Bug 2506809 (CVE-2026-64211)

Summary: CVE-2026-64211 kernel: srcu: Don't queue workqueue handlers to never-online CPUs
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
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Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel. During certain CPU state transitions, the kernel's srcu_struct mechanism may attempt to schedule tasks on processors that are not active or will never be online. This incorrect scheduling of workqueue handlers can lead to system unresponsiveness, specifically causing hangs on s390 architectures. This vulnerability could result in a denial of service, making the system unavailable.
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Description OSIDB Bzimport 2026-07-24 16:04:56 UTC 
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.
Comment 1 Mauro Matteo Cascella 2026-07-28 11:33:11 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026072414-CVE-2026-64211-f6d4@gregkh/T