CVE-2026-89438 in Linux
Summary
by MITRE • 09/12/2026
In the Linux kernel, the following vulnerability has been resolved:
platform/x86: ISST: Validate logical CPU id and clos id
Validate max CLOS ID and logical CPU ID for core power feature. Reject any clos level or logical CPU number greater than the supported maximum. These are used to calculate MMIO offset.
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Analysis
by VulDB Data Team • 09/12/2026
The Intel Speed Select Technology (ISST) interface within the Linux kernel provides a mechanism for userspace applications to manage processor performance and power states by interacting with specific Model-Specific Registers and memory-mapped I/O regions. A critical flaw was identified in how this subsystem validates input parameters before performing hardware operations, specifically concerning logical CPU identifiers and Cache Level Identifier (CLOS) values used for core power features. The vulnerability stems from insufficient boundary checking on these inputs prior to their use in calculating offsets into MMIO space. When a user or application supplies a CLOS ID or logical CPU number that exceeds the maximum value supported by the specific hardware generation, the kernel fails to reject this invalid input immediately. Instead, it proceeds with arithmetic operations using these out-of-bounds values to determine memory addresses for subsequent write commands.
This lack of validation creates a direct path for improper access to system resources and potential buffer overflows or arbitrary writes depending on how the calculated offset is utilized within the driver's MMIO handling routines. By allowing logical CPU IDs greater than the number of available cores, or CLOS levels beyond what the hardware architecture supports, the kernel may compute memory addresses that fall outside the intended register range for ISST control registers. This can lead to writing data to unintended hardware registers, potentially causing system instability, unpredictable behavior in other subsystems sharing those MMIO regions, or a denial of service condition if critical power management states are corrupted. In severe cases, such improper input validation could be exploited by local users with appropriate privileges to disrupt the stability of the operating system or escalate their privilege level if subsequent error handling routines contain additional flaws that can be triggered by the resulting hardware state corruption.
From a security architecture perspective, this issue is classified under CWE-20: Improper Input Validation and potentially CWE-787: Out-of-bounds Write depending on the exact memory layout and how the MMIO offset translates to physical addresses in different kernel configurations. The attack vector aligns with ATT&CK technique T1546.003, specifically Event Triggered Execution or Privilege Escalation through driver exploitation, as it involves manipulating system drivers via IOCTL interfaces that are typically accessible by users with specific group permissions. The operational impact is primarily localized to systems utilizing Intel processors that support ISST features, affecting the reliability and security of power management controls which are critical for both performance tuning and thermal regulation in modern server and desktop environments.
To mitigate this vulnerability, system administrators should ensure that their Linux kernels are updated to include patches from upstream vendors that implement strict range checking for logical CPU IDs and CLOS levels before they are processed by the ISST driver logic. The fix involves adding explicit conditional checks that compare incoming values against hardware-reported maximums retrieved during device initialization. If a value exceeds these limits, the operation must be aborted with an appropriate error code returned to the calling application rather than proceeding with MMIO calculations. Additionally, defense-in-depth strategies such as restricting access to ISST-related IOCTL interfaces through mandatory access control policies like SELinux or AppArmor can limit the attack surface by preventing unauthorized users from invoking these sensitive kernel functions even if a vulnerability exists in the driver implementation.