Guardat: Enforcing data policies at the storage layer

Anjo Vahldiek-Oberwagner1, Eslam Elnikety1, Aastha Mehta1, Deepak Garg1, Peter Druschel1,

Rodrigo Rodrigues2, Johannes Gehrke3,4, Ansley Post5

1MPI-SWS, 2NOVA LINCS/Nova University of Lisbon, 3Microsoft, 4Cornell, 5Google

2

Web attacks and leaks

3

httpd.conf .htaccess

Web Server

Content

Web Server State:

Operating SystemFile System users groups

Threat 1: Unauthorized Access

Clients Web Server Host

App

Unauthorized access due to configuration errors, bugs, attacks.

4

Web Server

App

Threat 2: State Corruption

Clients

Content

Web Server State:

Web Server Host

Operating SystemFile System

.htaccess

users groups

State corruption due to configuration errors, bugs, attacks.

httpd.conf

5

Threat 3: Trojans & log manipulation

Clients Web Server Host

Web Server

App

Logs Executables

Web Server State:

Operating SystemFile System

Trojans & log manipulation due to attacks.

6

Web Server

Content

Web Server State:

Operating SystemFile System

Threat 4: Unaccounted Provider Access

Clients Web Server Host

App

Providers bypass application access control protections.

Provider employee

7

• Confidentiality and integrity depend on large, fast evolving code base.

• Access control lists and access checks spread across system.

• Recovering corrupt persistent state is difficult.

Why is mitigation difficult?

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Per file policyFile attestation

GuardatStorageLayer

Stakeholder: User, provider, developer, privacy officer

Guardat: Storage Layer Compliance

Trusted Controller

Application

OSFile System

UNTR

USTE

D

9

Outline

Declarative Policies

Enforcement File Attestation

GuardatTransaction

Implementation & Evaluation

Design Principles

1

2

10

Policy language in a nutshell

Goals::• High level policy• Concise

• Easy to audit • Efficient enforcement

permission:- Boolean expression over predicates

read :- When to read a fileupdate :- When to update the filesetPolicy :- When to change policydestroy :- When to reuse the name

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Protecting files from unauthorized access

read :- sessionIs(Alice)update :- …setPolicy :- sessionIs(Alice)destroy :- sessionIs(Alice)

Assumption: Integrity of Alice’s key is maintainedGuarantee: Protected files may only be read with an authenticated

session.

Threat: Unauthorized access due to configuration error, bug or attack.

Private files:

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Protecting files from corruption

read :- …update :- sessionIs(Alice)setPolicy :- …destroy :- …

Assumption: Integrity of Alice’s key is maintained.Guarantee: Protected files may only be updated within an authenticated

session.

Threat: State corruption due to configuration error, bug or attack.

Private files:

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Protecting executables

read :- TRUEupdate :- fileNameIs(F) fileNewLenIs(L) ∧ (0, L) willHaveHash Nh ∧ keyIs(K, “Vendor”) ∧ ∧ K signs okHash(F, N, Nh) setPolicy:- FALSE

Assumption: Integrity of the vendor’s key is maintained.Guarantee: Protected files cannot be overwritten except with content signed by the vendor.

Threat: Attack installing a trojan.

Signed updates-only executables:

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Protecting log files from manipulation

read :- TRUEupdate :- [ fileCurrLenIs(Lc) fileNewLenIs(Ln) Ln ≥ Lc ∧ ∧ txUpdatedExAre(M)∧ listsAreDisjoint(M, [0, Lc])]∧ [sessionIs(Admin)]∨setPolicy :- FALSEdestroy :- FALSE

Assumption: Administrator key integrityGuarantee: Protected files cannot be overwritten, only appended.

Threat: Attack manipulating logs. Append-only log files:

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Protecting content from unaccounted provider access

read: - ((“profil”, SEQCNTLOC, _) says seqcnt(currseq)) ^ ((“acclog”, _, _) says read(currseq, _, offset, length)) ^ AccessOffIs(offset) ^ AccessLenIs(length)update:- ((“profil”, SEQCNTLOC, _) says seqcnt(currseq)) ^ ((“profil”, SEQCNTLOC, _) willSay seqcnt(nextseq)) ^ EQ(currseq + 1, nextseq) ^ ((“acclog”, gennb, _, _) says write(nextseq, _, newhash, offsetlist)) ^ ((offsetlist) willHaveHash newhash) ^ TxUpdateLocAre(offsetlist)setpolicy: FALSEdestroy: FALSE

Read :- TRUEupdate: [ FileCurrExAre(oe) ̂ FileNewExAre(ne) ̂ isPrefix(oe, ne) ̂ ((“acclog”, neo, _) willSay _(nseq, _. _. _)) ̂ ((“acclog”, neo - READENTRYLENGTH, _) says _(nseq-1, _, _, _)) ̂ FileCurrLenIs(currlen) ̂ LT(currlen, neo)] [FileCurrExAre(oe) ̂ FileNewExAre(ne) ̂ ∨ isPrefix(oe, ne) ̂ ((“acclog”, neo, _) willSays _(nseq, _. _. _)) ̂ ((“acclog”, gennb, neo - WRITEENTRYLENGTH, _) says _(nseq-1, _, _, _)) ̂ FileCurrLenIs(currlen) ̂ LT(currlen, neo) ] ∨ [ ((“acclog”, neo, nel) willSay write(nseq, _, _, _)) ̂ ((“acclog”,,neo, nel) says write(nseq, _, _, _) ̂ ((“profil”, gennb, SEQCNTLOC, _) says seqcnt(currseq)) ̂ LT(currseq, nseq) ̂ TxUpdateLocAre((neo, nel))]setpolicy:- FALSEdestroy:- FALSE

Assumption: Auditor key integrityGuarantee: Protected files cannot be accessed without corresponding log entry in the access log.

