CVE-2026-23354 Overview
A speculative execution safety vulnerability has been identified in the Linux kernel's x86/FRED (Flexible Return and Event Delivery) implementation. The vulnerability exists in the fred_extint() function where array_index_nospec() fails to provide adequate protection against speculative side-channel attacks when its result is spilled to the stack, making the believed safe-under-speculation value subject to memory predictions.
Critical Impact
This vulnerability could potentially allow attackers to bypass speculative execution mitigations in the Linux kernel's interrupt handling mechanism, potentially leading to information disclosure through side-channel attacks.
Affected Products
- Linux kernel versions with x86/FRED support
- x86 systems utilizing FRED (Flexible Return and Event Delivery) interrupt handling
Discovery Timeline
- 2026-03-25 - CVE CVE-2026-23354 published to NVD
- 2026-03-25 - Last updated in NVD database
Technical Details for CVE-2026-23354
Vulnerability Analysis
The vulnerability exists in the fred_extint() function within the Linux kernel's x86/FRED subsystem. The FRED (Flexible Return and Event Delivery) mechanism is designed to provide more efficient interrupt and event handling on modern x86 processors. The core issue stems from the improper placement of the array_index_nospec() macro, which is intended to prevent speculative execution attacks by sanitizing array indices.
When array_index_nospec() is called, its result is stored in a local variable (index) which gets spilled to the stack—specifically placed into %ebp across a function call to irqentry_enter(). This stack spilling defeats the purpose of the speculative safety mechanism because the CPU's memory prediction can speculatively load incorrect values from the stack, bypassing the bounds checking that array_index_nospec() was supposed to enforce.
Root Cause
The root cause is the separation between where array_index_nospec() is called and where the array access actually occurs. The intermediate storage of the index value (both in the index variable and in %ebp across the function call) creates a window where speculative execution can operate on potentially unsafe values.
For array_index_nospec() to be effective, it must be used directly in the expression that performs the array access, eliminating any opportunity for the sanitized value to be subject to memory predictions. The original code violated this principle by computing the index too early in the execution flow.
Attack Vector
An attacker could potentially exploit this vulnerability by targeting the speculative execution window created by the improper index handling. Since the index value is spilled to the stack before the array access, an attacker with the ability to influence memory predictions could potentially cause speculative out-of-bounds array accesses.
The vulnerability occurs during external interrupt handling via FRED, making it theoretically exploitable in scenarios where an attacker can trigger external interrupts while simultaneously influencing the kernel's memory prediction state. This could potentially lead to information disclosure through speculative side-channel techniques similar to Spectre-variant attacks.
Detection Methods for CVE-2026-23354
Indicators of Compromise
- Unusual interrupt handling patterns in x86/FRED enabled systems
- Anomalous memory access patterns during external interrupt processing
- Evidence of side-channel attack attempts targeting kernel memory
Detection Strategies
- Monitor kernel logs for unusual interrupt handling behavior on FRED-enabled systems
- Deploy kernel runtime integrity checking mechanisms to detect speculative execution anomalies
- Utilize hardware performance counters to identify potential side-channel attack patterns
- Review system configurations to identify vulnerable kernel versions
Monitoring Recommendations
- Enable kernel auditing for interrupt-related system calls
- Implement continuous monitoring of kernel updates and patch status
- Deploy endpoint detection and response (EDR) solutions capable of detecting kernel-level anomalies
- Monitor for unusual patterns in interrupt frequency and handling
How to Mitigate CVE-2026-23354
Immediate Actions Required
- Update the Linux kernel to a patched version that includes the fix
- Review and audit systems using x86/FRED functionality for potential exposure
- Prioritize patching for systems handling sensitive data or in high-security environments
- Monitor official Linux kernel security advisories for additional guidance
Patch Information
The Linux kernel development team has released patches to address this vulnerability. The fix removes the intermediate index variable and repositions array_index_nospec() to be calculated immediately before the array access, ensuring the speculative safety mechanism operates correctly.
Multiple commit fixes are available:
- Kernel Git Commit 3bc5887
- Kernel Git Commit 92caa52
- Kernel Git Commit aa280a0
- Kernel Git Commit e58f1a9
Workarounds
- Consider disabling FRED functionality if not required and reverting to legacy interrupt handling mechanisms
- Implement additional kernel hardening measures to reduce the attack surface
- Apply kernel lockdown mode where appropriate to limit potential exploitation vectors
- Monitor systems closely until patches can be applied
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
