{"schema":"threatline.record/2","record":{"id":"CVE-2026-78118","aliases":["GHSA-gm6w-pv43-m3vf"],"summary":"Race condition on kernel wait queues: unlocked z_waitq_head() read in the timer expiration handler can yield a stale or sentinel thread pointer on SMP","details":"The kernel helper z_waitq_head() (kernel/include/wait_q.h) reads the head of a thread wait queue, but on SMP systems it was called from several places without holding the scheduler spinlock _sched_spinlock that serializes wait-queue mutation. The consequential call site is z_timer_expiration_handler() in kernel/timer.c, which held only the timer subsystem lock timer_lock while reading timer->wait_q and then unpending, setting the return value of, and readying the thread it obtained. halt_thread() in kernel/sched.c removes an aborted thread from its wait queue while holding only _sched_spinlock, so the two paths are not mutually exclusive.\n\nWhen the last waiter is removed on another CPU inside that window, sys_dlist_peek_head() can return the list sentinel — a pointer into the k_timer object itself — typed as a struct k_thread * (with CONFIG_WAITQ_SCALABLE, rb_get_min() can return an arbitrary node of a tree being restructured), or it can return a pointer to a thread that has already been unpended and, for dynamically allocated threads, freed. The handler then calls unpend_thread_no_timeout(), which performs a sys_dlist_remove() through pointers read from that object, and arch_thread_return_value_set(), which writes unconditionally through the pointer.\n\nThe result is out-of-bounds and use-after-free writes in supervisor context, plus scheduler state corruption; a milder outcome is a thread that has re-pended elsewhere being woken with a success return value from an unrelated blocking call. All of the primitives needed to drive the interleaving — k_timer_start(), k_timer_status_sync(), k_thread_create() and k_thread_abort() — are system calls, so with CONFIG_USERSPACE enabled an unprivileged user-mode thread can attempt the race against kernel-owned data.\n\nExploitation requires a multi-CPU build (CONFIG_MP_MAX_NUM_CPUS greater than one; the affected region is a no-op on uniprocessor targets), local code execution, and winning a narrow timing window, so the practical outcome is most often a kernel crash rather than controlled corruption. The fix splits the helper into z_waitq_head_locked() for callers that already hold the scheduler spinlock and a z_waitq_head() that acquires it, and brings the timer expiration handler's head read, unpend and ready under a single LOCK_SCHED_SPINLOCK region.","categories":["Unclassified"],"product":"zephyr","vendor":"zephyrproject","ecosystem":null,"affected_ranges":[{"source":"CVE List","source_url":"https://www.cve.org/CVERecord?id=CVE-2026-78118","source_modified_at":"2026-10-23T21:28:10.701Z","fetched_at":"2026-10-23T21:47:51.402276Z","product":"zephyr","vendor":"zephyrproject","versions":[{"version":"2.2.0","status":"affected","lessThan":"4.5.0","versionType":"semver"}],"defaultStatus":"unaffected"}],"fixed_versions":[{"source":"CVE List","source_url":"https://www.cve.org/CVERecord?id=CVE-2026-78118","source_modified_at":"2026-10-23T21:28:10.701Z","fetched_at":"2026-10-23T21:47:51.402276Z","package":{"name":"zephyr","ecosystem":null},"version":"4.5.0","basis":"first unaffected version stated in the CVE record"}],"cvss":7,"severity":"High","kev":{"listed":false},"epss":{"probability":0.00083,"percentile":0.00223,"score_date":"2026-10-23"},"published_at":"2026-10-21T18:19:32.200Z","modified_at":"2026-10-23T21:28:10.701Z","provenance":[{"source":"CVE List","url":"https://www.cve.org/CVERecord?id=CVE-2026-78118","fetched_at":"2026-10-23T21:47:51.402276Z","source_modified_at":"2026-10-23T21:28:10.701Z"},{"source":"EPSS","url":"https://api.first.org/data/v1/epss?cve=CVE-2026-78118","fetched_at":"2026-10-23T18:42:43.628194Z","source_modified_at":"2026-10-23"}],"sources":["CVE List","EPSS"],"references":[{"source":"CVE List","url":"https://github.com/zephyrproject-rtos/zephyr/commit/a9b21fb354e8468dd15db307c3e599dc68b7ef33","tags":["patch"]},{"source":"CVE List","url":"https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-gm6w-pv43-m3vf"}],"locations":[],"motoko_assessment":{"status":"requires_architecture_review","automatic_remediation":false},"cve_state":"PUBLISHED","cvss_source":"CVE List","ssvc":{"exploitation":"none","automatable":"no","technical_impact":"total","assessed_at":"2026-10-23T20:45:38.419990Z"},"classification":{"method":"product-first software-layer rules","rules_version":"2.1","matched_on":null,"confidence":"unknown","reviewed":false},"change_timestamp":"2026-10-23T21:28:10.701Z","content_hash":"4e65a2ad84df8ddfecdc2a6a0a83a62070a2024f3d2683da2ddae4cdfe8fcbed"}}