CVE-2026-90208 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
clocksource/drivers/samsung_pwm: Switch to raw_spinlock_t type
Samsung PWM timer might be used as a clock source on some legacy systems. When PREEMPT_RT is enabled on ARM, regular spinlock is converted to a sleeping lock (mutex-based), which must not be used in atomic context such as hard interrupt handlers. Switch the samsung_pwm_lock to the raw_spinlock, which remains a true non-sleeping spinlock even under PREEMPT_RT.
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Analysis
by VulDB Data Team • 09/17/2026
The vulnerability described involves an improper use of synchronization primitives within the Linux kernel's Samsung Pulse Width Modulation driver, specifically affecting systems where this hardware serves as a clock source. In standard preemptible kernels, regular spinlocks function correctly for short critical sections that cannot be interrupted. However, when the PREEMPT_RT patch set is enabled on ARM architectures, these regular spinlocks are transformed into sleeping locks implemented via mutexes to improve real-time latency characteristics by allowing tasks to sleep while waiting for lock acquisition rather than busy-waiting. This transformation introduces a critical constraint: sleeping locks cannot be used in atomic contexts where preemption is disabled or interrupts are masked, such as within hard interrupt handlers.
The Samsung PWM timer driver utilizes the samsung_pwm_lock variable to protect access to hardware registers and internal state during operations that may occur in high-priority execution paths. Under PREEMPT_RT configurations on ARM platforms, this lock was incorrectly implemented using a standard spinlock type which gets converted to a sleeping mutex at runtime. When an interrupt handler or other atomic context attempts to acquire this lock, the kernel triggers a warning or panic because it detects that code is attempting to sleep in a non-sleepable context. This represents a classic concurrency violation where the synchronization mechanism does not match the execution context requirements, leading to system instability, crashes, or undefined behavior on affected legacy systems relying on Samsung PWM for timekeeping.
From a technical classification perspective, this issue aligns with CWE-829, which covers Inclusion of Functionality from Untrusted Control Sphere when considering the broader implications of improper kernel configuration handling, but more accurately maps to CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization (Race Condition). The specific failure mode is a violation of atomic context constraints, often categorized under timing-related concurrency errors. In terms of MITRE ATT&CK mapping for Linux systems, this relates to techniques involving exploitation of race conditions or improper handling of system resources that could potentially be leveraged in denial-of-service scenarios if triggered by maliciously timed interrupts, although the primary impact here is stability rather than direct security compromise like privilege escalation.
The operational impact of this vulnerability includes kernel panics and system crashes on embedded devices and legacy hardware running PREEMPT_RT kernels with ARM architecture support. Since clock sources are fundamental to timekeeping across the entire operating system, instability in this driver can lead to cascading failures affecting timers, scheduling decisions, and other subsystems dependent on accurate time synchronization. This is particularly severe for real-time applications where deterministic behavior is required, as the crash prevents any further execution of critical tasks.
To mitigate this vulnerability, the Samsung PWM timer driver must be updated to utilize raw_spinlock_t instead of regular spinlock_t for its internal locking mechanisms. The raw_spinlock type ensures that the lock remains a true non-sleeping spinlock even when PREEMPT_RT is enabled, thereby maintaining compatibility with atomic contexts such as hard interrupt handlers and bottom halves. This change preserves the necessary mutual exclusion properties without introducing illegal sleeping operations in restricted execution environments. System administrators should apply kernel updates containing this fix to ensure stability on affected platforms, particularly those deploying real-time extensions for low-latency requirements.