CVE-2024-50066

Estado: ModificadaAlta (7)—

In the Linux kernel, the following vulnerability has been resolved:

mm/mremap: fix move_normal_pmd/retract_page_tables race

In mremap(), move_page_tables() looks at the type of the PMD entry and the specified address range to figure out by which method the next chunk of page table entries should be moved.

At that point, the mmap_lock is held in write mode, but no rmap locks are held yet. For PMD entries that point to page tables and are fully covered by the source address range, move_pgt_entry(NORMAL_PMD, ...) is called, which first takes rmap locks, then does move_normal_pmd(). move_normal_pmd() takes the necessary page table locks at source and destination, then moves an entire page table from the source to the destination.

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The problem is: The rmap locks, which protect against concurrent page table removal by retract_page_tables() in the THP code, are only taken after the PMD entry has been read and it has been decided how to move it. So we can race as follows (with two processes that have mappings of the same tmpfs file that is stored on a tmpfs mount with huge=advise); note that process A accesses page tables through the MM while process B does it through the file rmap:

When this happens, move_normal_pmd() can end up creating bogus PMD entries in the line `pmd_populate(mm, new_pmd, pmd_pgtable(pmd))`. The effect depends on arch-specific and machine-specific details; on x86, you can end up with physical page 0 mapped as a page table, which is likely exploitable for user->kernel privilege escalation.

Fix the race by letting process B recheck that the PMD still points to a page table after the rmap locks have been taken. Otherwise, we bail and let the caller fall back to the PTE-level copying path, which will then bail immediately at the pmd_none() check.

Bug reachability: Reaching this bug requires that you can create shmem/file THP mappings - anonymous THP uses different code that doesn't zap stuff under rmap locks. File THP is gated on an experimental config flag (CONFIG_READ_ONLY_THP_FOR_FS), so on normal distro kernels you need shmem THP to hit this bug. As far as I know, getting shmem THP normally requires that you can mount your own tmpfs with the right mount flags, which would require creating your own user+mount namespace; though I don't know if some distros maybe enable shmem THP by default or something like that.

Bug impact: This issue can likely be used for user->kernel privilege escalation when it is reachable.

Detalles técnicos trazas, registros y código del informe original

process A                      process B
=========                      =========
mremap
  mremap_to
    move_vma
      move_page_tables
        get_old_pmd
        alloc_new_pmd
                      *** PREEMPT ***
                               madvise(MADV_COLLAPSE)
                                 do_madvise
                                   madvise_walk_vmas
                                     madvise_vma_behavior
                                       madvise_collapse
                                         hpage_collapse_scan_file
                                           collapse_file
                                             retract_page_tables
                                               i_mmap_lock_read(mapping)
                                               pmdp_collapse_flush
                                               i_mmap_unlock_read(mapping)
        move_pgt_entry(NORMAL_PMD, ...)
          take_rmap_locks
          move_normal_pmd
          drop_rmap_locks

CVSS

Versión: 3.1
Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
Puntuación base: 7

Probabilidad de explotación (EPSS)

Probabilidad de explotación en los próximos 30 días: 0.21%
Percentil entre todas las CVEs puntuadas: 10
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.

ExplotaciónT1068 Exploitation for Privilege Escalationprivilege escalation85 %

Vulnerabilidad de race condition en mremap() que permite escalada de privilegios local (user→kernel) mediante manipulación de entradas PMD; AV:L, PR:L, UI:N confirma acceso local sin interacción; CWE-362 (TOCTOU) sustenta la condición de carrera.

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

Threat Intelligence Program Execution Prevention Application Isolation and Sandboxing Exploit Protection Update Software

Tecnologías afectadas (1)

Linux — Linux Kernel

CWE

CWE-362
CWE-362

Referencias

JSON original (NVD)

