CVE-2026-64211: srcu: Don't queue workqueue handlers to never-online CPUs
In the Linux kernel, the following vulnerability has been resolved:
srcu: Don't queue workqueue handlers to never-online CPUs
While an srcustruct 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 cpupossiblemask. 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 callsrcu(), synchronizesrcu(), or synchronizesrcuexpedited() 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 callsrcu() than to warn about such invocations. This commit therefore also redirects callbacks queued on not-yet-fully-online CPUs to the boot CPU.
Affected Software
Remediation
Event History
Frequently Asked Questions
Which environments are most likely to experience impact?
The reported failure mode is a hang on s390 systems when workqueue handlers are scheduled on CPUs that have never been online. The condition arises while an SRCU structure transitions from CPU-0 to all-CPUs state.
What level of attacker access is required?
The CVSS vector rates the attack as local and requires low privileges, with no user interaction. The supplied information does not identify a remote attack path.
What can be done if updates cannot be applied immediately?
The available remediation is to apply the provided patch. The data does not provide a documented configuration workaround or mitigation other than the fix, which prevents queueing handlers to CPUs that have not yet come online and redirects early callbacks to the boot CPU.