CVE-2025-38434

In the Linux kernel, the following vulnerability has been resolved:<br /> <br /> Revert "riscv: Define TASK_SIZE_MAX for __access_ok()"<br /> <br /> This reverts commit ad5643cf2f69 ("riscv: Define TASK_SIZE_MAX for<br /> __access_ok()").<br /> <br /> This commit changes TASK_SIZE_MAX to be LONG_MAX to optimize access_ok(),<br /> because the previous TASK_SIZE_MAX (default to TASK_SIZE) requires some<br /> computation.<br /> <br /> The reasoning was that all user addresses are less than LONG_MAX, and all<br /> kernel addresses are greater than LONG_MAX. Therefore access_ok() can<br /> filter kernel addresses.<br /> <br /> Addresses between TASK_SIZE and LONG_MAX are not valid user addresses, but<br /> access_ok() let them pass. That was thought to be okay, because they are<br /> not valid addresses at hardware level.<br /> <br /> Unfortunately, one case is missed: get_user_pages_fast() happily accepts<br /> addresses between TASK_SIZE and LONG_MAX. futex(), for instance, uses<br /> get_user_pages_fast(). This causes the problem reported by Robert [1].<br /> <br /> Therefore, revert this commit. TASK_SIZE_MAX is changed to the default:<br /> TASK_SIZE.<br /> <br /> This unfortunately reduces performance, because TASK_SIZE is more expensive<br /> to compute compared to LONG_MAX. But correctness first, we can think about<br /> optimization later, if required.