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.

Weakness

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

Affected Software

Extended Description

A race condition occurs within concurrent environments, and it is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/3b95abab7369235a37b15eaec6e1a0b443bba7c7
• https://git.kernel.org/stable/c/3dfd1328c05234e8d8fa61948b2ba82680594988
• https://git.kernel.org/stable/c/4a9654a2b46cfdaae287fb8995f536245635e467
• https://git.kernel.org/stable/c/5ee01f1a7343d6a3547b6802ca2d4cdce0edacb1
• https://git.kernel.org/stable/c/7c85debc35e6d131bd29c64f2ae78c6ede0e55c4
• https://git.kernel.org/stable/c/86ceaccfdfa16dad05addb33dc206e03589bcfd1
• https://git.kernel.org/stable/c/9cca530c7cc1b3e02cb8fa7f80060dd4b38562ce
• https://git.kernel.org/stable/c/9dc76f6fc0d28d2382583715bc4ec22f28104845