CVE-2026-45942
In the Linux kernel, the following vulnerability has been resolved:
ext4: fix e4b bitmap inconsistency reports
A bitmap inconsistency issue was observed during stress tests under mixed huge-page workloads. Ext4 reported multiple e4b bitmap check failures like:
ext4_mb_complex_scan_group:2508: group 350, 8179 free clusters as per group info. But got 8192 blocks
Analysis and experimentation confirmed that the issue is caused by a race condition between page migration and bitmap modification. Although this timing window is extremely narrow, it is still hit in practice:
The root cause of this issue is that the fast path of load_buddy only increments the folio's reference count, which is insufficient to prevent concurrent folio migration.
Leer descripción completaMostrar menos
We observed that the folio migration process acquires the folio lock. Therefore, we can determine whether to take the fast path in load_buddy by checking the lock status. If the folio is locked, we opt for the slow path (which acquires the lock) to close this concurrency window.
Additionally, this change addresses the following issues:
When the DOUBLE_CHECK macro is enabled to inspect bitmap-related issues, the following error may be triggered:
corruption in group 324 at byte 784(6272): f in copy != ff on disk/prealloc
Analysis reveals that this is a false positive. There is a specific race window where the bitmap and the group descriptor become momentarily inconsistent, leading to this error report:
The original logic assumed that since mb_cmp_bitmaps is called when the bitmap is newly loaded from disk, the folio lock would be sufficient to prevent concurrent access. However, this overlooks a specific race condition: if another process attempts to load buddy and finds the folio is already in an uptodate state, it will immediately begin using it without holding folio lock.
Detalles técnicos trazas, registros y código del informe original
folio_lock ext4_mb_load_buddy
__migrate_folio
check ref count
folio_mc_copy __filemap_get_folio
folio_try_get(folio)
......
mb_mark_used
ext4_mb_unload_buddy
__folio_migrate_mapping
folio_ref_freeze
folio_unlock
ext4_mb_load_buddy ext4_mb_load_buddy
__filemap_get_folio(create|lock)
folio_lock
ext4_mb_init_cache
folio_mark_uptodate
__filemap_get_folio(no lock)
......
mb_mark_used
mb_mark_used_double
mb_cmp_bitmaps
mb_set_bits(e4b->bd_bitmap)
folio_unlockCVSS
- Versión: 3.1
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- Puntuación base: 7.8
Probabilidad de explotación (EPSS)
- Probabilidad de explotación en los próximos 30 días: 0.10%
- Percentil entre todas las CVEs puntuadas: 1
- Fecha de la puntuación: 6/10/2026
EPSS (Exploit Prediction Scoring System, de FIRST) estima la probabilidad de que una vulnerabilidad sea explotada en 30 días. Complementa a CVSS (impacto) y a CISA KEV (explotación confirmada).
🎯 Técnicas ATT&CK
Cómo se explota esta vulnerabilidad y qué consigue el atacante, en el lenguaje de MITRE ATT&CK.
- Explotación
T1068Exploitation for Privilege Escalationprivilege escalation85 % - Impacto secundario
T1565.001Stored Data Manipulationimpact70 %
Vulnerabilidad local (AV:L, PR:L) en el kernel Linux causada por race condition entre migración de páginas y modificación de bitmap en ext4. Permite a un atacante local con privilegios bajos conseguir escalada (C:H/I:H/A:H) y corrupción de datos del sistema de ficheros.
Inferido por nuestro agente de análisis a partir de la descripción oficial, el vector CVSS y la CWE, y comprobado por un supervisor. Puede contener errores.
🛡️ Mitigaciones ATT&CK que cubren estas técnicas
Tecnologías afectadas (1)
CWE
- CWE-362
Referencias
- https://git.kernel.org/stable/c/29a07d691d282faf38c33d4b61839b89399110f9
- https://git.kernel.org/stable/c/57e83bfbe1e412ac42daced2086f3c6f9a17bba0
- https://git.kernel.org/stable/c/bdc56a9c46b2a99c12313122b9352b619a2e719e
- https://git.kernel.org/stable/c/c05033cfc5c7699cd4df8d48cef94d01da755f24
- https://git.kernel.org/stable/c/f29709a7a3fc38f5015d850504762cdef0e151f9
JSON original (NVD)
Mostrar
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"value": "In the Linux kernel, the following vulnerability has been resolved:\n\next4: fix e4b bitmap inconsistency reports\n\nA bitmap inconsistency issue was observed during stress tests under\nmixed huge-page workloads. Ext4 reported multiple e4b bitmap check\nfailures like:\n\next4_mb_complex_scan_group:2508: group 350, 8179 free clusters as\nper group info. But got 8192 blocks\n\nAnalysis and experimentation confirmed that the issue is caused by a\nrace condition between page migration and bitmap modification. Although\nthis timing window is extremely narrow, it is still hit in practice:\n\nfolio_lock ext4_mb_load_buddy\n__migrate_folio\n check ref count\n folio_mc_copy __filemap_get_folio\n folio_try_get(folio)\n ......\n mb_mark_used\n ext4_mb_unload_buddy\n __folio_migrate_mapping\n folio_ref_freeze\nfolio_unlock\n\nThe root cause of this issue is that the fast path of load_buddy only\nincrements the folio's reference count, which is insufficient to prevent\nconcurrent folio migration. We observed that the folio migration process\nacquires the folio lock. Therefore, we can determine whether to take the\nfast path in load_buddy by checking the lock status. If the folio is\nlocked, we opt for the slow path (which acquires the lock) to close this\nconcurrency window.\n\nAdditionally, this change addresses the following issues:\n\nWhen the DOUBLE_CHECK macro is enabled to inspect bitmap-related\nissues, the following error may be triggered:\n\ncorruption in group 324 at byte 784(6272): f in copy != ff on\ndisk/prealloc\n\nAnalysis reveals that this is a false positive. There is a specific race\nwindow where the bitmap and the group descriptor become momentarily\ninconsistent, leading to this error report:\n\next4_mb_load_buddy ext4_mb_load_buddy\n __filemap_get_folio(create|lock)\n folio_lock\n ext4_mb_init_cache\n folio_mark_uptodate\n __filemap_get_folio(no lock)\n ......\n mb_mark_used\n mb_mark_used_double\n mb_cmp_bitmaps\n mb_set_bits(e4b->bd_bitmap)\n folio_unlock\n\nThe original logic assumed that since mb_cmp_bitmaps is called when the\nbitmap is newly loaded from disk, the folio lock would be sufficient to\nprevent concurrent access. However, this overlooks a specific race\ncondition: if another process attempts to load buddy and finds the folio\nis already in an uptodate state, it will immediately begin using it without\nholding folio lock."
}
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