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CVE-2025-38434

Estado: ModificadaMedia (5.5)—

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

Revert "riscv: Define TASK_SIZE_MAX for __access_ok()"

This reverts commit ad5643cf2f69 ("riscv: Define TASK_SIZE_MAX for __access_ok()").

This commit changes TASK_SIZE_MAX to be LONG_MAX to optimize access_ok(), because the previous TASK_SIZE_MAX (default to TASK_SIZE) requires some computation.

The reasoning was that all user addresses are less than LONG_MAX, and all kernel addresses are greater than LONG_MAX. Therefore access_ok() can filter kernel addresses.

Addresses between TASK_SIZE and LONG_MAX are not valid user addresses, but access_ok() let them pass. That was thought to be okay, because they are not valid addresses at hardware level.

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Unfortunately, one case is missed: get_user_pages_fast() happily accepts addresses between TASK_SIZE and LONG_MAX. futex(), for instance, uses get_user_pages_fast(). This causes the problem reported by Robert [1].

Therefore, revert this commit. TASK_SIZE_MAX is changed to the default: TASK_SIZE.

This unfortunately reduces performance, because TASK_SIZE is more expensive to compute compared to LONG_MAX. But correctness first, we can think about optimization later, if required.

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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      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nRevert \"riscv: Define TASK_SIZE_MAX for __access_ok()\"\n\nThis reverts commit ad5643cf2f69 (\"riscv: Define TASK_SIZE_MAX for\n__access_ok()\").\n\nThis commit changes TASK_SIZE_MAX to be LONG_MAX to optimize access_ok(),\nbecause the previous TASK_SIZE_MAX (default to TASK_SIZE) requires some\ncomputation.\n\nThe reasoning was that all user addresses are less than LONG_MAX, and all\nkernel addresses are greater than LONG_MAX. Therefore access_ok() can\nfilter kernel addresses.\n\nAddresses between TASK_SIZE and LONG_MAX are not valid user addresses, but\naccess_ok() let them pass. That was thought to be okay, because they are\nnot valid addresses at hardware level.\n\nUnfortunately, one case is missed: get_user_pages_fast() happily accepts\naddresses between TASK_SIZE and LONG_MAX. futex(), for instance, uses\nget_user_pages_fast(). This causes the problem reported by Robert [1].\n\nTherefore, revert this commit. TASK_SIZE_MAX is changed to the default:\nTASK_SIZE.\n\nThis unfortunately reduces performance, because TASK_SIZE is more expensive\nto compute compared to LONG_MAX. But correctness first, we can think about\noptimization later, if required."
    },
    {
      "lang": "es",
      "value": "En el kernel de Linux, se ha resuelto la siguiente vulnerabilidad: Revertir \"riscv: Define TASK_SIZE_MAX for __access_ok()\". Esto revierte el commit ad5643cf2f69 (\"riscv: Define TASK_SIZE_MAX for __access_ok()\"). Este commit cambia TASK_SIZE_MAX a LONG_MAX para optimizar access_ok(), ya que el valor predeterminado de TASK_SIZE_MAX (predeterminado) requiere cálculos. El razonamiento era que todas las direcciones de usuario son menores que LONG_MAX y todas las direcciones de kernel son mayores que LONG_MAX. Por lo tanto, access_ok() puede filtrar direcciones de kernel. Las direcciones entre TASK_SIZE y LONG_MAX no son direcciones de usuario válidas, pero access_ok() las deja pasar. Se consideró que esto era correcto, ya que no son direcciones válidas a nivel de hardware. Desafortunadamente, se omite un caso: get_user_pages_fast() acepta direcciones entre TASK_SIZE y LONG_MAX. futex(), por ejemplo, usa get_user_pages_fast(). Esto causa el problema reportado por Robert [1]. Por lo tanto, revierte este commit . TASK_SIZE_MAX se cambia al valor predeterminado: TASK_SIZE. Lamentablemente, esto reduce el rendimiento, ya que TASK_SIZE es más costoso de calcular que LONG_MAX. Pero primero la corrección; podemos pensar en la optimización más adelante, si es necesario."
    }
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  "lastModified": "2026-07-30T06:23:14.043",
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