Zephyr’s dynamic kernel-object disposal path unref_check() in kernel\/userspace\/userspace.c frees an object’s storage (k_free(dyn->data)) once its reference count reaches zero, after running a per-object-type cleanup. The cleanup switch handled only K_OBJ_MSGQ and K_OBJ_STACK; there was no K_OBJ_TIMER case. A dynamically-allocated, initialized, and armed k_timer keeps its embedded struct _timeout dnode linked in the global timeout queue (_timeout_q), so freeing the timer storage without cancelling the timeout leaves a dangling node in that queue.<\/p>\n
When the timer next expires, the timeout machinery walks _timeout_q and invokes z_timer_expiration_handler() on the freed node, dereferencing and writing freed (and reusable) kernel heap in kernel\/ISR context. This is a deterministic use-after-free that does not depend on SMP: the queued node is simply never unlinked at free time.<\/p>\n
The disposal is reachable from an unprivileged user thread under CONFIG_USERSPACE + CONFIG_DYNAMIC_OBJECTS: a thread that holds the last permission on such a timer drops it via the k_object_release() syscall (or by exiting, through k_thread_perms_all_clear()), and can arm the timer itself via the k_timer_start() syscall. The free and the expiration handler run at kernel privilege while the actor is a user thread, so the bug is a sandbox-escape memory-corruption primitive usable for privilege escalation. The fix adds k_timer_cleanup() (cancel the timeout and wait for any in-flight handler) and calls it for K_OBJ_TIMER before freeing.<\/p>\n","protected":false},"excerpt":{"rendered":"
Zephyr’s dynamic kernel-object disposal path unref_check() in kernel\/userspace\/userspace.c frees an object’s storage (k_free(dyn->data)) once its reference count reaches zero, after running a per-obje<\/p>\n","protected":false},"template":"","meta":[],"categories":[],"tags":[],"class_list":["post-6662","vuln","type-vuln","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/vuln\/6662","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/vuln"}],"about":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/types\/vuln"}],"wp:attachment":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=6662"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=6662"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=6662"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}