[PPT] - Presence of Fault Attacks Robert Schilling 1,2 , Mario Werner 1 , PowerPoint Presentation

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Robert Schilling1,2, Mario Werner1, Stefan Mangard1

1Graz University of Technology, 2Know-Center GmbH

March 22, 2018

Securing Conditional Branches in the Presence of Fault Attacks

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Introduction to control-flow integrity and data protection
Generic approach to protect conditional branches without

hardware extensions

Protected comparison algorithms based on AN-codes
Prototype compiler based on LLVM
Evaluation

Overview

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Fault attacks can modify the code and data
Control-flow integrity (CFI) restricts the control-flow

to valid execution traces

Data encoding to protect arithmetic
No protection for conditional branches
Conditional branches are critical instructions
Password checks, signature verification depend on

conditional branches

Preferred target for fault attacks

Motivation

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PW check Continue Enter System Raise Alarm

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Different CFI granularities
Program counter dependent state 𝑇𝑗
Depends on the previous state
Depends on currently executed instruction

Introduction to Control-Flow Integrity (CFI)

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 Instruction granularity

𝐽1 𝐽2 𝐽3 𝐽4 𝐽5 𝐽6 𝐽7 𝐽8 𝐽9

𝑇1 𝑇2 𝑇3 𝑇4 𝑇5 𝑇6 𝑇7 𝑇8 𝑇9

Conditional branches are not protected by means of CFI

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Arithmetic codes defined by: 𝑦𝑑= 𝐵 ∙ 𝑦
All code words are multiples of the encoding constant 𝐵
AN-code congruence: 0 ≡ 𝑦𝑑 𝑛𝑝𝑒 𝐵
Support different arithmetic operations
+, -, *, /
Closed under addition/subtraction
Adding two AN-code words results in another valid AN-code word
𝑨𝑑 = 𝑦𝑑 + 𝑧𝑑 = 𝐵 ∙ 𝑦 + 𝐵 ∙ 𝑧 = 𝐵 ∙ (𝑦 + 𝑧)

A Primer to AN-Codes

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1. First operation: Comparison
Takes two inputs x, y and comparison predicate P
Returns 1-bit signal if the comparison is true or false
2. Second operation: Branch
Determines how to update the program counter (PC1,PC2)

What is a Conditional Branch

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CFI introduces a program counter dependent state S
State is different if branch is taken or not
Decision if the branch is taken still relies on a 1-bit signal

Conditional Branch with CFI

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Multiple attack vectors to bypass conditional branches

1. Faulting the operands 2. Faulting the comparison 3. Faulting the branch

Generic Protected Conditional Branches

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Multiple attack vectors to bypass conditional branches

1. Faulting the operands  Add redundancy to x and y (AN-codes) 2. Faulting the comparison 3. Faulting the branch

Generic Protected Conditional Branches

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Multiple attack vectors to bypass conditional branches

1. Faulting the operands  Add redundancy to x and y (AN-codes) 2. Faulting the comparison  Encoded comparison in software 3. Faulting the branch

Generic Protected Conditional Branches

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Multiple attack vectors to bypass conditional branches

1. Faulting the operands  Add redundancy to x and y (AN-codes) 2. Faulting the comparison  Encoded comparison in software 3. Faulting the branch  Link the redundant condition value with the CFI state

Generic Protected Conditional Branches

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1. Compute the encoded compare
2. Perform a standard conditional

branch

3. Link the redundant condition value

with the CFI state

Example: Protected Conditional Branch

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𝐽1 𝐽2 𝑐𝑠 𝑑𝑝𝑜𝑒 = 𝑈𝑠𝑣𝑓 𝑉𝑞𝑒𝑏𝑢𝑓(𝑑𝑝𝑜𝑒) 𝐽6 𝐽7 𝑉𝑞𝑒𝑏𝑢𝑓(𝑑𝑝𝑜𝑒) 𝐽8 𝐽9

𝑇1 𝑇2 𝑇3 𝑇6

∗

𝑇6 𝑇7 𝑇8

∗

𝑇8 𝑇9

𝑑𝑝𝑜𝑒 = EncCmp

𝑇4

Wrong branch and wrong condition lead to invalid CFI state

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Problem:
𝑑𝑝𝑜𝑒𝑗𝑢𝑗𝑝𝑜 ← 𝐹𝑜𝑑𝑝𝑒𝑓𝑒𝐷𝑝𝑛𝑞𝑏𝑠𝑓 𝑄, 𝑦𝑑, 𝑧𝑑 𝑥𝑗𝑢ℎ

𝑑𝑝𝑜𝑒𝑗𝑢𝑗𝑝𝑜 ∈ 𝐷1, 𝐷2 𝑏𝑜𝑒 𝐼𝑏𝑛𝑛𝑗𝑜𝑕 𝐸𝑗𝑡𝑢𝑏𝑜𝑑𝑓 ≥ 𝐸

Find an algorithm for all comparison predicates: <, ≤, >, ≥, =, ≠
How to compute 𝑦𝑑< 𝑧𝑑 ?

