CVE-2026-43439

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

cgroup: fix race between task migration and iteration

When a task is migrated out of a css_set, cgroup_migrate_add_task()

first moves it from cset->tasks to cset->mg_tasks via:

list_move_tail(&task->cg_list, &cset->mg_tasks);

If a css_task_iter currently has it->task_pos pointing to this task,

css_set_move_task() calls css_task_iter_skip() to keep the iterator

valid. However, since the task has already been moved to ->mg_tasks,

the iterator is advanced relative to the mg_tasks list instead of the

original tasks list. As a result, remaining tasks on cset->tasks, as

well as tasks queued on cset->mg_tasks, can be skipped by iteration.

Fix this by calling css_set_skip_task_iters() before unlinking

task->cg_list from cset->tasks. This advances all active iterators to

the next task on cset->tasks, so iteration continues correctly even

when a task is concurrently being migrated.

This race is hard to hit in practice without instrumentation, but it

can be reproduced by artificially slowing down cgroup_procs_show().

For example, on an Android device a temporary

/sys/kernel/cgroup/cgroup_test knob can be added to inject a delay

into cgroup_procs_show(), and then:

1) Spawn three long-running tasks (PIDs 101, 102, 103).

2) Create a test cgroup and move the tasks into it.

3) Enable a large delay via /sys/kernel/cgroup/cgroup_test.

4) In one shell, read cgroup.procs from the test cgroup.

5) Within the delay window, in another shell migrate PID 102 by

writing it to a different cgroup.procs file.

Under this setup, cgroup.procs can intermittently show only PID 101

while skipping PID 103. Once the migration completes, reading the

file again shows all tasks as expected.

Note that this change does not allow removing the existing

css_set_skip_task_iters() call in css_set_move_task(). The new call

in cgroup_migrate_add_task() only handles iterators that are racing

with migration while the task is still on cset->tasks. Iterators may

also start after the task has been moved to cset->mg_tasks. If we

dropped css_set_skip_task_iters() from css_set_move_task(), such

iterators could keep task_pos pointing to a migrating task, causing

css_task_iter_advance() to malfunction on the destination css_set,

up to and including crashes or infinite loops.

The race window between migration and iteration is very small, and

css_task_iter is not on a hot path. In the worst case, when an

iterator is positioned on the first thread of the migrating process,

cgroup_migrate_add_task() may have to skip multiple tasks via

css_set_skip_task_iters(). However, this only happens when migration

and iteration actually race, so the performance impact is negligible

compared to the correctness fix provided here.