CVE-2026-10681 - SMP Race in Thread Index Allocation

CVE-2026-10681 - SMP Race in Thread Index Allocation

CVE-2026-10681 details an SMP race in thread_idx_alloc() that allows concurrent k_object_alloc(K_OBJ_THREAD) callers to share a kernel-object permission slot. This vulnerability occurs within Zephyr’s userspace dynamic-objects subsystem. Specifically, thread_idx_alloc() in kernel/userspace/userspace.c allocated a new thread permission index from the global _thread_idx_map[] bitmap without holding lists_lock. On SMP systems, two user-mode threads invoking the k_object_alloc(K_OBJ_THREAD) syscall concurrently can both observe the same low free bit, perform the same non-atomic RMW to clear it, and return the identical tidx.

The two newly created K_OBJ_THREAD objects are then assigned the same thread_id, causing the two user threads to alias a single bit position in every kernel object’s perms[] bitfield: any subsequent grant of access on a kernel object to one thread is implicitly a grant to the other, defeating userspace ACL isolation. A secondary lost-update window between the unlocked &=~BIT() in alloc and the locked |= BIT() in thread_idx_free() can also leak entries from the thread-index pool. The defect is reachable from any user-mode thread via the unrestricted __syscall k_object_alloc and is gated on CONFIG_USERSPACE, CONFIG_DYNAMIC_OBJECTS, and CONFIG_SMP.

The flaw was introduced when the per-thread permission index was added in 2018 and is present in every release up to and including v4.4.0. The flaw is fixed by holding lists_lock across the bitmap RMW and the permissions clear (and inlining the obj_list traversal that previously took the lock itself).

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