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      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nmm/mremap: fix move_normal_pmd/retract_page_tables race\n\nIn mremap(), move_page_tables() looks at the type of the PMD entry and the\nspecified address range to figure out by which method the next chunk of\npage table entries should be moved.\n\nAt that point, the mmap_lock is held in write mode, but no rmap locks are\nheld yet.  For PMD entries that point to page tables and are fully covered\nby the source address range, move_pgt_entry(NORMAL_PMD, ...) is called,\nwhich first takes rmap locks, then does move_normal_pmd(). \nmove_normal_pmd() takes the necessary page table locks at source and\ndestination, then moves an entire page table from the source to the\ndestination.\n\nThe problem is: The rmap locks, which protect against concurrent page\ntable removal by retract_page_tables() in the THP code, are only taken\nafter the PMD entry has been read and it has been decided how to move it. \nSo we can race as follows (with two processes that have mappings of the\nsame tmpfs file that is stored on a tmpfs mount with huge=advise); note\nthat process A accesses page tables through the MM while process B does it\nthrough the file rmap:\n\nprocess A                      process B\n=========                      =========\nmremap\n  mremap_to\n    move_vma\n      move_page_tables\n        get_old_pmd\n        alloc_new_pmd\n                      *** PREEMPT ***\n                               madvise(MADV_COLLAPSE)\n                                 do_madvise\n                                   madvise_walk_vmas\n                                     madvise_vma_behavior\n                                       madvise_collapse\n                                         hpage_collapse_scan_file\n                                           collapse_file\n                                             retract_page_tables\n                                               i_mmap_lock_read(mapping)\n                                               pmdp_collapse_flush\n                                               i_mmap_unlock_read(mapping)\n        move_pgt_entry(NORMAL_PMD, ...)\n          take_rmap_locks\n          move_normal_pmd\n          drop_rmap_locks\n\nWhen this happens, move_normal_pmd() can end up creating bogus PMD entries\nin the line `pmd_populate(mm, new_pmd, pmd_pgtable(pmd))`.  The effect\ndepends on arch-specific and machine-specific details; on x86, you can end\nup with physical page 0 mapped as a page table, which is likely\nexploitable for user->kernel privilege escalation.\n\nFix the race by letting process B recheck that the PMD still points to a\npage table after the rmap locks have been taken.  Otherwise, we bail and\nlet the caller fall back to the PTE-level copying path, which will then\nbail immediately at the pmd_none() check.\n\nBug reachability: Reaching this bug requires that you can create\nshmem/file THP mappings - anonymous THP uses different code that doesn't\nzap stuff under rmap locks.  File THP is gated on an experimental config\nflag (CONFIG_READ_ONLY_THP_FOR_FS), so on normal distro kernels you need\nshmem THP to hit this bug.  As far as I know, getting shmem THP normally\nrequires that you can mount your own tmpfs with the right mount flags,\nwhich would require creating your own user+mount namespace; though I don't\nknow if some distros maybe enable shmem THP by default or something like\nthat.\n\nBug impact: This issue can likely be used for user->kernel privilege\nescalation when it is reachable."
    },
    {
      "lang": "es",
      "value": "En el kernel de Linux, se ha resuelto la siguiente vulnerabilidad: mm/mremap: corrección de la ejecución move_normal_pmd/retract_page_tables En mremap(), move_page_tables() examina el tipo de entrada PMD y el rango de direcciones especificado para determinar mediante qué método se debe mover el siguiente fragmento de entradas de la tabla de páginas. En ese punto, el mmap_lock se mantiene en modo de escritura, pero aún no se mantienen bloqueos rmap. Para las entradas PMD que apuntan a tablas de páginas y están completamente cubiertas por el rango de direcciones de origen, se llama a move_pgt_entry(NORMAL_PMD, ...), que primero toma bloqueos rmap y luego realiza move_normal_pmd(). move_normal_pmd() toma los bloqueos de tabla de páginas necesarios en el origen y el destino, luego mueve una tabla de páginas completa desde el origen hasta el destino. El problema es el siguiente: los bloqueos de rmap, que protegen contra la eliminación simultánea de tablas de páginas por retract_page_tables() en el código THP, solo se toman después de que se haya leído la entrada PMD y se haya decidido cómo moverla. Por lo tanto, podemos competir de la siguiente manera (con dos procesos que tienen asignaciones del mismo archivo tmpfs que está almacenado en un montaje tmpfs con huge=advise); tenga en cuenta que el proceso A accede a las tablas de páginas a través del MM mientras que el proceso B lo hace a través del archivo rmap: proceso A proceso B ========= ========= mremap mremap_to move_vma move_page_tables get_old_pmd alloc_new_pmd *** PREEMPT *** madvise(MADV_COLLAPSE) do_madvise madvise_walk_vmas madvise_vma_behavior madvise_collapse hpage_collapse_scan_file collapse_file retract_page_tables i_mmap_lock_read(mapping) pmdp_collapse_flush i_mmap_unlock_read(mapping) move_pgt_entry(NORMAL_PMD, ...) take_rmap_locks move_normal_pmd drop_rmap_locks Cuando esto sucede, move_normal_pmd() puede terminar creando entradas PMD falsas en la línea `pmd_populate(mm, new_pmd, pmd_pgtable(pmd))`. El efecto depende de detalles específicos de la arquitectura y de la máquina; en x86, puede terminar con la página física 0 mapeada como una tabla de páginas, lo que probablemente sea explotable para la escalada de privilegios de usuario a kernel. Arregle la ejecución permitiendo que el proceso B vuelva a verificar que el PMD aún apunta a una tabla de páginas después de que se hayan tomado los bloqueos rmap. De lo contrario, abandonamos y dejamos que el llamador vuelva a la ruta de copia de nivel PTE, que luego abandonará inmediatamente en la verificación pmd_none(). Alcance del error: Alcanzar este error requiere que pueda crear asignaciones shmem/file THP - el THP anónimo usa un código diferente que no elimina cosas bajo bloqueos rmap. El THP de archivo está controlado por un indicador de configuración experimental (CONFIG_READ_ONLY_THP_FOR_FS), por lo que en los núcleos de distribución normales necesita shmem THP para alcanzar este error. Hasta donde yo sé, obtener shmem THP normalmente requiere que puedas montar tu propio tmpfs con los indicadores de montaje correctos, lo que requeriría crear tu propio espacio de nombres de usuario+montaje; aunque no sé si algunas distribuciones habilitan shmem THP de forma predeterminada o algo así. Impacto del error: es probable que este problema se pueda usar para la escalada de privilegios de usuario a kernel cuando sea posible."
    }
  ],
  "lastModified": "2026-08-04T11:21:14.440",
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