Threat: Provider accesses are not accounted for.

Private files accounting for provider access:

Log file with log entry check and append-only:

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Outline

Declarative Policies

Enforcement File Attestation

GuardatTransaction

Implementation & Evaluation

Design Principles

1

2

17

Storage Layer Enforcement

Web Server

Host

App

Operating SystemFile System

VMM

Library

OS abstractionFile System

Virtual device

Storage controller Decrease risk of circumvention

Storage Layer

DiskDiskDisk

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Storage Layer Enforcement

Web Server

Host

App

Operating SystemFile System

VMM

Physically protected Machine Room

NetworkUN

TRUS

TED

UNTR

USTE

D

Storage Area Network Server

DiskDiskDiskDiskDiskDisk

19

Outline

Declarative Policies

Enforcement File Attestation

GuardatTransaction

Implementation & Evaluation

Design Principles

1

2

20

Per file policy

GuardatStorageLayer

Trusted Controller

Data Disk

Attests:• File name• Policy• State• Content

NVM

Application

OSFile SystemUN

TRUS

TED

File Attestation: Bridging gap between file and block level enforcement

Stakeholder: User, provider, developer, policy officer

Metadata:

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Outline

Declarative Policies

Enforcement File Attestation

GuardatTransaction

Implementation & Evaluation

Design Principles

1

2

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• Atomic update of file• Bundle multiple accesses• Delays evaluation to commit operation• Transaction caches

Introducing Guardat Transactions

1

2

23

GuardatDevice

Trusted ControllerTransaction cache

Guardat Transaction by Example

Application

OSFile System

Metadata:

UNTR

USTE

D

N#

N#

N#

Shifting burden of proving complex policy compliance to untrusted code keeping policies concise and policy evaluation efficient.

Demonstrate policy compliance:1. Download binary + certificate2. Begin transaction tx (+ certificate)3. Write new binary4. Commit transaction tx`

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Outline

Declarative Policies

Enforcement File Attestation

GuardatTransaction

Implementation & Evaluation

Design Principles

1

2

25

ImplementationAlternatives:1. Hybrid Disk/RAID controller2. Microcontroller on SCSI/SATA adapter 3. VMM4. Storage area Network (SAN)Trusted controller in iSCSI Enterprise Target (IET) server

Guardat IETserver

Trusted Controller(~ 20,000 LoC)

Metadata(SSD)

Data Disk(HDD/SSD)

Network

SSD Throughput overhead: < 2%• 3.8 Million files• 40,000 policies

Seq. read Seq. write0

100200300400500600

iSCSIGuardat

Access type

Throughput in MB/S

Better

512B Access LatencyHDD latency overhead: < 1%

SSD latency overhead

Random Sequential Random Sequential0

0.10.20.3

iSCSIGuardat

Random Sequential Random Sequential0.1

1

10

iSCSIGuardat

Latency in ms (log)

Latency in ms

Reads Writes

Reads Writes

Better

Better

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Protection:• Append-only logs• Vendor-only update

executables• Owner-only update

to content pages

Scenario:• Modified Apache (added 51 Lines of code)• Hosting 220 GB English Wikipedia (~15M files)• Replay an hourly access distribution of Wikipedia

Protected Apache Web server

600

50100150200250

iSCSIGuardat

Concurrent HTTP Clients

Requests/s

2% overhead at peak throughput.

Better

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Conclusions• Guardat guarantees confidentiality and integrity of

persistent data and state of a system.

• No need to trust higher software layers.

• Guardat protects computer systems from unauthorized access, trojans and log manipulations.

• Efficient prototype implementation in SAN server.

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Anjo Vahldiek-Oberwagnervahldiek@mpi-sws.org

Guardat: Enforcing data policies at the storage layer

Eslam Elnikety

Aastha Mehta

Peter Druschel

Rodrigo Rodrigues

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Backup slides

Backup slides

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Guardat: Related work• TCG storage work group spec [2012]

– Architecture for sessions, access control policies– Concrete design, evaluation left to vendors– No object attestation

• TC: Semantic attestation [Haldar 2004], Excalibur [Santos 2012], Pasture [Kotla 2012]

• Integrity/confidentiality: self-encrypting disks, capability NAS [Aguilera 2003], type-safe disks [Sivathanu 2006], Venti [Quinlan 2002], S4 [Strunk 2000], NetApp SnapVault, PCFS[Garg 2010], PFS[Walsh 2012]

• Extended disk functionality: hybrid disks, object-based storage [Mesnier 2003], active disks [Riedel 2001], semantically smart disks [Sivathanu 2003], differentiated storage [Mesnier 2011]

• VMM/OS data protection: Overshadow [Chen 2008], InkTag [Hofmann 2013], Nexus [Sirer 2011], DCAC [Xu 2014]

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Related work: Trusted Computing

Mostly complementary; can be combined, e.g.,• Remotely attested external verifier• Tamper-resident persistent storage

Property Trusted computing Guardat

Root of trust TPM TGCAuthenticates Motherboard Guardat device

Certifies HW/SW configuration(remote attestation)

Object state + policy(object attestation)

Storage propertyprovided

Confidentiality +Read integrity

Confidentiality + Write integrity

Properties expressed in Trusted software Policy languageTCB TPM + trusted SW TGC (narrow API)Persistent secure state NVRAM Entire storage device

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Threats to persistent data

• Storage media failure (deterioration, obsolescence)• Natural disasters and physical attack• Operator error and negligence• Software/hardware bugs• Cyber attack and sabotage

Problem is real: Among most frequent causes of loss• Human error is (close) second to device failure• Software errors + viruses third

This talk

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Discussion: Enforcement layer

36

Legacy FS

37

Transaction API

38

Stateful policies