between static checking and run-time checking: PCC can be also used where run-time checking cannot (e.g., concurrency) Performance Penalty Static Checking Run-Time Checking SFI Proof Complexity

What Can We Check With PCC? Any property with a sound formalization if you can prove it, PCC can check it! One proof-checker for many such policies! Small commitment for open-ended extensibility Example: policies defined by safe interpreters E.g, security monitors, memory safety, resource usage limits, … … can be enforced statically with PCC Prove that all run-time checks would succeed No run-time penalty

PCC Beyond Memory Safety Example: database with access control Safety policy: host invokes the agent with: security clearance level item and price fields readable only if access £ level parent communication permitted only back to parent must terminate within MAX instructions wait BNDW * SIZE instructions before sending SIZE bytes level access item price agent pricing server PCC is not limited to memory safety!

Practical Difficulties Proof generation Similar to program verification But: done off-line can use run-time checks to simplify the proofs In restricted cases it is feasible (even automatable) Proof-size explosion It is exponential in the worst case Not a problem in our experiments

PCC Discussion A very promising framework for ensuring safety of untrusted code Limitations? What about generating proof on the fly? Could verify safety property based on specific arguments Related problem: how to secure mobile code from malicious host?