CVE-2024-44948 in Linux
Summary
by MITRE • 09/04/2024
In the Linux kernel, the following vulnerability has been resolved:
x86/mtrr: Check if fixed MTRRs exist before saving them
MTRRs have an obsolete fixed variant for fine grained caching control of the 640K-1MB region that uses separate MSRs. This fixed variant has a separate capability bit in the MTRR capability MSR.
So far all x86 CPUs which support MTRR have this separate bit set, so it went unnoticed that mtrr_save_state() does not check the capability bit before accessing the fixed MTRR MSRs.
Though on a CPU that does not support the fixed MTRR capability this results in a #GP. The #GP itself is harmless because the RDMSR fault is handled gracefully, but results in a WARN_ON().
Add the missing capability check to prevent this.
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Analysis
by VulDB Data Team • 10/10/2025
The vulnerability described in CVE-2024-44948 resides within the Linux kernel's x86 memory type range (MTRR) subsystem, specifically affecting how the kernel handles fixed MTRR (Memory Type Range Register) configuration during system state saving operations. This issue demonstrates a critical flaw in kernel-level hardware abstraction where the system fails to properly validate hardware capabilities before attempting to access specific machine-specific registers. The MTRR mechanism provides fine-grained control over memory caching behavior, particularly for the 640K-1MB region which is crucial for system stability and performance. The fixed MTRR variant represents an older hardware feature designed for precise caching control in this specific memory range, utilizing dedicated model-specific registers that require explicit capability verification before access.
The technical flaw manifests in the mtrr_save_state() function which is responsible for preserving MTRR configuration during system transitions such as suspend/resume operations or system shutdown. This function attempts to read fixed MTRR MSRs without first consulting the MTRR capability MSR to determine whether the fixed MTRR functionality is actually supported by the processor. The MTRR capability MSR contains a specific bit that indicates whether the processor supports the fixed MTRR variant, and this bit should be checked before attempting to access the fixed MTRR registers. When a processor lacks fixed MTRR capability but the kernel still attempts to read these registers, a general protection fault (#GP) occurs, which according to the x86 architecture specification triggers a system-level exception that terminates the current operation.
The operational impact of this vulnerability, while not immediately catastrophic, creates significant reliability issues in kernel operation and system diagnostics. The #GP exception is handled gracefully by the kernel's fault handling mechanisms, preventing system crashes or lockups, but triggers a WARN_ON() message that indicates a potential configuration error or hardware incompatibility. This warning message serves as an indicator that the kernel is encountering unexpected hardware behavior, which could mask more serious underlying issues or create confusion during system debugging and maintenance. The vulnerability affects systems running Linux kernels that implement the x86 MTRR subsystem, particularly those with processors that have disabled or lack support for the fixed MTRR capability bit, potentially leading to degraded system stability and performance monitoring issues.
The remediation for this vulnerability involves implementing a proper capability check within the mtrr_save_state() function before attempting to access fixed MTRR MSRs. This aligns with the fundamental security principle of capability-based access control and defensive programming practices that require explicit verification of hardware features before utilization. The fix ensures that the kernel only attempts to read fixed MTRR registers when the corresponding capability bit is set in the MTRR capability MSR, preventing the generation of unnecessary general protection faults and eliminating the associated warning messages. This approach follows established best practices for hardware abstraction layer design and adheres to the principle of least privilege in system programming, where system components only access hardware features they are explicitly designed to support. The vulnerability classification aligns with CWE-129, which addresses inadequate input validation, and relates to ATT&CK technique T1082, which involves system information discovery, as the issue affects how the system discovers and interacts with hardware capabilities during normal operation.
The root cause analysis reveals a fundamental gap in the kernel's hardware capability detection mechanisms, where the code assumes that all processors supporting MTRR functionality also support the fixed variant. This assumption breaks down on modern processors that may disable or remove certain legacy features while maintaining core MTRR functionality. The vulnerability demonstrates the importance of maintaining strict hardware compatibility checks in kernel code and underscores the need for comprehensive testing across different processor generations and configurations. The fix essentially implements a capability-based access control mechanism that prevents the kernel from attempting operations on hardware features that may not be present or enabled, thereby improving system robustness and reducing unnecessary error conditions that could interfere with system monitoring and diagnostic processes.