CVE-2026-43439

Description

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.

PUBLISHED Reserved 2026-05-01 | Published 2026-05-08 | Updated 2026-05-08 | Assigner Linux

Product status

References

git.kernel.org/...c/7c85debc35e6d131bd29c64f2ae78c6ede0e55c4

git.kernel.org/...c/3b95abab7369235a37b15eaec6e1a0b443bba7c7

git.kernel.org/...c/4a9654a2b46cfdaae287fb8995f536245635e467

git.kernel.org/...c/3dfd1328c05234e8d8fa61948b2ba82680594988

git.kernel.org/...c/9cca530c7cc1b3e02cb8fa7f80060dd4b38562ce

git.kernel.org/...c/86ceaccfdfa16dad05addb33dc206e03589bcfd1

git.kernel.org/...c/9dc76f6fc0d28d2382583715bc4ec22f28104845

git.kernel.org/...c/5ee01f1a7343d6a3547b6802ca2d4cdce0edacb1

cve.org (CVE-2026-43439)

nvd.nist.gov (CVE-2026-43439)

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