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CVE-2024-50102

Estado: ModificadaMedia (5.5)—

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

x86: fix user address masking non-canonical speculation issue

It turns out that AMD has a "Meltdown Lite(tm)" issue with non-canonical accesses in kernel space. And so using just the high bit to decide whether an access is in user space or kernel space ends up with the good old "leak speculative data" if you have the right gadget using the result:

Now, the kernel surrounds the access with a STAC/CLAC pair, and those instructions end up serializing execution on older Zen architectures, which closes the speculation window.

But that was true only up until Zen 5, which renames the AC bit [1]. That improves performance of STAC/CLAC a lot, but also means that the speculation window is now open.

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Note that this affects not just the new address masking, but also the regular valid_user_address() check used by access_ok(), and the asm version of the sign bit check in the get_user() helpers.

It does not affect put_user() or clear_user() variants, since there's no speculative result to be used in a gadget for those operations.

Detalles técnicos trazas, registros y código del informe original
  CVE-2020-12965 “Transient Execution of Non-Canonical Accesses“

CVSS

Probabilidad de explotación (EPSS)

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).

Tecnologías afectadas (1)

Linux — Linux Kernel

CWE

Referencias

JSON original (NVD)

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          "version": "2.0.3",
          "timestamp": "2025-10-01T20:22:29.275892Z"
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      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nx86: fix user address masking non-canonical speculation issue\n\nIt turns out that AMD has a \"Meltdown Lite(tm)\" issue with non-canonical\naccesses in kernel space.  And so using just the high bit to decide\nwhether an access is in user space or kernel space ends up with the good\nold \"leak speculative data\" if you have the right gadget using the\nresult:\n\n  CVE-2020-12965 “Transient Execution of Non-Canonical Accesses“\n\nNow, the kernel surrounds the access with a STAC/CLAC pair, and those\ninstructions end up serializing execution on older Zen architectures,\nwhich closes the speculation window.\n\nBut that was true only up until Zen 5, which renames the AC bit [1].\nThat improves performance of STAC/CLAC a lot, but also means that the\nspeculation window is now open.\n\nNote that this affects not just the new address masking, but also the\nregular valid_user_address() check used by access_ok(), and the asm\nversion of the sign bit check in the get_user() helpers.\n\nIt does not affect put_user() or clear_user() variants, since there's no\nspeculative result to be used in a gadget for those operations."
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      "value": "En el kernel de Linux, se ha resuelto la siguiente vulnerabilidad: x86: se soluciona el problema de especulación no canónica de enmascaramiento de direcciones de usuario Resulta que AMD tiene un problema de \"Meltdown Lite(tm)\" con los accesos no canónicos en el espacio del kernel. Y entonces, usar solo el bit alto para decidir si un acceso está en el espacio del usuario o en el espacio del kernel termina con la buena y vieja \"filtración de datos especulativos\" si tienes el gadget correcto usando el resultado: CVE-2020-12965 \"Ejecución transitoria de accesos no canónicos\" Ahora, el kernel rodea el acceso con un par STAC/CLAC, y esas instrucciones terminan serializando la ejecución en arquitecturas Zen más antiguas, lo que cierra la ventana de especulación. Pero eso era cierto solo hasta Zen 5, que renombra el bit AC [1]. Eso mejora mucho el rendimiento de STAC/CLAC, pero también significa que la ventana de especulación ahora está abierta. Tenga en cuenta que esto no solo afecta al nuevo enmascaramiento de dirección, sino también a la comprobación regular valid_user_address() utilizada por access_ok() y a la versión asm de la comprobación del bit de signo en los ayudantes get_user(). No afecta a las variantes put_user() o clear_user(), ya que no hay ningún resultado especulativo que se pueda utilizar en un gadget para esas operaciones."
    }
  ],
  "lastModified": "2026-06-17T08:03:35.533",
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