CVE-2021-47209

{"id": "CVE-2021-47209", "cveTags": [], "metrics": {"cvssMetricV31": [{"type": "Primary", "source": "nvd@nist.gov", "cvssData": {"scope": "UNCHANGED", "version": "3.1", "baseScore": 5.5, "attackVector": "LOCAL", "baseSeverity": "MEDIUM", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H", "integrityImpact": "NONE", "userInteraction": "NONE", "attackComplexity": "LOW", "availabilityImpact": "HIGH", "privilegesRequired": "LOW", "confidentialityImpact": "NONE"}, "impactScore": 3.6, "exploitabilityScore": 1.8}]}, "published": "2024-04-10T19:15:48.447", "references": [{"url": "https://git.kernel.org/stable/c/512e21c150c1c3ee298852660f3a796e267e62ec", "tags": ["Patch"], "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"}, {"url": "https://git.kernel.org/stable/c/b027789e5e50494c2325cc70c8642e7fd6059479", "tags": ["Patch"], "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"}, {"url": "https://git.kernel.org/stable/c/512e21c150c1c3ee298852660f3a796e267e62ec", "tags": ["Patch"], "source": "af854a3a-2127-422b-91ae-364da2661108"}, {"url": "https://git.kernel.org/stable/c/b027789e5e50494c2325cc70c8642e7fd6059479", "tags": ["Patch"], "source": "af854a3a-2127-422b-91ae-364da2661108"}], "vulnStatus": "Analyzed", "weaknesses": [{"type": "Primary", "source": "nvd@nist.gov", "description": [{"lang": "en", "value": "CWE-416"}]}], "descriptions": [{"lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nsched/fair: Prevent dead task groups from regaining cfs_rq's\n\nKevin is reporting crashes which point to a use-after-free of a cfs_rq\nin update_blocked_averages(). Initial debugging revealed that we've\nlive cfs_rq's (on_list=1) in an about to be kfree()'d task group in\nfree_fair_sched_group(). However, it was unclear how that can happen.\n\nHis kernel config happened to lead to a layout of struct sched_entity\nthat put the 'my_q' member directly into the middle of the object\nwhich makes it incidentally overlap with SLUB's freelist pointer.\nThat, in combination with SLAB_FREELIST_HARDENED's freelist pointer\nmangling, leads to a reliable access violation in form of a #GP which\nmade the UAF fail fast.\n\nMichal seems to have run into the same issue[1]. He already correctly\ndiagnosed that commit a7b359fc6a37 (\"sched/fair: Correctly insert\ncfs_rq's to list on unthrottle\") is causing the preconditions for the\nUAF to happen by re-adding cfs_rq's also to task groups that have no\nmore running tasks, i.e. also to dead ones. His analysis, however,\nmisses the real root cause and it cannot be seen from the crash\nbacktrace only, as the real offender is tg_unthrottle_up() getting\ncalled via sched_cfs_period_timer() via the timer interrupt at an\ninconvenient time.\n\nWhen unregister_fair_sched_group() unlinks all cfs_rq's from the dying\ntask group, it doesn't protect itself from getting interrupted. If the\ntimer interrupt triggers while we iterate over all CPUs or after\nunregister_fair_sched_group() has finished but prior to unlinking the\ntask group, sched_cfs_period_timer() will execute and walk the list of\ntask groups, trying to unthrottle cfs_rq's, i.e. re-add them to the\ndying task group. These will later -- in free_fair_sched_group() -- be\nkfree()'ed while still being linked, leading to the fireworks Kevin\nand Michal are seeing.\n\nTo fix this race, ensure the dying task group gets unlinked first.\nHowever, simply switching the order of unregistering and unlinking the\ntask group isn't sufficient, as concurrent RCU walkers might still see\nit, as can be seen below:\n\n CPU1: CPU2:\n : timer IRQ:\n : do_sched_cfs_period_timer():\n : :\n : distribute_cfs_runtime():\n : rcu_read_lock();\n : :\n : unthrottle_cfs_rq():\n sched_offline_group(): :\n : walk_tg_tree_from(\u2026,tg_unthrottle_up,\u2026):\n list_del_rcu(&tg->list); :\n (1) : list_for_each_entry_rcu(child, &parent->children, siblings)\n : :\n (2) list_del_rcu(&tg->siblings); :\n : tg_unthrottle_up():\n unregister_fair_sched_group(): struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];\n : :\n list_del_leaf_cfs_rq(tg->cfs_rq[cpu]); :\n : :\n : if (!cfs_rq_is_decayed(cfs_rq) || cfs_rq->nr_running)\n (3) : list_add_leaf_cfs_rq(cfs_rq);\n : :\n : :\n : :\n : :\n : \n---truncated---"}, {"lang": "es", "value": "En el kernel de Linux, se ha resuelto la siguiente vulnerabilidad: sched/fair: Evitar que los grupos de tareas inactivos recuperen cfs_rq Kevin informa fallos que apuntan a un use-after-free de un cfs_rq en update_blocked_averages(). La depuraci\u00f3n inicial revel\u00f3 que tenemos cfs_rq activos (on_list=1) en un grupo de tareas a punto de ser kfree() en free_fair_sched_group(). Sin embargo, no estaba claro c\u00f3mo puede suceder eso. Su configuraci\u00f3n del kernel result\u00f3 en un dise\u00f1o de struct sched_entity que coloca el miembro 'my_q' directamente en el medio del objeto, lo que hace que se superponga incidentalmente con el puntero de lista libre de SLUB. Eso, en combinaci\u00f3n con la manipulaci\u00f3n del puntero de lista libre de SLAB_FREELIST_HARDENED, conduce a una violaci\u00f3n de acceso confiable en forma de un #GP que hizo que el UAF fallara r\u00e1pidamente. Michal parece haberse topado con el mismo problema[1]. \u00c9l ya diagnostic\u00f3 correctamente que el commit a7b359fc6a37 (\"sched/fair: Insertar correctamente cfs_rq en la lista al desregular\") est\u00e1 causando que se cumplan las condiciones previas para que se produzca la UAF al volver a agregar cfs_rq tambi\u00e9n a los grupos de tareas que ya no tienen tareas en ejecuci\u00f3n, es decir, tambi\u00e9n a los que est\u00e1n inactivos. Sin embargo, su an\u00e1lisis no detecta la causa ra\u00edz real y no se puede ver solo desde el backtrace del bloqueo, ya que el verdadero infractor es tg_unthrottle_up() que se llama a trav\u00e9s de sched_cfs_period_timer() mediante la interrupci\u00f3n del temporizador en un momento inconveniente. Cuando unregister_fair_sched_group() desvincula todos los cfs_rq del grupo de tareas que est\u00e1 inactivo, no se protege a s\u00ed mismo de ser interrumpido. Si la interrupci\u00f3n del temporizador se activa mientras iteramos sobre todas las CPU o despu\u00e9s de que unregister_fair_sched_group() haya terminado pero antes de desvincular el grupo de tareas, sched_cfs_period_timer() se ejecutar\u00e1 y recorrer\u00e1 la lista de grupos de tareas, intentando liberar cfs_rq, es decir, volver a agregarlos al grupo de tareas moribundo. Estos ser\u00e1n posteriormente -- en free_fair_sched_group() -- kfree()'ed mientras siguen vinculados, lo que lleva a los fuegos artificiales que Kevin y Michal est\u00e1n viendo. Para solucionar esta ejecuci\u00f3n, aseg\u00farese de que el grupo de tareas moribundo se desvincule primero. Sin embargo, simplemente cambiar el orden de anulaci\u00f3n del registro y desvinculaci\u00f3n del grupo de tareas no es suficiente, ya que los caminantes de RCU concurrentes a\u00fan podr\u00edan verlo, como se puede ver a continuaci\u00f3n: CPU1: CPU2: : timer IRQ: : do_sched_cfs_period_timer(): : : : distributed_cfs_runtime(): : rcu_read_lock(); : : : unthrottle_cfs_rq(): sched_offline_group(): : : walk_tg_tree_from(\u2026,tg_unthrottle_up,\u2026): list_del_rcu(&tg->list); : (1) : list_for_each_entry_rcu(child, &parent->children, brothers) : : (2) list_del_rcu(&tg->siblings); : : tg_unthrottle_up(): anular_registro_justo_sched_group(): struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)]; : : list_del_leaf_cfs_rq(tg->cfs_rq[cpu]); : : : : si (!cfs_rq_est\u00e1_deca\u00eddo(cfs_rq) || cfs_rq->nr_en_ejecuci\u00f3n) (3) : lista_agregar_hoja_cfs_rq(cfs_rq); : : : : : : : : : ---truncado---"}], "lastModified": "2025-03-27T21:16:39.163", "configurations": [{"nodes": [{"negate": false, "cpeMatch": [{"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "vulnerable": true, "matchCriteriaId": "172C15F0-CF2B-47F2-8931-3368DC97E4E2", "versionEndExcluding": "5.15.5", "versionStartIncluding": "5.13"}], "operator": "OR"}]}], "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"}