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This can lead\nto a situation where the list_empty() sees a valid list entry, but the\nsubsequent list_first() sees a different view of list head state after a\nmodification.\n\nIn the case of dm-thin, this author had a production box crash from a GP\nfault in the process_deferred_bios path. This function saw a valid list\nhead in get_first_thin() but when it subsequently dereferenced that and\nturned it into a thin_c, it got the inside of the struct pool, since the\nlist was now empty and referring to itself. The kernel on which this\noccurred printed both a warning about a refcount_t being saturated, and\na UBSAN error for an out-of-bounds cpuid access in the queued spinlock,\nprior to the fault itself. 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This is because each of these\nfunctions performs its own READ_ONCE() of the list head. This can lead\nto a situation where the list_empty() sees a valid list entry, but the\nsubsequent list_first() sees a different view of list head state after a\nmodification.\n\nIn the case of dm-thin, this author had a production box crash from a GP\nfault in the process_deferred_bios path. This function saw a valid list\nhead in get_first_thin() but when it subsequently dereferenced that and\nturned it into a thin_c, it got the inside of the struct pool, since the\nlist was now empty and referring to itself. The kernel on which this\noccurred printed both a warning about a refcount_t being saturated, and\na UBSAN error for an out-of-bounds cpuid access in the queued spinlock,\nprior to the fault itself. When the resulting kdump was examined, it\nwas possible to see another thread patiently waiting in thin_dtr's\nsynchronize_rcu.\n\nThe thin_dtr call managed to pull the thin_c out of the active thins\nlist (and have it be the last entry in the active_thins list) at just\nthe wrong moment which lead to this crash.\n\nFortunately, the fix here is straight forward. Switch get_first_thin()\nfunction to use list_first_or_null_rcu() which performs just a single\nREAD_ONCE() and returns NULL if the list is already empty.\n\nThis was run against the devicemapper test suite's thin-provisioning\nsuites for delete and suspend and no regressions were observed."}, {"lang": "es", "value": "En el kernel de Linux, se ha resuelto la siguiente vulnerabilidad: dm thin: hacer que get_first_thin use la funci\u00f3n list first de rcu-safe La documentaci\u00f3n en rculist.h explica la ausencia de list_empty_rcu() y advierte a los programadores que no conf\u00eden en una secuencia list_empty() -> list_first() en el c\u00f3digo seguro de RCU. Esto se debe a que cada una de estas funciones realiza su propio READ_ONCE() del encabezado de la lista. Esto puede llevar a una situaci\u00f3n en la que list_empty() ve una entrada de lista v\u00e1lida, pero el list_first() posterior ve una vista diferente del estado del encabezado de lista despu\u00e9s de una modificaci\u00f3n. En el caso de dm-thin, este autor tuvo un bloqueo del cuadro de producci\u00f3n debido a una falla de GP en la ruta process_deferred_bios. Esta funci\u00f3n vio un encabezado de lista v\u00e1lido en get_first_thin() pero cuando posteriormente desreferenciaba eso y lo convert\u00eda en un thin_c, obtuvo el interior del grupo de estructuras, ya que la lista ahora estaba vac\u00eda y se refer\u00eda a s\u00ed misma. El n\u00facleo en el que esto ocurri\u00f3 imprimi\u00f3 una advertencia sobre la saturaci\u00f3n de un refcount_t y un error UBSAN por un acceso a cpuid fuera de los l\u00edmites en el spinlock en cola, antes del fallo en s\u00ed. Cuando se examin\u00f3 el kdump resultante, fue posible ver otro hilo esperando pacientemente en elsynchronous_rcu de thin_dtr. La llamada thin_dtr logr\u00f3 sacar el thin_c de la lista de thins activa (y hacer que sea la \u00faltima entrada en la lista de active_thins) justo en el momento equivocado, lo que provoc\u00f3 este bloqueo. Afortunadamente, la soluci\u00f3n aqu\u00ed es sencilla. Cambie la funci\u00f3n get_first_thin() para usar list_first_or_null_rcu() que realiza solo un \u00fanico READ_ONCE() y devuelve NULL si la lista ya est\u00e1 vac\u00eda. Esto se ejecut\u00f3 contra las suites de aprovisionamiento fino del conjunto de pruebas devicemapper para eliminar y suspender y no se observaron regresiones."