Hardware Assisted Control Flow Obfuscation for Embedded

ProcessorsXiaoton Zhuang, Tao Zhang, Hsien-Hsin S. Lee, Santosh

PandeHIDE: An Infrastructure for

Efficiently Protecting Information Leakage on the Address Bus

Xiaoton Zhuang, Tao Zhang, Santosh Pande

Overview

• Software Obfuscation• Obfuscate - v - render obscure, unclear, or

unintelligible - bewilder (someone)

• Information Leakage• Layout leakage• Recurrence leakage

• Hardware Obfuscation Techniques

Assumptions

• XOM model • Everything outside the processor chip is

assumed to be insecure

• Memory contents are encrypted

Software Obfuscation(and why it doesn’t work)

• Lacks of theoretical foundation• It has been proven the perfect

obfuscation does not exist• May incur large overheads in code size• Performance may be penalized due to

carrying out extra computations• History has proven it inefficient

How is Software Obfuscation Vulnerable to Attack ?

• Layout Leakage• Spatial vicinity

• Recurrence Leakage• Recurring addresses

Layout Leakage100

101

102 103

104

Recurrence Leakage100

101

102 103

104

So What? It’s just Control Flow.

• Control flow info is the essential part of algorithms

• Competing company ex.

• Can help identify reuse code

• Control obfuscation techniques are well known and can be reversed

Hardware Obfuscation Overview (paper 1)

• Encrypt the Address Bus (layout leakage)

• Relocate blocks every time they are written out to memory (recurrence leakage)

Address Bus Encryption

Equates to a fixed mapping

Shuffle Buffer

• Designed to reorder all writes to memory• Exclusive to external memory

Shuffle Buffer• Indexed array through

the block address table• No address tag• Smaller size / cheaper

• Blocks can be stored anywhere

• Blocks can be randomly replaced (circuit white noise)

• Assume program binary updatable then multi-run recurrence prevented

Block Address Table (BAT) & Cache

• Records the current location of blocks• Use original block address to index into

BAT to get new address• Worst case scenario 10% overhead in

virtual memory space• Each access request from cache

checks with BAT use BAT cache to speed things up

How Secure Is This?

• With a shuffle buffer of 128 blocks 0.8% chance of guessing one recurrence correctly

• For n-recurrences the chance of guessing all correctly is 1/(M^n) where M is the size of the shuffle buffer

Performance/Cost Summary

• Performance degradation can be below 1%

• Hardware costs consist of small on chip shuffle buffer and BAT cache

HIDE(Hardware-support for Leakage-Immune

Dynamic Execution)

• Basic idea is to break the correlation between repeated memory addresses

• Achieved by permuting the address space at suitable intervals during execution

Hide Cache

• A cache same as a normal cache except that that blocks fetched after the previous permutation are all locked

• A locked block cannot be replaced until the memory space they belong to is permuted again

How The Hide Cache Works

Other Details

• When evicting a block choose the least recently used block among the unlocked blocks

• A separately stored bitmap is used to record whether a block is locked or not

Hardware Flowgraph

HIDE at Chunk Level• Chunk - one or more pages that are protected and

permuted together• Designed to limit size of permutation

• Large memory permutations = performance cost• At chunk level the permutation unit only permutes all the blocks

within a chunk

• With the smallest chunk size (a page) 75% of transition from one address to the next are intra-chunk

• Chunks can be specified in the code or at runtime with instructions inserted into the header of the binary code

Page Info Cache

• Stores the Page Info Record to speed up access

How Secure Is this?

• With 64K chunk protection and layout optimizations, 87% of address sequence is protected, in which 95% of the accesses to code and static data are hidden

• Interfaces are provided for the compiler or the user to increase the security to achieve almost complete protection

Performance/Cost Summary

• The performance overhead in their experiments was at most 1.5% mainly due to permutations

• Most on chip components are small

References

• Xiaotong Zhuang, Tao Zhang, Hsien-Hsin Lee and Santosh Pande. Hardware Assisted Control Flow Obfuscation for Embedded Processors. CASES, Washington DC, Sept. 2004.

• Zhuang, X., Zhang, T. and Pande, S. HIDE: An Infrastructure for Efficiently Protecting Information Leakage on the Address Bus. International Conference on Architectural Support for Programming Languages and Operating Systems, Boston, MA., Oct 2004.