Wind River Support Network

Description

In the Linux kernel, the following vulnerability has been resolved:  cgroup: Avoid iteration of dying tasks with zero refcount  The commit 260fbcb92bbea ("cgroup: Move dying_tasks cleanup from cgroup_task_release() to cgroup_task_free()") extended the lifetime of tasks on the dying_tasks list. The iterators have provision to go through dying_tasks because of dying threadgroup leaders or explicit CSS_TASK_ITER_WITH_DEAD, however, it was expected that such tasks can obtain a new reference (that is possible before cgroup_task_release()/put_task_struct_rcu_user()). The tasks after cgroup_task_release() and before cgroup_task_free() are subject to race when they may or may not have ->usage count > 0.  The race window is between css_task_iter_next() invocations when css_set_lock is released and we may arrive at a new ->task_pos. The iterator should not attempt to resurrect tasks whose ->usage count dropped to zero. (When that happens, __put_task_struct_rcu_cb() is already imminent and the returned task_struct would could be used after free.)  As for the fix, we cannot simply check the signal->live count of a task on the dying list because that won't distinguish regular zombies waiting to be reaped from RCU remnant tasks that are going to be free'd. Therefore add an extra check to rule out ->usage==0 tasks from any iteration.  The repeat: loop in css_task_iter_advance() doesn't consider ->usage count, so add a new loop to css_task_iter_next() to skip de-used tasks on the dying_list.  Rough illustration of the possible race    R (reader of cgroup.procs)         T (thread)                       L (group leader)   ---------------------------------  -------------------------------- --------------------------------                                                                       L exits, signal->live > 0                                                                       cgroup_task_dead(L)                                                                         css_set_skip_task_iters() // skips only cset->tasks                                                                         list_add_tail(&L->cg_list, &cset->dying_tasks)   css_task_iter_next()     take css_set_lock     css_task_iter_advance()       leader && signal->live != 0       => it->task_pos = &L->cg_list     release css_set_lock                                      T exits                                      --signal->live == 0 				     cgroup_task_dead(T) // css_set_lock                                      release_task(T)                                        cgroup_task_release(T)                                        release_task(L) // zap_leader                                          cgroup_task_release(L)                                          put_task_struct_rcu_user(L)                                          ...RCU...                                          put_task_struct(L)                                            L->usage = 0                                            /* L still on dying_tasks */                                            ...RCU...                                            __put_task_struct(L)   css_task_iter_next() // another iteration     take css_set_lock     it->task_pos = &L->cg_list     get_task_struct(L)       => addition on 0     drop css_set_lock                                            cgroup_task_free(L)                                              css_set_skip_task_iters() // dying skip comes too late                                            free_task(L)   cgroup_procs_show()     task_pid_vnr(L)

CVEs

• CVE-2026-98163