Lecture 8 - Hardware Security

Question 1

Introduction

Answer 1

• In early 2018, two significant hardware security bugs, Meltdown and Spectre, were reported.
• These bugs affect almost all modern CPUs and are considered a major hardware security issue.

Question 2

Meltdown (Overview)

Answer 2

• Meltdown breaks down the barrier between kernel space and user space in memory.
• It allows arbitrary memory locations to be read from user space, making passwords, encryption keys, and DRM systems vulnerable.
• It exploits out-of-order execution behavior in modern processors.
• Meltdown leverages cache side-channel attacks, specifically Flush+Reload, to determine the values in instructions after an exception.
• The attack involves loading the content of an inaccessible memory location into a register, accessing a cache line based on the secret content, and using Flush+Reload to determine the accessed cache line.

Question 3

Meltdown (Mechanics)

Answer 3

• Out-of-order execution: Instructions are processed but not committed by the Reorder Buffer (ROB) if a branch is incorrectly predicted.
• These uncommitted instructions can have microarchitectural effects, such as filling and emptying cache lines.
• Flush+Reload is used to determine if the cache was loaded, revealing values in instructions after an exception.

Question 4

Steps of Meltdown Attack

Answer 4

1. Load the content of an inaccessible memory location into a register.
2. A transient instruction accesses a cache line based on the secret content of the register.
3. The attacker uses Flush+Reload to determine the accessed cache line and hence the secret.

Question 5

Spectre

Answer 5

• Similar to Meltdown but does not deal with the boundary between protected and non-protected memory.
• It has no good mitigation.
• It involves three stages: Setup, Speculative Execution, and Read.

Question 6

Stages of Spectre Attack

Answer 6

1. Setup: Prime the branch predictor to mispredict.
2. Speculative Execution: Execute transient instructions speculatively.
3. Read: Read the results using a side-channel attack.

Question 7

Spectre Mechanics

Answer 7

• The attacker controls conditional branch misprediction.
• The branch predictor is trained to expect a branch to be not taken.
• Speculative execution occurs, and pointer calculation and access happen.
• The cache is still affected even when the processor state is unwound.
• Flush+Reload is used to measure the location in array2 that was loaded into the cache.