CVE-2026-10681 SMP race in `thread_idx_alloc()` lets concurrent `k_object_alloc(K_OBJ_THREAD)` callers share a kernel-object permission slot

In Zephyr's userspace dynamic-objects subsystem, threadidxalloc in kernel/userspace/userspace.c allocated a new thread permission index from the global threadidxmap bitmap without holding listslock. On SMP systems, two user-mode threads invoking the kobjectallocKOBJTHREAD syscall concurrently can...