KASLR Exploited: Breaking macOS Apple Silicon Kernel Hardening Techniques

Security researchers from Korea University have unveiled a new vulnerability in macOS systems running on Apple Silicon processors. 

Dubbed “SysBumps,” this attack successfully circumvents Kernel Address Space Layout Randomization (KASLR), a critical security mechanism designed to protect kernel memory from exploitation. 

The findings, presented at the 2024 ACM SIGSAC Conference on Computer and Communications Security (CCS ’24), expose significant weaknesses in Apple’s advanced kernel isolation techniques.

What is KASLR?

KASLR is a kernel hardening technique that randomizes memory addresses during system boot to prevent attackers from predicting the location of key kernel structures. 

This randomness is essential for mitigating memory corruption vulnerabilities, as it forces attackers to guess the kernel’s base address—a task made exponentially harder by high entropy levels. 

Apple has further reinforced KASLR on macOS for Apple Silicon by implementing “double map” kernel isolation, which separates user-space and kernel-space address layouts.

However, the SysBumps attack demonstrates that even these advanced defenses can be bypassed, with researchers achieving a 96.28% success rate across various M-series processors, including the M1, M2, and their Pro and Max variants.

How SysBumps Works

SysBumps exploits speculative execution vulnerabilities in macOS system calls. Speculative execution is a performance optimization technique in modern processors that predicts and executes instructions ahead of time. 

While beneficial for speed, it has been shown to leave traces in microarchitectural components like the Translation Lookaside Buffer (TLB), which attackers can exploit as side channels.

The attack unfolds in three key steps:

Triggering Speculative Execution: Certain macOS system calls perform validation checks on user-supplied arguments. By deliberately mistraining branch predictors, SysBumps induces speculative execution of invalid inputs. 

This transient execution accesses kernel addresses, leaving detectable traces in the TLB if the address is valid.

TLB Side-Channel Analysis: Using a reverse-engineered understanding of Apple Silicon’s TLB architecture, attackers employ a “prime+probe” technique to monitor TLB state changes. By measuring access latency, they can distinguish between valid and invalid kernel addresses.

Breaking KASLR: By systematically probing memory regions, SysBumps identifies valid kernel address ranges and calculates the kernel’s base address with high accuracy.

Apple’s double map kernel isolation was designed to prevent such attacks by ensuring that kernel addresses are inaccessible from user space. 

However, SysBumps bypasses this barrier by exploiting speculative execution during system calls. 

Researchers reverse-engineered the TLB architecture of Apple’s M-series processors using Performance Monitoring Units (PMUs), uncovering critical details such as its shared design between user and kernel processes. 

This knowledge enabled them to construct an attack primitive capable of distinguishing valid from invalid kernel addresses.

The SysBumps attack undermines macOS’s core defense against memory corruption exploits by exposing the randomized layout of the kernel. 

With an average execution time of just three seconds, the attack is both practical and efficient for real-world scenarios. 

The implications are severe: once KASLR is broken, attackers can more easily exploit other vulnerabilities to gain unauthorized access or execute arbitrary code.

Mitigations and Industry Response

The researchers responsibly disclosed their findings to Apple in April 2024.

Apple has acknowledged the vulnerability (tracked as CVE-2024-54531) and is investigating mitigation strategies. The study proposes several countermeasures:

Partitioning TLBs: Separating TLB entries for user and kernel processes could eliminate shared contention, reducing side-channel leakage.

Speculative Execution Fencing: Inserting serializing instructions like DSB and ISB before conditional branches can prevent speculative execution of sensitive code paths.

TLB Behavior Modification: Allocating TLB entries for invalid addresses would make it harder for attackers to distinguish valid from invalid addresses.

As Apple continues its transition to ARM-based silicon, addressing vulnerabilities like this will be critical to maintaining user trust and system security.

In light of these findings, macOS users are urged to keep their systems updated with the latest security patches as they become available. 

While no immediate fix exists yet, Apple’s response will likely shape future defenses against speculative execution attacks on custom silicon platforms.

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