CVE-2026-64211

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.