CVE-2025-38434 Information

Jul 26, 2025 cve

Description

In the Linux kernel the following vulnerability has been resolved:

Revert iscv: Define TASK_SIZE_MAX for __access_ok()\n This reverts commit ad5643cf2f69 ( iscv: 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.

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.

Reference

https://git.kernel.org/stable/c/890ba5be6335dbbbc99af14ea007befb5f83f174 https://git.kernel.org/stable/c/f8b1898748dfeb4f9b67b6a6d661f354b9de3523 https://git.kernel.org/stable/c/fe30c30bf3bb68d4a4d8c7c814769857b5c973e6

CNNVD-202507-3264 (Published: 2025-07-25)

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