CVE-2026-97904 in Linux
Summary
by MITRE • 09/25/2026
In the Linux kernel, the following vulnerability has been resolved:
cpufreq: initialize policy rwsem before sysfs publication
cpufreq_policy_alloc() initializes policy->rwsem after kobject_init_and_add() has created the policy sysfs directory and its default attributes. A sysfs access can therefore reach a policy callback before the semaphore has been initialized.
Initialize policy->rwsem before publishing the policy kobject so sysfs callbacks always see an initialized semaphore.
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Analysis
by VulDB Data Team • 09/25/2026
The vulnerability identified in the Linux kernel's cpufreq subsystem represents a critical race condition arising from improper initialization sequencing during device object creation. Specifically, within the cpufreq_policy_alloc function, the read-write semaphore associated with the CPU frequency policy was being initialized after the kobject had already been published to the sysfs filesystem via kobject_init_and_add. This ordering error creates a window of vulnerability where the kernel exposes system interface attributes before ensuring that the underlying synchronization primitives are ready for use. In Linux kernel development, sysfs serves as the primary mechanism for exposing device and driver state to user space applications, allowing configuration changes through file reads and writes. When an object is published in this manner, it becomes immediately accessible to any process with appropriate permissions, triggering callback functions that interact with the underlying data structures without guaranteeing their full initialization status.
The technical flaw lies in the temporal gap between sysfs publication and semaphore initialization. Once kobject_init_and_add completes its operation, the policy directory and its default attributes become visible in the virtual file system hierarchy. At this precise moment, any user space application or kernel thread can attempt to access these files. Such access invokes specific callback handlers designed to manage CPU frequency scaling policies. However, because the rwsem has not yet been initialized by cpufreq_policy_alloc at this stage, subsequent attempts to acquire locks for reading or writing policy data may operate on uninitialized memory structures or encounter undefined behavior. This lack of synchronization can lead to kernel panics, data corruption within the power management subsystem, or potentially allow an attacker to exploit the race condition to execute arbitrary code with kernel privileges if they can precisely time their access to coincide with this initialization gap.
From a security perspective, this vulnerability aligns closely with CWE-362, which describes concurrent execution issues and race conditions where shared resources are accessed without proper synchronization mechanisms in place. The operational impact is severe because it affects the stability and integrity of the operating system's power management infrastructure. An attacker who can trigger sysfs accesses rapidly or concurrently could potentially crash the kernel by causing a null pointer dereference or memory corruption due to the uninitialized semaphore state. Furthermore, depending on the specific implementation details of how the rwsem interacts with other locking mechanisms in the cpufreq driver, this race condition might be leveraged for privilege escalation attacks. By manipulating the timing of sysfs interactions, an unprivileged local user could potentially bypass security checks or corrupt kernel memory structures that are protected by expectations of proper lock acquisition.
Mitigation strategies primarily involve correcting the initialization order within the source code to ensure strict sequential integrity during object lifecycle management. The definitive fix requires moving the initialization of policy->rwsem to occur before calling kobject_init_and_add, thereby guaranteeing that any sysfs callback invoked immediately after publication will find a fully initialized and operational synchronization primitive. This ensures that all concurrent access attempts are properly serialized from the very first moment the interface becomes available. For system administrators unable to apply immediate kernel patches, limiting user space access to cpufreq sysfs attributes through strict file permissions can reduce the attack surface. Additionally, enabling Kernel Self-Protection features such as KASLR and stack protector may provide additional layers of defense against exploitation attempts derived from this race condition, although these are mitigations rather than fixes for the root cause.
This issue highlights the importance of adhering to established kernel development practices regarding object lifecycle management and synchronization primitives. It serves as a reminder that sysfs publication should always be treated as an irreversible point of no return where external access becomes possible. Consequently, all necessary internal state initialization must precede this step to prevent exposure of partially constructed objects. The resolution reinforces the need for rigorous code review processes focused on timing dependencies in driver development, particularly those involving hardware abstraction layers and power management subsystems which are frequently targeted by attackers seeking low-level system control.