FKIE_CVE-2026-43439

Vulnerability from fkie_nvd - Published: 2026-05-08 15:16 - Updated: 2026-05-21 17:26
Severity
4.7 (Medium) - CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
Summary
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.
References
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/3b95abab7369235a37b15eaec6e1a0b443bba7c7Patch
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/3dfd1328c05234e8d8fa61948b2ba82680594988Patch
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/4a9654a2b46cfdaae287fb8995f536245635e467Patch
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/5ee01f1a7343d6a3547b6802ca2d4cdce0edacb1Patch
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/7c85debc35e6d131bd29c64f2ae78c6ede0e55c4Patch
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/86ceaccfdfa16dad05addb33dc206e03589bcfd1Patch
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/9cca530c7cc1b3e02cb8fa7f80060dd4b38562cePatch
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/9dc76f6fc0d28d2382583715bc4ec22f28104845Patch
Impacted products
Vendor Product Version
linux linux_kernel *
linux linux_kernel *
linux linux_kernel *
linux linux_kernel *
linux linux_kernel *
linux linux_kernel *
linux linux_kernel *
linux linux_kernel *
linux linux_kernel *
linux linux_kernel 5.2
linux linux_kernel 5.2
linux linux_kernel 5.2
linux linux_kernel 5.2
linux linux_kernel 7.0
linux linux_kernel 7.0
To clipboard 

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      "lang": "en",
      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\ncgroup: fix race between task migration and iteration\n\nWhen a task is migrated out of a css_set, cgroup_migrate_add_task()\nfirst moves it from cset-\u003etasks to cset-\u003emg_tasks via:\n\n    list_move_tail(\u0026task-\u003ecg_list, \u0026cset-\u003emg_tasks);\n\nIf a css_task_iter currently has it-\u003etask_pos pointing to this task,\ncss_set_move_task() calls css_task_iter_skip() to keep the iterator\nvalid. However, since the task has already been moved to -\u003emg_tasks,\nthe iterator is advanced relative to the mg_tasks list instead of the\noriginal tasks list. As a result, remaining tasks on cset-\u003etasks, as\nwell as tasks queued on cset-\u003emg_tasks, can be skipped by iteration.\n\nFix this by calling css_set_skip_task_iters() before unlinking\ntask-\u003ecg_list from cset-\u003etasks. This advances all active iterators to\nthe next task on cset-\u003etasks, so iteration continues correctly even\nwhen a task is concurrently being migrated.\n\nThis race is hard to hit in practice without instrumentation, but it\ncan be reproduced by artificially slowing down cgroup_procs_show().\nFor example, on an Android device a temporary\n/sys/kernel/cgroup/cgroup_test knob can be added to inject a delay\ninto cgroup_procs_show(), and then:\n\n  1) Spawn three long-running tasks (PIDs 101, 102, 103).\n  2) Create a test cgroup and move the tasks into it.\n  3) Enable a large delay via /sys/kernel/cgroup/cgroup_test.\n  4) In one shell, read cgroup.procs from the test cgroup.\n  5) Within the delay window, in another shell migrate PID 102 by\n     writing it to a different cgroup.procs file.\n\nUnder this setup, cgroup.procs can intermittently show only PID 101\nwhile skipping PID 103. Once the migration completes, reading the\nfile again shows all tasks as expected.\n\nNote that this change does not allow removing the existing\ncss_set_skip_task_iters() call in css_set_move_task(). The new call\nin cgroup_migrate_add_task() only handles iterators that are racing\nwith migration while the task is still on cset-\u003etasks. Iterators may\nalso start after the task has been moved to cset-\u003emg_tasks. If we\ndropped css_set_skip_task_iters() from css_set_move_task(), such\niterators could keep task_pos pointing to a migrating task, causing\ncss_task_iter_advance() to malfunction on the destination css_set,\nup to and including crashes or infinite loops.\n\nThe race window between migration and iteration is very small, and\ncss_task_iter is not on a hot path. In the worst case, when an\niterator is positioned on the first thread of the migrating process,\ncgroup_migrate_add_task() may have to skip multiple tasks via\ncss_set_skip_task_iters(). However, this only happens when migration\nand iteration actually race, so the performance impact is negligible\ncompared to the correctness fix provided here."
    }
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  "id": "CVE-2026-43439",
  "lastModified": "2026-05-21T17:26:45.540",
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          "baseScore": 4.7,
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          "confidentialityImpact": "NONE",
          "integrityImpact": "NONE",
          "privilegesRequired": "LOW",
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          "userInteraction": "NONE",
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  "published": "2026-05-08T15:16:56.263",
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