CVE-2026-68421 in Linux
Summary
by MITRE • 08/10/2026
In the Linux kernel, the following vulnerability has been resolved:
sched_ext: Don't warn on core-sched forced idle in put_prev_task_scx()
put_prev_task_scx() warns when a runnable task drops to a lower sched_class without SCX_OPS_ENQ_LAST, on the assumption that balance_one() would have kept it running. Core scheduling breaks that: a forced-idle SMT sibling reschedules through the core_pick fast path in pick_next_task(), which skips pick_task_scx() and thus balance_one(), so a runnable task can drop to idle with ENQ_LAST unset.
Gate the warning on sched_cpu_cookie_match(): a cookie mismatch means core scheduling forced the idle, while a match (or core scheduling off) still catches a genuine missing-ENQ_LAST drop.
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Analysis
by VulDB Data Team • 08/10/2026
The vulnerability resides within the Linux kernel's scheduler extension subsystem, specifically in the sched_ext module where the put_prev_task_scx() function generates unwarranted warnings under certain core scheduling conditions. This issue manifests when a runnable task transitions from a higher scheduling class to a lower one without the SCX_OPS_ENQ_LAST flag being set, triggering a warning that assumes the balance_one() function would have maintained its execution. However, this assumption breaks down in core scheduling scenarios where tasks can be forced into idle states through specific pathways that bypass normal scheduling logic.
The technical flaw occurs due to the interaction between core scheduling mechanisms and the scheduler's expectation about task state transitions. When core scheduling is active, a forced-idle SMT sibling can reschedule through the core_pick fast path in pick_next_task(), which directly skips the pick_task_scx() function and consequently bypasses balance_one(). This direct transition pathway means that a runnable task can legitimately drop to an idle state without having its ENQ_LAST flag set, as the normal balancing process is circumvented. The warning mechanism fails to account for this core scheduling behavior, leading to false positive alerts.
The operational impact of this vulnerability affects system administrators and developers who rely on accurate scheduler warnings to identify genuine configuration issues or implementation bugs. The spurious warnings can obscure real problems in the scheduler's operation, making debugging more difficult and potentially leading to misdiagnosis of actual performance or stability issues. Systems utilizing core scheduling features may experience increased noise in their monitoring and logging systems, as legitimate forced-idle transitions are incorrectly flagged as errors.
The mitigation strategy involves gating the warning mechanism through a sched_cpu_cookie_match() check that differentiates between genuine scheduling errors and core-scheduling-induced transitions. When a cookie mismatch occurs, it indicates that core scheduling has forced the idle state, while matching cookies or disabled core scheduling still correctly identify actual missing-ENQ_LAST drops. This approach aligns with common security practices for distinguishing legitimate system behavior from potential security issues, similar to how ATT&CK framework categorizes different types of system interactions to avoid false positives in threat detection mechanisms.
This vulnerability demonstrates the complexity inherent in modern scheduler implementations where multiple subsystems must interact seamlessly while maintaining appropriate error reporting boundaries. The issue reflects the challenges of maintaining backward compatibility and accurate error reporting when introducing new scheduling paradigms such as core scheduling that fundamentally alter expected execution patterns. The solution implemented follows established principles for handling conditional logic in kernel code, ensuring that warnings only appear when genuine implementation errors occur rather than legitimate system behaviors.
The fix addresses a specific CWE category related to improper handling of system state transitions and warning generation, where the vulnerability falls under weak error handling or incorrect assumptions about system behavior. It also relates to operational security principles where accurate logging and warning systems are crucial for maintaining system integrity and performance monitoring. The core scheduling feature's interaction with the scheduler's expectation management highlights the importance of considering all possible execution paths when designing kernel subsystems, particularly those that modify fundamental scheduling behaviors.
This technical analysis reveals how seemingly minor implementation details in kernel schedulers can have significant implications for system monitoring and debugging operations. The vulnerability showcases the complexity of modern kernel development where new features must not only function correctly but also maintain appropriate interfaces with existing diagnostic systems. The solution demonstrates proper defensive programming practices by explicitly checking conditions that distinguish between different types of system transitions rather than making assumptions about execution flow patterns.
The resolution approach mirrors common security engineering principles for maintaining accurate telemetry and logging systems, where false positives can be as problematic as missing genuine issues. By implementing the cookie-based matching mechanism, the scheduler maintains its ability to detect actual implementation bugs while avoiding spurious warnings that could mislead system operators during troubleshooting activities. This type of fix is particularly important in production environments where accurate monitoring signals are essential for maintaining system reliability and performance optimization efforts.