CVE-2026-46223

In the Linux kernel, the following vulnerability has been resolved:

cgroup: Defer css percpu_ref kill on rmdir until cgroup is depopulated

A chain of commits going back to v7.0 reworked rmdir to satisfy the

controller invariant that a subsystem's ->css_offline() must not run while

tasks are still doing kernel-side work in the cgroup.

[1] d245698d727a ("cgroup: Defer task cgroup unlink until after the task is done switching out")

[2] a72f73c4dd9b ("cgroup: Don't expose dead tasks in cgroup")

[3] 1b164b876c36 ("cgroup: Wait for dying tasks to leave on rmdir")

[4] 4c56a8ac6869 ("cgroup: Fix cgroup_drain_dying() testing the wrong condition")

[5] 13e786b64bd3 ("cgroup: Increment nr_dying_subsys_* from rmdir context")

[1] moved task cset unlink from do_exit() to finish_task_switch() so a

task's cset link drops only after the task has fully stopped scheduling.

That made tasks past exit_signals() linger on cset->tasks until their final

context switch, which led to a series of problems as what userspace expected

to see after rmdir diverged from what the kernel needs to wait for. [2]-[5]

tried to bridge that divergence: [2] filtered the exiting tasks from

cgroup.procs; [3] had rmdir(2) sleep in TASK_UNINTERRUPTIBLE for them; [4]

fixed the wait's condition; [5] made nr_dying_subsys_* visible

synchronously.

The cgroup_drain_dying() wait in [3] turned out to be a dead end. When the

rmdir caller is also the reaper of a zombie that pins a pidns teardown (e.g.

host PID 1 systemd reaping orphan pids that were re-parented to it during

the same teardown), rmdir blocks in TASK_UNINTERRUPTIBLE waiting for those

pids to free, the pids can't free because PID 1 is the reaper and it's stuck

in rmdir, and the system A-A deadlocks. No internal lock ordering breaks

this; the wait itself is the bug.

The css killing side that drove the original reorder, however, can be made

cleanly asynchronous: ->css_offline() is already async, run from

css_killed_work_fn() driven by percpu_ref_kill_and_confirm(). The fix is to

make that chain start only after all tasks have left the cgroup. rmdir's

user-visible side then returns as soon as cgroup.procs and friends are

empty, while ->css_offline() still runs only after the cgroup is fully

drained.

Verified by the original reproducer (pidns teardown + zombie reaper, runs

under vng) which hangs vanilla and succeeds here, and by per-commit

deterministic repros for [2], [3], [4], [5] with a boot parameter that

widens the post-exit_signals() window so each state is reliably reachable.

Some stress tests on top of that.

cgroup_apply_control_disable() has the same shape of pre-existing race:

when a controller is disabled via subtree_control, kill_css() ran

synchronously while tasks past exit_signals() could still be linked to

the cgroup's csets, and ->css_offline() could fire before they drained.

This patch preserves the existing synchronous behavior at that call site

(kill_css_sync() + kill_css_finish() back-to-back) and a follow-up patch

will defer kill_css_finish() there using a per-css trigger.

This seems like the right approach and I don't see problems with it. The

changes are somewhat invasive but not excessively so, so backporting to

-stable should be okay. If something does turn out to be wrong, the fallback

is to revert the entire chain ([1]-[5]) and rework in the development branch

instead.

v2: Pin cgrp across the deferred destroy work with explicit

cgroup_get()/cgroup_put() around queue_work() and the work_fn. v1

wasn't actually broken (ordered cgroup_offline_wq + queue_work order

in cgroup_task_dead() saved it) but the explicit ref removes the

dependency on those non-obvious invariants. Also note the

pre-existing cgroup_apply_control_disable() race in the description;

a follow-up will defer kill_css_finish() there.