CVE-2026-98163

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)