[PATCH] cgroup: Avoid iteration of dying tasks with zero refcount

["Michal Koutný" <mkoutny@suse.com>]

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 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 a band-aid, filter tasks returned from css_task_iter_next() only to
those that have positive usage count (whose reference's lifetime can be
extended).

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
                                     T exits
                                     --signal->live == 0
                                     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)
    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)

cgroup_task_release() is not synced via css_set_lock hence the race
possibility. (I'm not 100% convinced about this LLM-assisted
interleaving, multiple css_task_iter_next() calls may be involved with
css_set_lock released.)

Fixes: 260fbcb92bbea ("cgroup: Move dying_tasks cleanup from cgroup_task_release() to cgroup_task_free()")
Cc: stable@vger.kernel.org # v6.19+
Link: https://lists.debian.org/debian-kernel/2026/08/msg00220.html
Reported-by: Noah Elias Feldt <N.Feldt@mittwald.de>
Reported-by: Salvatore Bonaccorso <carnil@debian.org>
Signed-off-by: Michal Koutný <mkoutny@suse.com>
---
 kernel/cgroup/cgroup.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

Salvatore,
I was able to trigger the issue with your reproducer on 7.1.8
(occassionally, not always). With the patch on top of v7.3-rc1, I cannot
reproduce it (different base though). If you can confirm that in your
env, it'd be good.

Thanks,
Michal

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index c3a12fee7528f..112d68fe28778 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -5303,10 +5303,13 @@ struct task_struct *css_task_iter_next(struct css_task_iter *it)
 	if (it->flags & CSS_TASK_ITER_SKIPPED)
 		css_task_iter_advance(it);
 
-	if (it->task_pos) {
+	while (it->task_pos && !it->cur_task) {
 		it->cur_task = list_entry(it->task_pos, struct task_struct,
 					  cg_list);
-		get_task_struct(it->cur_task);
+		/* a task on dying_tasks with zero refcount is only valid for
+		 * RCU readers, not even interesting for
+		 * CSS_TASK_ITER_WITH_DEAD, find another one */
+		it->cur_task = tryget_task_struct(it->cur_task);
 		css_task_iter_advance(it);
 	}
 
-- 
2.55.0