Protected Comparisons with AN-Codes

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Step 1: Subtract 𝑦𝑑 − 𝑧𝑑
𝑦𝑑 − 𝑧𝑑 ቊpositve if 𝑦𝑑 ≥ 𝑧𝑑

negative if 𝑦𝑑 < 𝑧𝑑

Sign bit determines the comparison  No redundancy
Returns a valid AN-code word because AN-codes are closed under

subtractions

AN-code congruence true
How to map the sign bit to a redundant condition value?

Protected < Comparison with AN-Codes

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Step 2: Condition mapping
𝑦𝑑 − 𝑧𝑑 ቊpositve if 𝑦𝑑 ≥ 𝑧𝑑

negative if 𝑦𝑑 < 𝑧𝑑

Map the difference to redundant condition values
Trick: Cast difference to unsigned

Protected < Comparison with AN-Codes

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Step 2: Condition mapping
𝑦𝑑 − 𝑧𝑑 ቊpositve if 𝑦𝑑 ≥ 𝑧𝑑

negative if 𝑦𝑑 < 𝑧𝑑

AN-code congruence still true
0 ≡ 𝑦𝑑 − 𝑧𝑑 𝑣 𝑛𝑝𝑒 𝐵
No change for positive differences due to the cast

Protected < Comparison with AN-Codes

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Step 2: Condition mapping
𝑦𝑑 − 𝑧𝑑 ቊpositve if 𝑦𝑑 ≥ 𝑧𝑑

negative if 𝑦𝑑 < 𝑧𝑑

(𝑦𝑑 − 𝑧𝑑)𝑣= 232 + 𝑦𝑑 − 𝑧𝑑
AN-code congruence not true anymore
(𝑦𝑑 − 𝑧𝑑)𝑣 𝑛𝑝𝑒 𝐵

Protected < Comparison with AN-Codes

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ቐ

cancels out

= 232 𝑛𝑝𝑒 𝐵 = 232 + 𝐵 ∙ (𝑦 − 𝑧) = (232+𝐵 ∙ 𝑦 − 𝑧 ) 𝑛𝑝𝑒 𝐵

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Condition mapping
(𝑦𝑑 − 𝑧𝑑)𝑣 𝑛𝑝𝑒 𝐵 ቊ0

if 𝑦𝑑 ≥ 𝑧𝑑 232 𝑛𝑝𝑒 𝐵 if 𝑦𝑑 < 𝑧𝑑

To avoid a zero condition value add a constant 𝐷

Protected < Comparison with AN-Codes

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Condition mapping
(𝑦𝑑 − 𝑧𝑑 + C)𝑣 𝑛𝑝𝑒 𝐵 ቊ

C if 𝑦𝑑 ≥ 𝑧𝑑 𝐷 + 232 𝑛𝑝𝑒 𝐵 if 𝑦𝑑 < 𝑧𝑑

To avoid a zero condition value add a constant 𝐷
Final algorithm:

Protected < Comparison with AN-Codes

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Applicable to ≤, >, ≥ by
Swapping the operands of the first subtraction
Swapping the true and false constants
=/≠ equal comparison assembled using ≤ and ≥

Protected Comparisons with AN-Codes

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Annotate functions using attribute protect_branches
Transformation operates on LLVM IR and is target independent

1. Searches conditional branches 2. Slice operands 3. Transform all dependent operations into the AN-code domain 4. Insert protected comparison algorithm

Backend links comparison with CFI mechanism

LLVM Compiler Prototype

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LLVM Compiler Prototype

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Frontend Source Code Binary

Back End

Middle End

IR IR

IR Optimizers Instruction Selection Code Emission Instruction Scheduling

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LLVM Compiler Prototype

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Frontend Source Code Binary

Back End

Middle End

IR IR

IR Optimizers Instruction Selection AN Coder Lower Switch Lower Select Loop Decoupler

CFI Instrumentation

Instruction Scheduling Code Emission

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ARMv7-M instruction set simulator
Software-centered CFI scheme
State updates via store to the memory-

mapped CFI unit

AN-code with 6-bit Hamming distance
Compare with duplication (5 times)
Benchmarks: integer comparison,

memcmp, bootloader

Evaluation Setting

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CMP CMP CMP CMP CMP Error Error

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