CVE-2026-64211 in Linux
Summary
by MITRE • 07/24/2026
In the Linux kernel, the following vulnerability has been resolved:
srcu: Don't queue workqueue handlers to never-online CPUs
While an srcu_struct structure is in the midst of switching from CPU-0 to all-CPUs state, it can attempt to invoke callbacks for CPUs that have never been online. Worse yet, it can attempt in invoke callbacks for CPUs that never will be online, even including imaginary CPUs not in cpu_possible_mask. This can cause hangs on s390, which is not set up to deal with workqueue handlers being scheduled on such CPUs. This commit therefore causes Tree SRCU to refrain from queueing workqueue handlers on CPUs that have not yet (and might never) come online.
Because callbacks are not invoked on CPUs that have not been online, it is an error to invoke call_srcu(), synchronize_srcu(), or synchronize_srcu_expedited() on a CPU that is not yet fully online. However, it turns out to be less code to redirect the callbacks from too-early invocations of call_srcu() than to warn about such invocations. This commit therefore also redirects callbacks queued on not-yet-fully-online CPUs to the boot CPU.
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Analysis
by VulDB Data Team • 07/27/2026
The vulnerability in question affects the Linux kernel's Source Reference Counting Update (SRCU) mechanism, specifically within the Tree SRCU implementation that manages concurrent access to data structures across multiple CPUs. The issue manifests when an srcu_struct structure transitions from a CPU-0 only state to an all-CPUs state during system operation. During this transitional phase, the kernel attempts to invoke callback functions on CPUs that have never been brought online or may never become available, including virtual CPU identifiers that exceed the bounds of cpu_possible_mask. This problematic behavior creates potential system hangs particularly on s390 architecture systems where the workqueue handler scheduling mechanism lacks proper handling for such scenarios.
The technical flaw stems from inadequate validation within the SRCU subsystem's callback invocation logic. When transitioning between CPU states, the system fails to properly check whether target CPUs are actually online or have ever been brought online before queuing workqueue handlers. This oversight allows for execution attempts on non-existent or unavailable CPU resources, creating a condition where the kernel may hang indefinitely while attempting to schedule work items on processors that cannot execute them. The vulnerability specifically impacts systems where the workqueue infrastructure is not designed to handle scheduling operations on CPUs that have never been online, which represents a fundamental architectural mismatch in the kernel's concurrency management.
The operational impact of this vulnerability extends beyond simple system hangs to potentially destabilize entire system operations, particularly on mainframe architectures like s390 where such CPU states are more prevalent and the workqueue handling is less flexible. Systems that rely heavily on SRCU mechanisms for synchronization may experience complete lockups during CPU state transitions, rendering critical services unavailable until manual intervention occurs. The vulnerability affects core kernel functionality that underpins many subsystems including network processing, file system operations, and device driver management that depend on SRCU for safe concurrent access patterns.
The fix implemented addresses this by modifying the Tree SRCU behavior to prevent queuing workqueue handlers on CPUs that have not yet come online or might never become available. This solution redirects callback execution from prematurely invoked call_srcu(), synchronize_srcu(), or synchronize_srcu_expedited() calls on non-ready CPUs to the boot CPU, effectively preventing system hangs while maintaining functional correctness. The approach follows established security principles by implementing defensive programming techniques that prevent invalid state transitions rather than attempting to detect and warn about them, which would require additional complexity in the kernel's warning infrastructure.
This vulnerability aligns with CWE-129, which addresses improper validation of array indices and resource access in kernel space operations, and relates to ATT&CK technique T1490, which covers resource hijacking through system instability. The mitigation strategy represents a proactive approach to preventing invalid CPU scheduling operations while maintaining backward compatibility for legitimate use cases, though it does introduce a performance overhead of redirecting callbacks to the boot CPU rather than allowing direct execution on target processors.
The fix demonstrates sound kernel security engineering principles by addressing the root cause rather than merely providing workarounds, ensuring that SRCU operations remain robust across all system configurations while maintaining the expected concurrency guarantees for legitimate use cases. This approach prevents the exploitation of invalid state transitions that could otherwise be leveraged to create denial-of-service conditions or potentially more sophisticated attacks targeting kernel stability and resource management mechanisms.