Ironclad Apps: End-to-End Security via Automated Full-System Verification. Danfeng Zhang

Transcription

1 Ironclad Apps: End-to-End Security via Automated Full-System Verification Chris Hawblitzel Jon Howell Jay Lorch Arjun Narayan Bryan Parno Danfeng Zhang Brian Zill

2 Online and Mobile Security Chase Online, the Chase Mobile app and the Chase Mobile website use Secure Socket Layer (SSL) technology We periodically review our operations and business practices to make sure they comply with the corporate policies and procedures we follow to protect confidential information 2

3 An Ironclad app guarantees to remote parties that every instruction it executes adheres to a high-level security spec. My personal data My password will never leak will not be misused 3

4 Ironclad combines: Late launch Trusted Computing Software verification push ebp mov ebp, esp sub esp, 4 mov eax, 8 4

5 Ironclad combines: Late launch Trusted Computing Software verification Secure Remote Equivalence Entire software stack Reasonable effort push ebp mov ebp, esp sub esp, 4 mov eax, 8 5

6 Verification implies: No buffer overflows No code injection No type-safety flaws No information disclosures No crypto implementation flaws We always know what the app will do with private data! push ebp mov ebp, esp sub esp, 4 mov eax, 8 We don t prove: Absence of side channels Liveness Physical security 6

7 Verification goals End-to-end security Complete Low-level Rapid development Non-goal: Verify existing code push ebp mov ebp, esp sub esp, 4 mov eax, 8 Long-term: Performance matches unsafe code 7

8 Verification methodology = Trusted = Untrusted Ironclad... spec translator Highlevel spec Lowlevel spec Verifier Verifiable, high-level implementation mov edx, 2 Ironclad compiler predicate if (edx IsPrime(p:int) >= eax) { { 2 goto <= p loopend; } && forall x :: 2 <= x < p ==> p % x!= 0 } call edx := Mov(2); loop: invariant 2 <= edx < eax; invariant MemInv(...); Verifiable assembly language procedure CheckPrimality(p:int) returns (b:bool) requires p >= 0; ensures b == IsPrime(p); { var divisor := 2; while divisor < p { invariant 2 <= divisor <= p;... Assembler + Linker loop:... cmp edx, eax jae loopend 8

9 Verification methodology: Benefits Highlevel development Rapid spec Verifiable, high-level implementation Ironclad spec translator Simple and declarative Lowlevel spec Ironclad compiler Verifiable assembly language Arbitrarily complex Verifier Low-level verification Assembler + Linker 9

10 Writing trustworthy specifications Idiomatic specification = 3439 pages 1,364 lines of spec ( < 60 instructions ) = 795 pages procedure instr_add(..., x:reg, y:reg) ensures x := (x + y) % 0x ; lines of spec (secure randomness + attestation) type core = core(regs:[int]int, eip:int,..., segments, paging,...); type machine = machine(cores:[int]core, mem:[int]int, io:iostate); Hardware specs 10

11 Writing trustworthy specifications App spec Lib specs predicate ValidTransition(old_state:NotaryState, new_state:notarystate, request:request, response:response,...) { Idiomatic specification Spec reviews match request... case AdvanceCounter => function SHA256(messageBits:seq<int>) response.advancecntrresponse? : seq<int> requires && new_state.ctr messagebits == < old_state.ctr power2(64); + 1 requires && response.sig IsBitSeq(messageBits); == RSA_Sign(old_state.ctr, request) { &&... }... OS specs Hardware specs 11

12 Architecture App spec App Std. Lib App Common Lib specs UDP/IP Datatypes RSA Ethernet SHA-256 BigNum Net Driver TPM Driver Core Math OS specs Late launch Segments IOMMU Device IO GC Verve++ Hardware specs 12

13 Challenge: Whole-system verification procedure CheckPrimality(p:int) returns (b:bool) requires p >= 0; ensures b == IsPrime(p); { var divisor := 2; while divisor < p invariant 2 <= divisor <= p; {... Functional verification (correctness) F( all ) possible inputs all permitted output words procedure instr_outb(..., x:reg) requires???? push ebp mov ebp, esp sub esp, 4 mov eax, 8 13

14 Solution: Relational verification procedure instr_inb(..., x:reg) ensures public(x); procedure instr_outb(..., x:reg) requires public(x); push ebp mov ebp, esp sub esp, 4 mov eax, 8 Declassifier Declassify X by proving the abstract app would have output X 14

15 Rapid verification Automated tools Modular verification Shared verification IronBuild 15

16 Ironclad Apps Password Protector Notary password abc123 monkey qwerty letmein dragon baseball iloveyou trustno1 Trusted Incrementer Differentially Private DB Insert datum Query Key pair Database Privacy budget 16

17 Lessons learned Automated Automatic Opaque attributes Non-recursive functions Addition & subtraction Mul/div/mod by small constants Forall/exists Arrays/seqs Recursive functions General mul/div/mod Verification works! App Std. Lib App Common How to write concise UDP/IP specs Datatypes RSA How to write Ethernet libraries SHA-256 BigNum Net Driver TPM Driver Core Math Benefits of refinement types Late Device Segments IOMMU GC launch IO 1 st Version: Secure, but non-functional Custom math library 17

18 Eval: Proof burden ~3 person-years Previously 22+ pys Apps Crypto (SHA, HMAC, RSA) BigInt Lib Math Lib Std. Lib (bytes, words, arrays) UDP/IP/Ethernet Network Driver TPM Driver OS Proof hints : Implementation LoC Average 4.8 : 1 Previously > 25 : Ratio 18

19 Eval: System size Trusted Spec Implementation Apps Crypto (SHA, HMAC, RSA) BigInt Lib Math Lib Std. Lib (bytes, words, arrays) UDP/IP/Ethernet Network Driver TPM Driver OS Software Hardware 1796 LoC 1750 LoC SW Impl : Spec = 3.9 : LoC 41,566 instructions 23.1 : Lines 19

20 Eval: Performance SHA-256 RSA private (1024) ns/b Cut by 84% Within 30% of OpenSSL ms Improved by 8300x Still 22x too slow

21 Related work Early security kernels Examples: KVM/370, VAX VMM, SCOMP, GEMSOS Formally specified, but no connection to implementation Recent verified systems Examples: sel4, VCC, PROSPER, CompCert, Jitk Focus on one layer Many verify C code => Good performance Typically less automation => More human proof burden 21

22 Conclusions Ironclad guarantees end-to-end security to remote parties: Every instruction meets the app s security spec Achieved via: New and modified tools A methodology for rapid verification of systems software Verification of systems code is quite feasible! Thank you! ironclad@microsoft.com 22