Linux Kernel EUVD-2026-39849

The padata subsystem (kernel/padata.c) provides a parallelization framework used primarily by Linux's crypto layer (e.g., IPsec XFRM) to distribute workloads across CPUs. It registers CPU hotplug callbacks to manage work queues during CPU topology changes. The Linux kernel hotplug state machine enforces that callbacks registered in teardown phases at or below CPUHP_TEARDOWN_CPU must not return errors - the framework has no mechanism to handle failure at that stage. padata_cpu_dead(), introduced at commit 894c9ef9780c, was placed in such a phase, violating this invariant. When the callback returns an error, the kernel triggers a WARNING at kernel/cpu.c:1463. The root cause is an incorrect hotplug section registration - the callback must reside in the ONLINE section where failure is an accepted outcome. CWE is not formally assigned, but the class corresponds to improper error handling in a constrained execution context (closest: CWE-703). CPE confirms the affected software is cpe:2.3:a:linux:linux across multiple version ranges.