We found a refcount UAF bug as follows:

refcount_t: addition on 0; use-after-free.
WARNING: CPU: 1 PID: 342 at lib/refcount.c:25 refcount_warn_saturate+0xa0/0x148
Workqueue: events cpuset_hotplug_workfn
Call trace:
 refcount_warn_saturate+0xa0/0x148
 __refcount_add.constprop.0+0x5c/0x80
 css_task_iter_advance_css_set+0xd8/0x210
 css_task_iter_advance+0xa8/0x120
 css_task_iter_next+0x94/0x158
 update_tasks_root_domain+0x58/0x98
 rebuild_root_domains+0xa0/0x1b0
 rebuild_sched_domains_locked+0x144/0x188
 cpuset_hotplug_workfn+0x138/0x5a0
 process_one_work+0x1e8/0x448
 worker_thread+0x228/0x3e0
 kthread+0xe0/0xf0
 ret_from_fork+0x10/0x20

then a kernel panic will be triggered as below:

Unable to handle kernel paging request at virtual address 00000000c0000010
Call trace:
 cgroup_apply_control_disable+0xa4/0x16c
 rebind_subsystems+0x224/0x590
 cgroup_destroy_root+0x64/0x2e0
 css_free_rwork_fn+0x198/0x2a0
 process_one_work+0x1d4/0x4bc
 worker_thread+0x158/0x410
 kthread+0x108/0x13c
 ret_from_fork+0x10/0x18

The race that cause this bug can be shown as below:

(hotplug cpu)                | (umount cpuset)
mutex_lock(&cpuset_mutex)    | mutex_lock(&cgroup_mutex)
cpuset_hotplug_workfn        |
 rebuild_root_domains        |  rebind_subsystems
  update_tasks_root_domain   |   spin_lock_irq(&css_set_lock)
   css_task_iter_start       |    list_move_tail(&cset->e_cset_node[ss->id]
   while(css_task_iter_next) |                  &dcgrp->e_csets[ss->id]);
   css_task_iter_end         |   spin_unlock_irq(&css_set_lock)
mutex_unlock(&cpuset_mutex)  | mutex_unlock(&cgroup_mutex)

Inside css_task_iter_start/next/end, css_set_lock is hold and then
released, so when iterating task(left side), the css_set may be moved to
another list(right side), then it->cset_head points to the old list head
and it->cset_pos->next points to the head node of new list, which can't
be used as struct css_set.

To fix this issue, switch from all css_sets to only scgrp's css_sets to
patch in-flight iterators to preserve correct iteration, and then
update it->cset_head as well.
Reported-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Link: https://www.spinics.net/lists/cgroups/msg37935.htmlSuggested-by: Michal Koutný <mkoutny@suse.com>
Link: https://lore.kernel.org/all/20230526114139.70274-1-xiujianfeng@huaweicloud.com/Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Fixes: 2d8f243a ("cgroup: implement cgroup->e_csets[]")
Cc: stable@vger.kernel.org # v3.16+
Signed-off-by: Tejun Heo <tj@kernel.org>