}], "id": "CVE-2025-21664", "lastModified": "2025-11-03T21:19:03.373", "metrics": {"cvssMetricV31": [{"cvssData": {"attackComplexity": "LOW", "attackVector": "LOCAL", "availabilityImpact": "HIGH", "baseScore": 5.5, "baseSeverity": "MEDIUM", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "privilegesRequired": "LOW", "scope": "UNCHANGED", "userInteraction": "NONE", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H", "version": "3.1"}, "exploitabilityScore": 1.8, "impactScore": 3.6, "source": "nvd@nist.gov", "type": "Primary"}]}, "published": "2025-01-21T13:15:10.053", "references": [{"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"], "url": "https://git.kernel.org/stable/c/12771050b6d059eea096993bf2001da9da9fddff"}, {"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"], "url": "https://git.kernel.org/stable/c/6b305e98de0d225ccebfb225730a9f560d28ecb0"}, {"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"], "url": "https://git.kernel.org/stable/c/802666a40c71a23542c43a3f87e3a2d0f4e8fe45"}, {"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"], "url": "https://git.kernel.org/stable/c/80f130bfad1dab93b95683fc39b87235682b8f72"}, {"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"], "url": "https://git.kernel.org/stable/c/cbd0d5ecfa390ac29c5380200147d09c381b2ac6"}, {"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"], "url": "https://git.kernel.org/stable/c/cd30a3960433ec2db94b3689752fa3c5df44d649"}, {"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"], "url": "https://git.kernel.org/stable/c/ec037fe8c0d0f6140e3d8a49c7b29cb5582160b8"}, {"source": "af854a3a-2127-422b-91ae-364da2661108", "url": "https://lists.debian.org/debian-lts-announce/2025/03/msg00001.html"}, {"source": "af854a3a-2127-422b-91ae-364da2661108", "url": "https://lists.debian.org/debian-lts-announce/2025/03/msg00002.html"}], "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "vulnStatus": "Modified", "weaknesses": [{"description": [{"lang": "en", "value": "NVD-CWE-noinfo"}], "source": "nvd@nist.gov", "type": "Primary"}]}

{"bugzilla": {"description": "kernel: dm thin: make get_first_thin use rcu-safe list first function", "id": "2339132", "url": "https://bugzilla.redhat.com/show_bug.cgi?id=2339132"}, "csaw": false, "cvss3": {"cvss3_base_score": "4.4", "cvss3_scoring_vector": "CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H", "status": "draft"}, "cwe": "CWE-20", "details": ["In the Linux kernel, the following vulnerability has been resolved:\ndm thin: make get_first_thin use rcu-safe list first function\nThe documentation in rculist.h explains the absence of list_empty_rcu()\nand cautions programmers against relying on a list_empty() ->\nlist_first() sequence in RCU safe code. This is because each of these\nfunctions performs its own READ_ONCE() of the list head. This can lead\nto a situation where the list_empty() sees a valid list entry, but the\nsubsequent list_first() sees a different view of list head state after a\nmodification.\nIn the case of dm-thin, this author had a production box crash from a GP\nfault in the process_deferred_bios path. This function saw a valid list\nhead in get_first_thin() but when it subsequently dereferenced that and\nturned it into a thin_c, it got the inside of the struct pool, since the\nlist was now empty and referring to itself. The kernel on which this\noccurred printed both a warning about a refcount_t being saturated, and\na UBSAN error for an out-of-bounds cpuid access in the queued spinlock,\nprior to the fault itself. When the resulting kdump was examined, it\nwas possible to see another thread patiently waiting in thin_dtr's\nsynchronize_rcu.\nThe thin_dtr call managed to pull the thin_c out of the active thins\nlist (and have it be the last entry in the active_thins list) at just\nthe wrong moment which lead to this crash.\nFortunately, the fix here is straight forward. Switch get_first_thin()\nfunction to use list_first_or_null_rcu() which performs just a single\nREAD_ONCE() and returns NULL if the list is already empty.\nThis was run against the devicemapper test suite's thin-provisioning\nsuites for delete and suspend and no regressions were observed."], "name": "CVE-2025-21664", "package_state": [{"cpe": "cpe:/o:redhat:enterprise_linux:10", "fix_state": "Fix deferred", "package_name": "kernel", "product_name": "Red Hat Enterprise Linux 10"}, {"cpe": "cpe:/o:redhat:enterprise_linux:6", "fix_state": "Out of support scope", "package_name": "kernel", "product_name": "Red Hat Enterprise Linux 6"}, {"cpe": "cpe:/o:redhat:enterprise_linux:7", "fix_state": "Out of support scope", "package_name": "kernel", "product_name": "Red Hat Enterprise Linux 7"}, {"cpe": "cpe:/o:redhat:enterprise_linux:7", "fix_state": "Out of support scope", "package_name": "kernel-rt", "product_name": "Red Hat Enterprise Linux 7"}, {"cpe": "cpe:/o:redhat:enterprise_linux:8", "fix_state": "Out of support scope", "package_name": "kernel", "product_name": "Red Hat Enterprise Linux 8"}, {"cpe": "cpe:/o:redhat:enterprise_linux:8", "fix_state": "Out of support scope", "package_name": "kernel-rt", "product_name": "Red Hat Enterprise Linux 8"}, {"cpe": "cpe:/o:redhat:enterprise_linux:9", "fix_state": "Fix deferred", "package_name": "kernel", "product_name": "Red Hat Enterprise Linux 9"}, {"cpe": "cpe:/o:redhat:enterprise_linux:9", "fix_state": "Fix deferred", "package_name": "kernel-rt", "product_name": "Red Hat Enterprise Linux 9"}], "public_date": "2025-01-21T00:00:00Z", "references": ["https://www.cve.org/CVERecord?id=CVE-2025-21664\nhttps://nvd.nist.gov/vuln/detail/CVE-2025-21664\nhttps://lore.kernel.org/linux-cve-announce/2025012135-CVE-2025-21664-3744@gregkh/T"], "threat_severity": "Moderate"}

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